Modification of the active site of alkaline phosphatase by site-directed mutagenesis

Essential Functional Interplay of the Catalytic Groups in Acid Phosphatase

The cooperative interplay between the functional devices of a preorganized active site is fundamental to enzyme catalysis. An in-depth understanding of this phenomenon is central to elucidating the remarkable efficiency of natural enzymes and provides an essential benchmark for enzyme design and engineering. Here, we study the functional interconnectedness of the catalytic nucleophile (His18) in an acid phosphatase by analyzing the consequences of its replacement with aspartate. We present crystallographic, biochemical, and computational evidence for a conserved mechanistic pathway via a phospho-enzyme intermediate on Asp18. Linear free-energy relationships for phosphoryl transfer from phosphomonoester substrates to His18/Asp18 provide evidence for the cooperative interplay between the nucleophilic and general-acid catalytic groups in the wild-type enzyme, and its substantial loss in the H18D variant. As an isolated factor of phosphatase efficiency, the advantage of a histidine compared to an aspartate nucleophile is ∼10⁴-fold. Cooperativity with the catalytic acid adds ≥10²-fold to that advantage. Empirical valence bond simulations of phosphoryl transfer from glucose 1-phosphate to His and Asp in the enzyme explain the loss of activity of the Asp18 enzyme through a combination of impaired substrate positioning in the Michaelis complex, as well as a shift from early to late protonation of the leaving group in the H18D variant. The evidence presented furthermore suggests that the cooperative nature of catalysis distinguishes the enzymatic reaction from the corresponding reaction in solution and is enabled by the electrostatic preorganization of the active site. Our results reveal sophisticated discrimination in multifunctional catalysis of a highly proficient phosphatase active site.

Molecular differences between a mutase and a phosphatase: investigations of the activation step in Bacillus cereus phosphopentomutase

Prokaryotic phosphopentomutases (PPMs) are di-Mn(2+) enzymes that catalyze the interconversion of α-D-ribose 5-phosphate and α-D-ribose 1-phosphate at an active site located between two independently folded domains. These prokaryotic PPMs belong to the alkaline phosphatase superfamily, but previous studies of Bacillus cereus PPM suggested adaptations of the conserved alkaline phosphatase catalytic cycle. Notably, B. cereus PPM engages substrates when the active site nucleophile, Thr-85, is phosphorylated. Further, the phosphoenzyme is stable throughout purification and crystallization. In contrast, alkaline phosphatase engages substrates when the active site nucleophile is dephosphorylated, and the phosphoenzyme reaction intermediate is only stably trapped in a catalytically compromised enzyme. Studies were undertaken to understand the divergence of these mechanisms. Crystallographic and biochemical investigations of the PPM(T85E) phosphomimetic variant and the neutral corollary PPM(T85Q) determined that the side chain of Lys-240 underwent a change in conformation in response to active site charge, which modestly influenced the affinity for the small molecule activator α-D-glucose 1,6-bisphosphate. More strikingly, the structure of unphosphorylated B. cereus PPM revealed a dramatic change in the interdomain angle and a new hydrogen bonding interaction between the side chain of Asp-156 and the active site nucleophile, Thr-85. This hydrogen bonding interaction is predicted to align and activate Thr-85 for nucleophilic addition to α-D-glucose 1,6-bisphosphate, favoring the observed equilibrium phosphorylated state. Indeed, phosphorylation of Thr-85 is severely impaired in the PPM(D156A) variant even under stringent activation conditions. These results permit a proposal for activation of PPM and explain some of the essential features that distinguish between the catalytic cycles of PPM and alkaline phosphatase.

