Refined Empirical Force Field to Model Protein-Self-Assembled Monolayer Interactions Based on AMBER14 and GAFF

Understanding protein interaction with material surfaces is important for the development of nanotechnological devices. The structures and dynamics of proteins can be studied via molecular dynamics (MD) if the protein-surface interactions can be accurately modeled. To answer this question, we computed the adsorption free energies of peptides (representing eleven different amino acids) on a hydrophobic self-assembled monolayer (CH₃-SAM) and compared them to the benchmark experimental data set. Our result revealed that existing biomolecular force fields, GAFF and AMBER ff14sb, cannot reproduce the experimental peptide adsorption free energies by Wei and Latour (Langmuir, 2009, 25, 5637-5646). To obtain the improved force fields, we systematically tuned the Lennard-Jones parameters of selected amino acid sidechains and the functional group of SAM with repeated metadynamics and umbrella sampling simulations. The final parameter set has yielded a significant improvement in the free energy values with R = 0.83 and MSE = 0.65 kcal/mol. We applied the refined force field to predict the initial adsorption orientation of lysozyme on CH₃-SAM. Two major orientations-face-down and face-up-were predicted. Our analysis on the protein structure, solvent accessible surface area, and binding of native ligand NAG₃ suggested that lysozyme in the face-up orientation can remain active after initial adsorption. However, because of its weaker affinity (ΔΔG = 7.86 kcal/mol) for the ligand, the bioactivity of the protein is expected to reduce. Our work facilitates the use of MD for the study of protein-SAM systems. The refined force field compatible with GROMACS is available at https://cbbio.cis.um.edu.mo/software/SAMFF .

The Role of Buffers in Wild-Type HEWL Amyloid Fibril Formation Mechanism

Amyloid fibrils, highly ordered protein aggregates, play an important role in the onset of several neurological disorders. Many studies have assessed amyloid fibril formation under specific solution conditions, but they all lack an important phenomena in biological solutions-buffer specific effects. We have focused on the formation of hen egg-white lysozyme (HEWL) fibrils in aqueous solutions of different buffers in both acidic and basic pH range. By means of UV-Vis spectroscopy, fluorescence measurements and CD spectroscopy, we have managed to show that fibrillization of HEWL is affected by buffer identity (glycine, TRIS, phosphate, KCl-HCl, cacodylate, HEPES, acetate), solution pH, sample incubation (agitated vs. static) and added excipients (NaCl and PEG). HEWL only forms amyloid fibrils at pH = 2.0 under agitated conditions in glycine and KCl-HCl buffers of high enough ionic strength. Phosphate buffer on the other hand stabilizes the HEWL molecules. Similar stabilization effect was achieved by addition of PEG12000 molecules to the solution.

Integrating Proteins in Layer-by-Layer Assemblies Independently of their Electrical Charge

Layer-by-layer (LbL) assembly is an attractive method for protein immobilization at interfaces, a much wanted step for biotechnologies and biomedicine. Integrating proteins in LbL thin films is however very challenging due to their low conformational entropy, heterogeneous spatial distribution of charges, and polyampholyte nature. Protein-polyelectrolyte complexes (PPCs) are promising building blocks for LbL construction owing to their standardized charge and polyelectrolyte (PE) corona. In this work, lysozyme was complexed with poly(styrenesulfonate) (PSS) at different ionic strengths and pH values. The PPCs size and electrical properties were investigated, and the forces driving complexation were elucidated, in the light of computations of polyelectrolyte conformation, with a view to further unravel LbL construction mechanisms. Quartz crystal microbalance and atomic force microscopy were used to monitor the integration of PPCs compared to the one of bare protein molecules in LbL assemblies, and colorimetric assays were performed to determine the protein amount in the thin films. Layers built with PPCs show higher protein contents and hydration levels. Very importantly, the results also show that LbL construction with PPCs mainly relies on standard PE-PE interactions, independent of the charge state of the protein, in contrast to classical bare protein assembly with PEs. This considerably simplifies the incorporation of proteins in multilayers, which will be beneficial for biosensing, heterogeneous biocatalysis, biotechnologies, and medical applications that require active proteins at interfaces.

Synthetic Nanochaperones Facilitate Refolding of Denatured Proteins

The folding process of a protein is inherently error-prone, owing to the large number of possible conformations that a protein chain can adopt. Partially folded or misfolded proteins typically expose hydrophobic surfaces and tend to form dysfunctional protein aggregates. Therefore, materials that can stabilize unfolded proteins and then efficiently assist them refolding to its bioactive form are of significant interest. Inspired by natural chaperonins, we have synthesized a series of polymeric nanochaperones that can facilitate the refolding of denatured proteins with a high recovery efficiency (up to 97%). Such nanochaperones possess phase-separated structure with hydrophobic microdomains on the surface. This structure allows nanochaperones to stabilize denatured proteins by binding them to the hydrophobic microdomains. We have also investigated the mechanism by which nanochaperones assist the protein refolding and established the design principles of nanochaperones in order to achieve effective recovery of a certain protein from their denatured forms. With a carefully designed composition of the microdomains according to the surface properties of the client proteins, the binding affinity between the hydrophobic microdomain and the denatured protein molecules can be tuned precisely, which enables the self-sorting of the polypeptides and the refolding of the proteins into their bioactive states. This work provides a feasible and effective strategy to recover inclusion bodies to their bioactive forms, which has potential to reduce the cost of the manufacture of recombinant proteins significantly.

Scalable organic solvent free supercritical fluid spray drying process for producing dry protein formulations

In this study, we evaluated the influence of supercritical carbon dioxide (scCO2) spray drying conditions, in the absence of organic solvent, on the ability to produce dry protein/trehalose formulations at 1:10 and 1:4 (w/w) ratios. When using a 4L drying vessel, we found that decreasing the solution flow rate and solution volume, or increasing the scCO2 flow rate resulted in a significant reduction in the residual water content in dried products (Karl Fischer titration). The best conditions were then used to evaluate the ability to scale the scCO2 spray drying process from 4L to 10L chamber. The ratio of scCO2 and solution flow rate was kept constant. The products on both scales exhibited similar residual moisture contents, particle morphologies (SEM), and glass transition temperatures (DSC). After reconstitution, the lysozyme activity (enzymatic assay) and structure (circular dichroism, HP-SEC) were fully preserved, but the sub-visible particle content was slightly increased (flow imaging microscopy, nanoparticle tracking analysis). Furthermore, the drying condition was applicable to other proteins resulting in products of similar quality as the lysozyme formulations. In conclusion, we established scCO2 spray drying processing conditions for protein formulations without an organic solvent that holds promise for the industrial production of dry protein formulations.

Structural fingerprints and their evolution during oligomeric vs. oligomer-free amyloid fibril growth

Deposits of fibrils formed by disease-specific proteins are the molecular hallmark of such diverse human disorders as Alzheimer's disease, type II diabetes, or rheumatoid arthritis. Amyloid fibril formation by structurally and functionally unrelated proteins exhibits many generic characteristics, most prominently the cross β-sheet structure of their mature fibrils. At the same time, amyloid formation tends to proceed along one of two separate assembly pathways yielding either stiff monomeric filaments or globular oligomers and curvilinear protofibrils. Given the focus on oligomers as major toxic species, the very existence of an oligomer-free assembly pathway is significant. Little is known, though, about the structure of the various intermediates emerging along different pathways and whether the pathways converge towards a common or distinct fibril structures. Using infrared spectroscopy we probed the structural evolution of intermediates and late-stage fibrils formed during in vitro lysozyme amyloid assembly along an oligomeric and oligomer-free pathway. Infrared spectroscopy confirmed that both pathways produced amyloid-specific β-sheet peaks, but at pathway-specific wavenumbers. We further found that the amyloid-specific dye thioflavin T responded to all intermediates along either pathway. The relative amplitudes of thioflavin T fluorescence responses displayed pathway-specific differences and could be utilized for monitoring the structural evolution of intermediates. Pathway-specific structural features obtained from infrared spectroscopy and Thioflavin T responses were identical for fibrils grown at highly acidic or at physiological pH values and showed no discernible effects of protein hydrolysis. Our results suggest that late-stage fibrils formed along either pathway are amyloidogenic in nature, but have distinguishable structural fingerprints. These pathway-specific fingerprints emerge during the earliest aggregation events and persist throughout the entire cascade of aggregation intermediates formed along each pathway.