The 1.9 A crystal structure of heat-labile shrimp alkaline phosphatase

Alkaline phosphatases are non-specific phosphomonoesterases that are distributed widely in species ranging from bacteria to man. This study has concentrated on the tissue-nonspecific alkaline phosphatase from arctic shrimps (shrimp alkaline phosphatase, SAP). Originating from a cold-active species, SAP is thermolabile and is used widely in vitro, e.g. to dephosphorylate DNA or dNTPs, since it can be inactivated by a short rise in temperature. Since alkaline phosphatases are zinc-containing enzymes, a multiwavelength anomalous dispersion (MAD) experiment was performed on the zinc K edge, which led to the determination of the structure to a resolution of 1.9 A. Anomalous data clearly showed the presence of a zinc triad in the active site, whereas alkaline phosphatases usually contain two zinc and one magnesium ion per monomer. SAP shares the core, an extended beta-sheet flanked by alpha-helices, and a metal triad with the currently known alkaline phosphatase structures (Escherichia coli structures and a human placental structure). Although SAP lacks some features specific for the mammalian enzyme, their backbones are very similar and may therefore be typical for other higher organisms. Furthermore, SAP possesses a striking feature that the other structures lack: surface potential representations show that the enzyme's net charge of -80 is distributed such that the surface is predominantly negatively charged, except for the positively charged active site. The negatively charged substrate must therefore be directed strongly towards the active site. It is generally accepted that optimization of the electrostatics is one of the characteristics related to cold-adaptation. SAP demonstrates this principle very clearly.

Functional interrelationships in the alkaline phosphatase superfamily: phosphodiesterase activity of Escherichia coli alkaline phosphatase

Escherichia coli alkaline phosphatase (AP) is a proficient phosphomonoesterase with two Zn(2+) ions in its active site. Sequence homology suggests a distant evolutionary relationship between AP and alkaline phosphodiesterase/nucleotide pyrophosphatase, with conservation of the catalytic metal ions. Furthermore, many other phosphodiesterases, although not evolutionarily related, have a similar active site configuration of divalent metal ions in their active sites. These observations led us to test whether AP could also catalyze the hydrolysis of phosphate diesters. The results described herein demonstrate that AP does have phosphodiesterase activity: the phosphatase and phosphodiesterase activities copurify over several steps; inorganic phosphate, a strong competitive inhibitor of AP, inhibits the phosphodiesterase and phosphatase activities with the same inhibition constant; a point mutation that weakens phosphate binding to AP correspondingly weakens phosphate inhibition of the phosphodiesterase activity; and mutation of active site residues substantially reduces both the mono- and diesterase activities. AP accelerates the rate of phosphate diester hydrolysis by 10(11)-fold relative to the rate of the uncatalyzed reaction [(k(cat)/K(m))/k(w)]. Although this rate enhancement is substantial, it is at least 10(6)-fold less than the rate enhancement for AP-catalyzed phosphate monoester hydrolysis. Mutational analysis suggests that common active site features contribute to hydrolysis of both phosphate monoesters and phosphate diesters. However, mutation of the active site arginine to serine, R166S, decreases the monoesterase activity but not the diesterase activity, suggesting that the interaction of this arginine with the nonbridging oxygen(s) of the phosphate monoester substrate provides a substantial amount of the preferential hydrolysis of phosphate monoesters. The observation of phosphodiesterase activity extends the previous observation that AP has a low level of sulfatase activity, further establishing the functional interrelationships among the sulfatases, phosphatases, and phosphodiesterases within the evolutionarily related AP superfamily. The catalytic promiscuity of AP could have facilitated divergent evolution via gene duplication by providing a selective advantage upon which natural selection could have acted.