Synthesis and characterization of CLEA-lysozyme immobilized PS/PSMA nanofiber

Using 0.2% glutaraldehyde as the cross-linker, lysozyme was covalently immobilized onto electrospun polystyrene/poly(styrene-co-maleic anhydride) (PS/PSMA) nanofibers as cross-linked enzyme aggregates (CLEA). The lysozyme capacity of PS/PSMA nanofibers under optimal condition was 57.6 mg/g of nanofibers. Various parameters were used to evaluate the stability of the immobilized CLEA-lysozyme. Compared to free enzyme, the immobilized CLEA-lysozyme exhibited its optimal enzymatic activity at higher temperature and pH. The immobilized CLEA-lysozyme maintained more than 78% of its initial activity during 30 days of storage period. Additionally, the immobilized CLEA-lysozyme presented a high antibacterial activity against Staphylococcus aureus. The durability determinations of such nanofibers showed 90.3% retention of the initial lysozyme activity after 80 consecutive reuses, and 81.2% of bacteriostasis ratio after 10 cycles. The results of this study suggest that CLEA-lysozyme immobilized nanofiber which can stabilize its enzymatic activity through cross-linking immobilization can be beneficial for various antibacterial processes.

Convergent chemical synthesis and high-resolution x-ray structure of human lysozyme

In this article, we report the total chemical synthesis of human lysozyme. Lysozyme serves as a widespread model system in various fields of biochemical research, including protein folding, enzyme catalysis, and amyloidogenesis. The 130-aa wild-type polypeptide chain of the human enzyme was assembled from four polypeptide segments by using native chemical ligation in a fully convergent fashion. Key to the assembly strategy is the application of the recently developed kinetically controlled ligation methodology, which provides efficient control over the ligation of two peptide (alpha)thioesters to yield a unique product. This result enables the facile preparation of a 64-residue peptide (alpha)thioester; this segment is joined by native chemical ligation to a 66-aa Cys peptide, to yield the target 130-aa polypeptide chain. The synthetic polypeptide chain was folded in vitro into a defined tertiary structure with concomitant formation of four disulfides, as shown by 2D TOCSY NMR spectroscopy. The structure of the synthetic human lysozyme was confirmed by high-resolution x-ray diffraction, giving the highest-resolution structure (1.04 A) observed to date for this enzyme. Synthetic lysozyme was obtained in good yield and excellent purity and had full enzymatic activity. This facile and efficient convergent synthesis scheme will enable preparation of unique chemical analogs of the lysozyme molecule and will prove useful in numerous areas of lysozyme research in the future.

Amyloid fibril formation by peptide LYS (11-36) in aqueous trifluoroethanol

Peptide LYS (11-36), derived from the beta-sheet region of T4 lysozyme, forms an amyloid fibril in aqueous trifluoroethanol (TFE) at elevated temperature. The peptide has a moderate alpha-helix content in 20 and 50% (v/v) TFE solution; large quantities of fibrils were formed after incubation at 55 degrees C for 2 weeks as monitored by a thioflavin T fluorescence assay. No fibrils were observed when the peptide initially existed predominantly as a random coil or as a complete alpha helix. Our results suggest that a moderate amount of alpha helix and random coil present in the peptide initially facilitates the fibril-formation process, but a high alpha-helix content inhibits fibril formation. Transmission electron microscopy revealed several types of fibril morphologies at different TFE concentrations. The fibrils were highly twisted and consisted of interleaved protofilaments in 50% TFE, while smooth and flat ribbonlike fibrils were found in 20% TFE. In 50% TFE, the fibril growth rate of LYS (11-36) was found to depend strongly on peptide concentration and seeding but was insensitive to solution pH and ionic strength.

RENAL-SELECTIVE DELIVERY AND ANGIOTENSIN-CONVERTING ENZYME INHIBITION BY SUBCUTANEOUSLY ADMINISTERED CAPTOPRIL-LYSOZYME

In previous studies, we have demonstrated that the low molecular weight protein lysozyme can be used as a renal-selective drug carrier for delivery of the angiotensin-converting enzyme (ACE) inhibitor captopril. Typically, such macromolecular drug-targeting preparations are administered intravenously. In the present study, we investigated the fate of captopril-lysozyme following subcutaneous administration, a convenient route for long-term treatment. The absorption from the subcutaneous injection site and renal uptake of lysozyme were determined by gamma scintigraphy in rats. Bioavailability, renal accumulation, and stability of the captopril-lysozyme conjugate were evaluated by high performance liquid chromatography analysis and by ACE activity measurements. Lysozyme was absorbed gradually and completely from the subcutaneous injection site within 24 h and accumulated specifically in kidneys. After subcutaneous injection of the captopril-lysozyme conjugate, higher renal captopril levels and lower captopril-lysozyme levels in urine indicated the improved renal accumulation in comparison with intravenous administration of the conjugate, as well as its stability at the injection site. After both treatments, captopril-lysozyme conjugate effectuated renal ACE inhibition, whereas plasma ACE was not inhibited. In conclusion, our results demonstrate that we can use the subcutaneous route to administer drug delivery preparations like the captopril-lysozyme conjugate.

Protein Dissection Experiments Reveal Key Differences in the Equilibrium Folding of α-Lactalbumin and the Calcium Binding Lysozymes

The alpha-lactalbumins and c-type lysozymes have virtually identical structure but exhibit very different folding behavior. All alpha-lactalbumins form a well populated molten globule state, while most of the lysozymes do not. alpha-Lactalbumin consists of two subdomains, and the alpha-subdomain is considerably more structured in the molten globule state than the beta-subdomain. Constructs derived from the alpha-subdomain of human alpha-lactalbumin containing the A, B, D, and 3(10) helices are known to form a molten globule state in the absence of the rest of the protein (Demarest, S. et al. (1999) J. Mol. Biol. 294, 213-221). Here we reported comparative studies of constructs derived from the same regions of canine and equine lysozymes. These proteins form two of the most stable molten globule states among all the lysozymes. A construct containing the A, B, D, and 3(10) helices of equine lysozyme is partially helical but is less structured than the corresponding human alpha-lactalbumin peptide. Addition of the C-helix leads to a construct that is still less structured and less stable than the alpha-lactalbumin construct. The corresponding construct from canine lysozyme is also less structured and less stable than the alpha-lactalbumin peptide. Thus, molten globule formation in human alpha-lactalbumin can be driven by the isolated alpha-subdomain, while more extensive interactions are required to generate a stable molten globule in the two lysozymes. The stability of the canine and equine lysozyme constructs is similar, indicating that the extraordinary stability of the canine lysozyme molten globule is not due to an unusually stable isolated alpha-subdomain.