Nucleotide sequence of the gene for alkaline phosphatase of Thermus caldophilus GK24 and characteristics of the deduced primary structure of the enzyme

The gene encoding Thermus caldophilus GK24 (Tca) alkaline phosphatase was cloned into Escherichia coli. The primary structure of Tca alkaline phosphatase was deduced from its nucleotide sequence. The Tca alkaline phosphatase precursor, including the signal peptide sequence, was comprised of 501 amino acid residues. Its molecular mass was determined to be 54¿ omitted¿760 Da. On the alignment of the amino acid sequence, Tca alkaline phosphatase showed sequence homology with the microbial alkaline phosphatases, 20% identity with E. coli alkaline phosphatase and 22% Bacillus subtilis (Bsu) alkaline phosphatases. High sequence identity was observed in the regions containing the Ser-102 residue of the active site, the zinc and magnesium binding sites of E. coli alkaline phosphatase. Comparison of Tca alkaline phosphatase and E. coli alkaline phosphatase structures suggests that the reduced activity of the Tca alkaline phosphatase, in the presence of zinc, is directly involved in some of the different metal binding sites. Heat-stable Tca alkaline phosphatase activity was detected in E. coli YK537, harboring pJRAP.

Hyperalkaline and thermostable phosphatase in Thermus thermophilus

The phosphatases existing in the extreme thermophilic bacterium Thermus thermophilus have been studied. Utilizing ion exchange, hydrophobic, pseudoaffinity, and affinity chromatography, a number of distinct phosphatase activities were identified. At least four phosphatases, with optimum pH ranging between 5.0 and 11.5, were assayed with p-nitrophenylphosphate, and two with optimum pH between 7.0 and 11.0, with 32P-casein as substrate. The authors have focused on the hyperalkaline phosphatase and have tried to purify and characterize it. This hyperalkaline phosphatase reaches a maximal level at the stationary phase of the growth, and is co-purified with alkaline phosphatase with optimum pH of 10.2. The enzymes present a relative mol wt of 65 and 58 kDa, respectively, as judged by SDS-PAGE and Sephadex G-150 column, and possess similar properties, indicating that they are isoforms. These enzymes barely function in the presence of tartrate, and are inhibited by EDTA, pyrophosphate, and molybdate. Among the metals tested, Hg2+ appeared as the strongest inhibitor of the hyperalkaline phosphatase. The two enzymes are thermostable and, upon treatment at 90 degrees C for 10 min, 75% of their activity remains. The physiological role and function of these phosphatases need further investigation.

Kinetic and X-ray structural studies of three mutant E. coli alkaline phosphatases: insights into the catalytic mechanism without the nucleophile Ser102

Escherichia coli alkaline phosphatase (EC 3.1.3.1) is a non-specific phosphomonoesterase that catalyzes the hydrolysis reaction via a phosphoseryl intermediate to produce inorganic phosphate and the corresponding alcohol. We investigated the nature of the primary nucleophile, fulfilled by the deprotonated Ser102, in the catalytic mechanism by mutating this residue to glycine, alanine and cysteine. The efficiencies of the S102G, S102A and S102C enzymes were 6 x 10(5)-fold, 10(5)-fold and 10(4)-fold lower than the wild-type enzyme, respectively, as measured by the kcat/Km ratio, still substantially higher than the non-catalyzed reaction. In order to investigate the structural details of the altered active site, the enzymes were crystallized and their structures determined. The enzymes crystallized in a new crystal form corresponding to the space group P6322. Each structure has phosphate at each active site and shows little departure from the wild-type model. For the S102G and S102A enzymes, the phosphate occupies the same position as in the wild-type enzyme, while in the S102C enzyme it is displaced by 2.5 A. This kinetic and structural study suggests an explanation for differences in catalytic efficiency of the mutant enzymes and provides a means to study the nature and strength of different nucleophiles in the same environment. The analysis of these results provides insight into the mechanisms of other classes of phosphatases that do not utilize a serine nucleophile.