A simplified counter diffusion method combined with a 1D simulation program for optimizing crystallization conditions

We developed a new protein crystallization method has been developed using a simplified counter-diffusion method for optimizing crystallization condition. It is composed of only a single capillary, the gel in the silicon tube and the screw-top test tube, which are readily available in the laboratory. The one capillary can continuously scan a wide range of crystallization conditions (combination of the concentrations of the precipitant and the protein) unless crystallization occurs, which means that it corresponds to many drops in the vapor-diffusion method. The amount of the precipitant and the protein solutions can be much less than in conventional methods. In this study, lysozyme and alpha-amylase were used as model proteins for demonstrating the efficiency of this method. In addition, one-dimensional (1-D) simulations of the crystal growth were performed based on the 1-D diffusion model. The optimized conditions can be applied to the initial crystallization conditions for both other counter-diffusion methods with the Granada Crystallization Box (GCB) and for the vapor-diffusion method after some modification.

Protein crystallization by using porous glass substrate

The effects of a commercially available porous glass substrate (Corning Porous Glass No.7930) on the heterogeneous nucleation of proteins [hen egg-white lysozyme (HEWL), thaumatin and apoferritin] have been investigated in order to develop an improved method to facilitate the nucleation of protein crystals. It was found that the porous glass substrate could promote the nucleation at lower supersaturations. The induction time for nucleation decreased, and the crystals obtained from porous glass substrates were larger than those from normal glass substrates. Many pores and channels of 10-100 nm in diameter were observed on the porous glass surface by atomic force microscopy (AFM). It is believed that these pores and channels are crucial for facilitating the nucleation process in this work.

Improvement of protein crystal quality by forced flow solution

Flow experiments in growing protein crystals were conducted to clarify the influence of the solution flow on the crystal quality. Lysozyme crystals grown under various flow velocities were analyzed by using synchrotron radiation to assess the quality. As a result, the crystals grown under forced flow were of better quality than those grown in quiescent conditions.

Protein crystal growth with a two-liquid system and stirring solution

We developed two novel methods for growing large, high-quality protein crystals. A two-liquid system enables the convenient extraction of protein crystals without causing mechanical damage due to growth at the interface between two liquids. Since this system does not require limitations on solution volume, it is also suitable for the seed technique, and for growing large crystals. Another new concept is the mild stirring of the solution using the Floating And Stirring Technique (FAST) and the Micro-stirring technique. When compared to conventional techniques, both techniques result in a reduced number of crystals, as well as the growth of large crystals.

Impurity effects on lysozyme crystal growth

Atomic force microscopy (AFM) and X-ray diffraction experiments have been combined to study the correlation between impurity incorporation, crystal surface morphology and crystal quality. Hen egg-white lysozyme has been used as a model protein, and covalently bound lysozyme dimer as a model impurity. AFM observation of the [101] crystal face revealed that the crystal surface clearly became rough when 5% impurity was added, and the steps disappeared as the impurity concentration increased to 10%. The crystal quality was evaluated by four factors: maximum resolution limit, <I>/<sigmaI>, Rmerge, and overall B factor. In every index, the crystal quality tended to degrade as the impurity concentration increased. The B-factor dropped significantly at 5% impurity; at the same time the step roughening was observed. This strongly suggested that the impurity incorporation affected the step growth mechanism and degraded the crystal quality.

Precipitation of lysozyme nanoparticles from dimethyl sulfoxide using carbon dioxide as antisolvent

The protein lysozyme has been precipitated as amorphous nanoparticles from a DMSO solution using dense carbon dioxide as antisolvent, by applying the so-called gas antisolvent recrystallization technique in a 400-mL precipitator. The objective is to investigate the possibility of tuning the particle properties by changing the key process parameters, namely, antisolvent addition rate, initial solute concentration, and temperature. It is shown that none of these operating parameters has a major effect on the average particle size or the particle size distribution. The former is mostly between 200 and 300 nm and exhibits no evident trend. The latter is always unimodal and rather narrow and exhibits increasing agglomeration at higher temperature and initial solute concentration. Up to 75% of the protein activity measured in the starting crystalline material is retained by the precipitated amorphous nanoparticles. The present experimental results compare well with data about the same system obtained in a different experimental setup, which were previously reported in the literature, thus pointing at the reproducibility and robustness of GAS antisolvent recrystallization. Moreover, these are consistent with the theoretical understanding of gas antisolvent recrystallization as achieved by using a recently developed model of the process.

A comparative study of peptide models of the alpha-domain of alpha-lactalbumin, lysozyme, and alpha-lactalbumin/lysozyme chimeras allows the elucidation of critical factors that contribute to the ability to form stable partially folded states

alpha-Lactalbumin (alpha LA) forms a well-populated equilibrium molten globule state, while the homologous protein hen lysozyme does not. alpha LA is a two-domain protein and the alpha-domain is more structured in the molten globule state than is the beta-domain. Peptide models derived from the alpha-subdomain that contain the A, B, D, and 3(10) helices of alpha LA are capable of forming a molten globule state in the absence of the remainder of the protein. Here we report comparative studies of a peptide model derived from the same region of hen lysozyme and a set of chimeric alpha-lactalbumin--lysozyme constructs. Circular dichroism, dynamic light scattering, sedimentation equilibrium, and fluorescence experiments indicate that the lysozyme construct does not fold. Chimeric constructs were prepared to probe the origins of the difference in the ability of the two isolated subdomains to fold. The first consists of the A and B helices of alpha LA cross-linked to the D and C-terminal 3(10) helices of lysozyme. This construct is highly helical, while a second construct that contains the A and B helices of lysozyme cross-linked to the D and 3(10) helices of alpha LA does not fold. Furthermore, the disulfide cross-linked homodimer of the alpha LA AB peptide is helical, while the homodimer of the lysozyme AB peptide is unstructured. Thus, the AB helix region of alpha LA appears to have an intrinsic ability to form structure as long as some relatively nonspecific interactions can be made with other regions of the protein. Our studies show that the A and B helices plays a key role in the ability of the respective alpha-subdomains to fold.

Identification of initiation sites for T4 lysozyme folding using CD and NMR spectroscopy of peptide fragments

Using CD and 2D (1)H NMR spectroscopy, we have identified potential initiation sites for the folding of T4 lysozyme by examining the conformational preferences of peptide fragments corresponding to regions of secondary structure. CD spectropolarimetry showed most peptides were unstructured in water, but adopted partial helical conformations in TFE and SDS solution. This was also consistent with the (1)H NMR data which showed that the peptides were predominantly disordered in water, although in some cases, nascent or small populations of partially folded conformations could be detected. NOE patterns, coupling constants, and deviations from random coil Halpha chemical shift values complemented the CD data and confirmed that many of the peptides were helical in TFE and SDS micelles. In particular, the peptide corresponding to helix E in the native enzyme formed a well-defined helix in both TFE and SDS, indicating that helix E potentially forms an initiation site for T4 lysozyme folding. The data for the other peptides indicated that helices D, F, G, and H are dependent on tertiary interactions for their folding and/or stability. Overall, the results from this study, and those of our earlier studies, are in agreement with modeling and HD-deuterium exchange experiments, and support an hierarchical model of folding for T4 lysozyme.

An improved method for preparing lysozyme with chemically 13C-enriched methionine residues using 2-aminothiophenol as a reagent of thiolysis

Jones et al. have reported that the epsilon-carbons of methionine residues in myoglobin can be enriched with stable isotope (13C) in two steps, i.e., methylation of methionine residues with 13CH3I in the protein and thiolysis using dithiothreitol [Jones, W.C., Rothgeb, T.M., and Gurd, F.R.N. (1976) J. Biol. Chem. 251,7452-7460]. Using their method, we failed to prepare active lysozyme in which the epsilon-carbons of methionine residues are enriched with 13C, because many side reactions took place under the thiolysis condition (pH 10.5, 37 degrees C). When we employed 2-aminothiophenol as a reagent for thiolysis, the reduction proceeded under a weakly acidic condition to afford fully active lysozyme, in which the epsilon-carbons of two methionine residues were enriched with 13C, in a 30% yield. Analysis of the 13C-edited NOESY spectra of 13C-enriched methionine lysozyme in the absence and presence of a substrate analogue indicated the occurrence of conformational change around Met 105 in lysozyme.