Enhanced catalysis by active-site mutagenesis at aspartic acid 153 in Escherichia coli alkaline phosphatase

Bacterial alkaline phosphatase catalyzes the hydrolysis and transphosphorylation of phosphate monoesters. Site-directed mutagenesis was used to change the active-site residue Asp-153 to Ala and Asn. In the wild-type enzyme Asp-153 forms a second-sphere complex with Mg2+. The activity of mutant enzymes D153N and D153A is dependent on the inclusion of Mg2+ in the assay buffer. The steady-state kinetic parameters of the D153N mutant display small enhancements, relative to wild type, in buffers containing 10 mM Mg2+. In contrast, the D153A mutation gives rise to a 6.3-fold increase in kcat, a 13.7-fold increase in kcat/Km (50 mM Tris, pH 8), and a 159-fold increase in Ki for Pi (1 M Tris, pH 8). In addition, the activity of D153A increases 25-fold as the pH is increased from 7 to 9. D153A hydrolyzes substrates with widely differing pKa's of their phenolic leaving groups (PNPP and DNPP), at similar rates. As with wild type, the rate-determining step takes place after the initial nucleophilic displacement (k2). The increase in kcat for the D153A mutant indicates that the rate of release of phosphate from the enzyme product complex (k4) has been enhanced.

Cloning, expression, and catalytic mechanism of the low molecular weight phosphotyrosyl protein phosphatase from bovine heart

The first representative of a group of mammalian, low molecular weight phosphotyrosyl protein phosphatases was cloned, sequenced and expressed in Escherichia coli. Using a 61-mer oligonucleotide probe based on the amino acid sequence of the purified enzyme, several overlapping cDNA clones were isolated from a bovine heart cDNA library. A full-length clone was obtained consisting of a 27-bp 5' noncoding region, an open reading frame encoding the expected 157 amino acid protein, and an extensive 3' nontranslated sequence. The identification of the clone as full length was consistent with results obtained in mRNA blotting experiments using poly(A)+ mRNA from bovine heart. The coding sequence was placed downstream of a bacteriophage T7 promoter, and protein was expressed in E. coli. The expressed enzyme was soluble, and catalytically active and was readily isolated and purified. The recombinant protein had the expected Mr of 18,000 (estimated by SDS-PAGE), and it showed cross-reactivity with antisera that had been raised against both the bovine heart and the human placenta enzymes. The amino acid sequence of the N-terminal region of the expressed protein showed that methionine had been removed, resulting in a sequence identical to that of the enzyme isolated from the bovine tissue, with the exception that the N-terminal alanine of the protein from tissue is acetylated. A kinetically competent phosphoenzyme intermediate was trapped from a phosphatase-catalyzed reaction. Using 31P NMR, the covalent intermediate was identified as a cysteinyl phosphate. By analogy with the nomenclature used for serine esterases, these enzymes may be called cysteine phosphatases.

Identification of two cysteine residues forming a pair of vicinal thiols in glucosamine-6-phosphate deaminase from Escherichia coli and a study of their functional role by site-directed mutagenesis

The nucleotide sequence of the nagB gene in Escherichia coli, encoding glucosamine-6-phosphate deaminase, located four cysteinyl residues at positions 118, 219, 228, and 239. Chemical modification studies performed with the purified enzyme had shown that the sulfhydryl groups of two of these residues form a vicinal pair in the enzyme and are easily modified by thiol reagents. The allosteric transition to the more active conformer (R), produced by the binding of homotropic (D-glucosamine 6-phosphate or 2-deoxy-2-amino-D-glucitol 6-phosphate) or heterotropic (N-acetyl-D-glucosamine 6-phosphate) ligands, completely protected these thiols against chemical modification. Selective cyanylation of the vicinal thiols with 2-nitro-5-(thiocyanato)benzoate, followed by alkaline hydrolysis to produce chain cleavage at the modified cysteines, gave a pattern of polypeptides which allowed us to identify Cys118 and Cys239 as the residues forming the thiol pair. Subsequently, three mutated forms of the gene were constructed by oligonucleotide-directed mutagenesis, in which one or both of the cysteine codons were changed to serine. The mutant proteins were overexpressed and purified, and their kinetics were studied. The dithiol formed by Cys118 and Cys239 was necessary for maximum catalytic activity. The single replacements and the double mutation affected catalytic efficiency in a similar way, which was also identical to the effect of the chemical block of the thiol pair. However, only one of these cysteinyl residues, Cys239, had a significant role in the allosteric transition, and its substitution for serine reduced the allosteric interaction energy, due to a lower value of KT.