Drug-targeting to the kidney: renal delivery and degradation of a naproxen-lysozyme conjugate in vivo

A renal-specific controlled release of an active drug may enable a reduction of the required dose and may provide a reduction of extra-renal toxicity. To achieve renal specific targeting of the NSAID naproxen, the low-molecular-weight protein (LMWP) lysozyme was employed as carrier since it is mainly taken up and catabolized in the proximal tubules of the kidney. A conjugate was synthesized with an average coupling degree of 2 mol naproxen per 1 mol lysozyme in which the drug was directly coupled to the protein via a peptide bond. First, we investigated whether naproxen conjugation affects the renal disposition of lysozyme. As native lysozyme, the conjugate was predominantly and rapidly (within 20 min) taken up by the kidney. The subsequent decrease in renal content reflecting the renal degradation of the conjugated lysozyme molecules appeared also to be similar to that of native lysozyme with a half life of four hours. Second, the effect of lysozyme conjugation on the body distribution of naproxen was studied. An important observation with regard to the aimed reduction in extra-renal side effects was that no detectable amounts of free naproxen were present in the plasma after administration of conjugate. Conjugation of naproxen to lysozyme resulted in a pronounced (70-fold) increase of naproxen accumulation in the kidney. In agreement with the protein disposition study, the conjugate was rapidly taken up by the kidney and subsequently degraded. In conclusion, renal selective targeting of the NSAID naproxen can be obtained by conjugation with the LMWP lysozyme. This concept of drug delivery to the kidney has the potential to improve drug efficacy and safety.

Preparation and properties of a lysozyme derivative in which two domains are cross-linked intramolecularly between Trp62 and Asp101

A lysozyme derivative in which two domains were cross-linked intramolecularly was newly prepared by means of a two-step reaction. First, the beta-carboxyl group of Asp101 in lysozyme was selectively modified with 2-(2-pyridyldithio)ethylamine in the presence of 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide hydrochloride. After reduction of the pyridyldithio moiety of Asp101 modified lysozyme at pH 4.5 with dithiothreitol, the derivative was allowed to cross-link intramolecularly by reaction with 1,3-dichloroacetone at pH 7. Intramolecularly cross-linked lysozyme thus formed was purified by gel chromatography followed by ion-exchange chromatography. Based on the results of 1H-NMR and peptide analyses, it was concluded that Asp101 was cross-linked to Trp62 with a -CH2COCH2SCH2CH2NH-bridge in this derivative. The derivative showed minor but distinct activity against Micrococcus lysodeikticus and glycol chitin. Its melting temperature for thermal denaturation was higher by 7.3 degrees than that of native lysozyme at pH 3.

Engineering of the active site of human lysozyme: conversion of aspartic acid 53 to glutamic acid and tyrosine 63 to tryptophan or phenylalanine

Three human lysozymes containing a mutation either at Asp-53 to Glu or at Tyr-63 to Trp or Phe were synthesized and examined for their immunological and enzymatical activities in comparison with the native one. All mutants were immunologically indistinguishable from native human lysozyme. The [Trp63] and [Phe63] mutants catalysed the hydrolysis of Micrococcus lysodeikticus cell wall and glycol chitin effectively, while the [Glu53] mutant displayed very low activity toward M. lysodeikticus cells and no detectable activity toward glycol chitin.