Crystal structure of rat trypsin-S195C at -150 degrees C. Analysis of low activity of recombinant and semisynthetic thiol proteases

The X-ray crystal structure of trypsin-S195C, a rat anionic trypsin mutant in which the active site serine has been replaced by cysteine, was determined at -150 degrees C and room temperature to 1.6 A resolution, R = 15.4% and 1.8 A resolution, R = 15.0%, respectively. Cryo-crystallography was employed to improve the quality of the diffraction data and the resulting structure by eliminating radiation damage and decreasing atomic thermal motion. The average temperature factor decreased by 10 A2 relative to that of the room temperature structure. No radiation-induced decay of the data was detected. The side-chains of the catalytic cysteine and histidine of trypsin-S195C are found with 25% occupancy in secondary orientations rotated 104 degrees and 90 degrees out of the active site, respectively. These alterations, as well as more subtle changes in the active site may be caused by the oxidation of the catalytic sulfur to sulfenic acid. The position of the carbonyl carbon of the tetrahedral intermediate analog, p-amidinophenylpyruvic acid, modeled into trypsin-S195C, is 1.1 A from the catalytic sulfur. The large size and altered approach of the catalytic sulfur to substrates could account for the observed low catalytic activity relative to wild-type trypsin. In addition to the benzamidine in the specificity pocket, two additional binding sites for benzamidine are characterized. One of these mediates an intermolecular contact that appears to maintain the crystal lattice.

Isolation of two glycosylated forms of membrane-bound alkaline phosphatase from avian growth plate cartilage matrix vesicle-enriched microsomes

Isolation of two membrane-bound alkaline phosphatase (AP) species from avian growth plate cartilage matrix vesicle (MV) fractions is described. AP was first released from the membranes by phosphatidylinositol-specific phospholipase C (PIase C), followed by chromatography on DEAE-Bio-Gel A and Reactive-Red agarose. Two AP species having apparent Mr of 81.5 and 77 kDa by SDS-PAGE were purified in high yield and specific activity by this simple method. Treatment with neuraminidase to remove sialic acid residues reduced their size slightly, but did not diminish the difference in Mr between the two species. Digestion with N-glycanase, however, decreased both AP species to a common size of 59 kDa. This reveals that both enzymes are highly glycosylated and suggests that the two forms may result from differences in degree of glycation. The amino acid compositions of the two avian enzyme forms are very similar, but are markedly enriched in serine, glycine and glutamate when compared to those reported for mammalian liver-kidney-bone AP. Possible differences in amino acid sequence between the two avian forms have not been excluded. The cross-reactivity of polyclonal antibodies to these enzymes with bovine kidney, but not intestinal AP, indicate that the avian cartilage APs are of the liver-kidney-bone isozyme type.

Role of the histidine 176 residue in glyceraldehyde-3-phosphate dehydrogenase as probed by site-directed mutagenesis

The catalytically essential amino acid, histidine 176, in the active site of Escherichia coli glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has been replaced with an asparagine residue by site-directed mutagenesis. The role of histidine 176 as a chemical activator, enhancing the reactivity of the thiol group of cysteine 149, has been demonstrated, with iodoacetamide as a probe. The esterolytic properties of GAPDH, illustrated by the hydrolysis of p-nitrophenyl acetate, have been also studied. The kinetic results favor a role for histidine 176 not only as a chemical activator of cysteine 149 but also as a hydrogen donor facilitating the formation of tetrahedral intermediates. These results support the hypothesis that histidine 176 plays a similar role during the oxidative phosphorylation of glyceraldehyde 3-phosphate.

Stereochemistry of phospho group transfer catalyzed by a mutant alkaline phosphatase

The stereochemical course of the phospho group transfer catalyzed by mutant (S102C) alkaline phosphatase from Escherichia coli was investigated by using 31P nuclear magnetic resonance spectroscopy. Transphosphorylation from 4-nitrophenyl (Rp)-[16O, 17O, 18O]phosphate to (S)-propane-1,2-diol occurs with overall retention of configuration at phosphorus. This result is consistent with the view that the hydrolysis of substrates by this mutant enzyme proceeds by way of a covalent phosphoenzyme intermediate in the same manner as the wild-type alkaline phosphatase.

Use of site-directed mutagenesis to elucidate the role of arginine-166 in the catalytic mechanism of alkaline phosphatase

The guanidinium group of arginine-166 has been postulated to act as an electrophilic species during phosphorylation of alkaline phosphatase. Its role could be either to stabilize the developing negative charge on the oxygen of the leaving group or the pentacoordinate transition state or to help bind the -PO2-3 group. We have produced via site-directed mutagenesis two Escherichia coli alkaline phosphatase mutants (lysine-166 and glutamine-166) to test whether the guanidinium group plays a critical role in catalysis. Comparative kinetic characterization of the lysine-166 and glutamine-166 mutants indicates that the charge at residue 166 is not required for the hydrolysis of phosphate monoesters. Small decreases in kcat are observed for both the lysine and glutamine mutants, relative to the wild-type enzyme, but the value for the uncharged glutamine mutant is only one-third that of lysine. Thus, the stabilizing effect of the positively charged guanidinium group does not appear to play a major role in the rate-limiting step for substrate hydrolysis. A significant effect on the Km value is seen only for the glutamine mutant.

Tinkering with enzymes: what are we learning?

It is now possible, by site-directed mutagenesis of the gene, to change any amino acid residue in a protein to any other. In enzymology, application of this technique is leading to exciting new insights both into the mechanism of catalysis by particular enzymes, and into the basis of catalysis itself. The precise and often delicate changes that are being made in and near the active sites of enzymes are illuminating the interdependent roles of catalytic groups, and are allowing the first steps to be taken toward the rational alteration of enzyme specificity and reactivity.

Lysine-156 and serine-119 are required for LexA repressor cleavage: a possible mechanism

LexA repressor of Escherichia coli is inactivated in vivo by a specific cleavage reaction requiring activated RecA protein. In vitro, cleavage requires activated RecA at neutral pH and proceeds spontaneously at alkaline pH. These two cleavage reactions have similar specificities, suggesting that RecA acts indirectly to stimulate self-cleavage, rather than directly as a protease. We have studied the chemical mechanism of cleavage by using site-directed mutagenesis to change selected amino acid residues in LexA, chosen on the basis of kinetic data, homology to other cleavable repressors, and potential similarity of the mechanism to that of proteases. Serine-119 and lysine-156 were changed to alanine, a residue with an unreactive side chain, resulting in two mutant proteins that had normal repressor function and apparently normal structure, but were completely deficient in both types of cleavage reaction. Serine-119 was also changed to cysteine, another residue with a nucleophilic side chain, resulting in a protein that was cleaved at a significant rate. These and other observations suggest that hydrolysis of the scissile peptide bond proceeds by a mechanism similar to that of serine proteases, with serine-119 being a nucleophile and lysine-156 being an activator. Possible roles for RecA are discussed.