The Bakerian lecture. Towards synthesis of proteins

Proteins, as their name indicates, occupy the prime place in Nature’s architecture. It is worth remarking that Nature uses the structural type for diverse purposes, in addition to those familiar in enzymology, immunology, etc. A striking example is provided by the recent publication (Bradbury et al . 1976), showing that the C fragment of lipotropin (residues 61‒91) is firmly bound by opiate receptors and is an analgesic 200 times as potent as morphine in molar terms. The pentapeptide comprising residues 61‒65 of this sequence is already well known as enkephalin (Hughes et al . 1975). Reference should also be made to the host of hormones from the pituitary gland, hypothalamus, and intestinal tract. An example of the last is gastrin which has been studied at Liverpool, principally by R. A. Gregory in the Department of Physiology. Recently the structure of the big gastrins, isolated by Gregory & Tracy (1972), has been elucidated by Harris & Runswick (1974) following preliminary work in the Robert Robinson Laboratories (Barton, Choudhury, Dancsi & Kenner 1973) (figure 1). This structure typifies the way in which small fragments are carved from a protein chain in order to provide an active hormone (Smyth 1975). Thus the gastrins originally described (Gregory & Tracy 1964; Kenner & Sheppard 1968) correspond to the sequence 18‒34 of the big gastrins. The lysyl glutaminyl bond is enzymically cleaved, as can be achieved in the laboratory by trypsin, and then the N-terminal glutaminyl residue cyclizes to the pyroglutamyl structure (pyrrolidone carbonyl). The pyroglutamyl structure at the N-terminus of the big gastrins must have arisen in the same way. It should also be noted that the C-terminal amide, which is essential for biological activity (Tracy & Gregory 1964), very likely arises from enzymatic aminolysis of a protein chain extending further to the right; the extensive studies by Morley (1968) of structure‒activity relations in the gastrin tetrapeptide indicate that the gastrin tetrapeptide embedded in a protein chain would not have hormonal activity until aminolysis had occurred, and therefore the gastrin sequence can be stored in a prohormone. The sequence of human big gastrin 1 (figure 1) has been synthesized by a colla­borative effort between our laboratory at Liverpool and that of Professor E. Wünsch in the Max Planck Institute at Munich (Choudhury et al . 1976). Two points concerning our experience of synthesis in the gastrin series are relevant to the main theme of this lecture. Firstly, although the synthesis was accomplished by the combination of section 1‒19 synthesized in Liverpool with section 20‒34 made in Munich according to the original design, additional superior routes have been devised in both laboratories. In other words, even those skilled Glp 1 -Leu-Gly-Pro-Gln-Gly-His-Pro-Ser-Leu-Val-Ala-Asp-Pro-Ser-Lys-Lys 17 -Gln 18 - Gly-Pro-Trp-Leu-Glu-Glu-Glu-Glu-Glu-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH 2 34

The semisynthesis of portions of hen's-egg lysozyme by fragment condensation

1. We have evaluated several methods for coupling protected tryptic peptides and have selected one that is satisfactory in terms of efficiency and degree of racemization. 2. We report the use of the chosen method (which must be slightly modified for each application in the light of the result of micro-scale pilot experiments) for the preparation of five fragments of hen's-egg lysozyme ranging in length from 31 to 51 residues. 3. We report methods for the complete deprotection of these fragments. They have been characterized by end-group determination, amino acid analysis and tryptic digestion.

Antibodies reactive with native lysozyme elicited by a completely synthetic antigen

A synthetic peptide consisting of the aminoacid sequence of residues 64-82 of lysozyme, with alanine replacing cysteine as residue 76, was prepared by the solid-phase technique. Mild reduction followed by reoxidation in air of the deprotected peptide led to the formation of a closed loop containing an intrachain disulfide bond. A conjugate consisting of this "loop" attached to multi-poly(DL-alanyl)-poly(L-lysine) elicited, in rabbits and goats, the formation of antibodies capable of reacting with lysozyme and with the loop peptide prepared from it. These immunological interactions can be inhibited by either lysozyme or the loop peptide, but not by the performic acid-oxidized open-chain peptide. Thus, the antibodies elicited by the completely synthetic antigen show specificity toward the "loop" structure (residues 64-80) of native lysozyme.