Rat alkaline phosphatase. I. Purification and characterization of the enzyme from osteosarcoma: generation of monoclonal and polyclonal antibodies

Alkaline phosphatase (AP) was purified to over 90% homogeneity from rat osteosarcoma by acetone precipitation followed by chromatography on DEAE-cellulose, Sephacryl S-200, and hydroxyapatite. The purified enzyme had a specific activity of 759 units/mg protein at its optimal pH (10.5), and a Km of 0.8 mM for p-nitrophenylphosphate. The enzyme's apparent subunit molecular mass on sodium dodecyl sulfate-polyacrylamide gel electrophoresis was 82,000 Da. The heat-inactivation profile and homoarginine inhibition were characteristic of the bone-liver-kidney AP isoenzyme. Monoclonal and polyclonal anti-AP antibodies were prepared and characterized. Polyclonal rabbit antiserum quantitatively precipitated the activity from purified AP preparations and tissue extracts but did not inhibit AP catalytic activity. This antiserum was almost 10-fold less active against heat-inactivated enzyme when tested in a competition assay using 125I-AP. Two distinct monoclonal antibodies were each partly effective in immunoprecipitating AP when tested individually; however, together they precipitated over 90% of the AP activity.

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.

Protein engineering

The techniques of protein engineering are proving to be a revolutionary experimental tool for understanding protein structure-function relationships. Even at this early stage, proteins of improved characteristics for specific industrial and therapeutic uses have already been produced. Tailoring enzymatic properties for non-physiological substrate conditions, altering pH optima, changing substrate specificity, and improving stability have already been demonstrated to be feasible. Nevertheless, the ability to make useful proteins which radically differ from a natural structure or designing altogether new structures exceeds present understanding.

Advantages to mutagenesis techniques generating populations containing the complete spectrum of single codon changes

The limitations of current mutagenesis techniques are analyzed in terms of the number and kinds of codon changes they make and in terms of the population size needed to produce all single or multiple amino acid variants. It is shown how a technique that can alter a single codon of a gene, producing all possible variant codons without affecting the rest of the gene, has certain advantages, if it can be used at each place in the gene in one experiment. Such a technique has advantages when the goals are to understand: (1) how specific structural alterations in a mutant protein cause it to function in a different but specific way, (2) how to predict which amino acids in a protein contact or interact with each other, and (3) why a protein is more or less sensitive to mutational disruption, depending upon the specific mutation. This is because it would generate the maximum number of (1) mutant proteins with different functions, (2) intracistronic suppressor for any starting mutation, and (3) random amino acid substitutions at random places. Furthermore, such a technique could produce useful variants more quickly and on a smaller scale than either evolution or current methods.

Importance of a hydrophobic residue in binding and catalysis by dihydrofolate reductase

A conserved residue at the dihydrofolate binding site of dihydrofolate reductase (EC 1.5.1.3), leucine-54, was replaced with glycine to ascertain the role of this hydrophobic amino acid. The effect of the mutation is both to increase the dissociation rate of dihydrofolate and decrease the rate of hydride transfer thus changing the rate-limiting step in catalysis from product loss (leucine-54) to hydride transfer (glycine-54). The total stabilization by leucine-54 of the transition state for hydride transfer is ca. 10(4)-fold (delta delta G approximately 5.4 kcal/mol) at subsaturating dihydrofolate levels relative to free enzyme despite its location some 10 A from the site of chemical reaction.

Idealization of the hydrophobic segment of the alkaline phosphatase signal peptide

Proteins secreted by prokaryotic cells are synthesized as precursors containing an amino-terminal extension sequence or signal peptide. Although these signal peptides share little primary sequence homology, recent studies suggest that they function via common pathways during the transport process and that a common element may reside in their secondary structural characteristics. We are investigating the role of an idealized hydrophobic sequence with high potential for alpha-helix formation in the Escherichia coli alkaline phosphatase signal peptide. Here, amino-acid substitutions were made using site-directed mutagenesis to produce a mutant signal sequence containing nine consecutive leucine residues in the hydrophobic core segment. Transport studies with this mutant precursor indicate that mature alkaline phosphatase is correctly targeted to the E. coli periplasm and that processing of the precursor to the mature form of the enzyme is extremely rapid. In contrast, processing is slowed when the mutant signal sequence is lengthened by the insertion of five additional leucine residues and one serine.