Molten globular characteristics of the native state of apomyoglobin

Apomyoglobin, myoglobin lacking the haem group, is a natural intermediate in biosynthesis of myoglobin, and has some structural features in common with the haem-containing native state. Unfolding or refolding studies of apomyoglobin have identified a molten globule intermediate at acid pH. We show here that both the native state of apomyoglobin and the molten globule intermediate have highly plastic structures. Substitution of single amino acids on the surface or in the interior of helices in the native protein produce dramatic changes in the helix content and tryptophan emission of apomyoglobin at neutral and acidic pH. The signals from the intermediate and native apomyoglobin correlate closely suggesting that apomyoglobin itself has a molten globule-like character, its structure representing a population of interconverting substates rather than a fixed conformation.

Alpha-helix-forming propensities in peptides and proteins

Much effort has been invested in seeking to understand the thermodynamic basis of helix stability in both peptides and proteins. Recently, several groups have measured the helix-forming propensities of individual residues (Lyu, P.C., Liff, M.I., Marky, L.A., Kallenbach, N.R. Science 250:669-673, 1990; O'Neil, K.T., DeGrado, W.F. Science 250:646-651, 1990; Padmanabhan, S., Marqusee, S., Ridgeway, T., Laue, T.M., Baldwin, R.L. Nature (London) 344:268-270, 1990). Using Monte Carlo computer simulations, we tested the hypothesis that these differences in measured helix-forming propensity are due primarily to loss of side chain conformational entropy upon helix formation (Creamer, T.P., Rose, G.D. Proc. Natl. Acad. Sci. U.S.A. 89:5937-5941, 1992). Our previous study employed a rigid helix backbone, which is here generalized to a completely flexible helix model in order to ensure that earlier results were not a methodological artifact. Using this flexible model, side chain rotamer distributions and entropy losses are calculated and shown to agree with those obtained earlier. We note that the side chain conformational entropy calculated for Trp in our previous study was in error; a corrected value is presented. Extending earlier work, calculated entropy losses are found to correlate strongly with recent helix propensity scales derived from substitutions made within protein helices (Horovitz, A., Matthews, J.M., Fersht, A.R. J. Mol. Biol. 227:560-568, 1992; Blaber, M., Zhang, X.-J., Matthews, B.M. Science 260:1637-1640, 1993). In contrast, little correlation is found between these helix propensity scales and the accessible surface area buried upon formation of a model polyalanyl alpha-helix. Taken in sum, our results indicate that loss of side chain entropy is a major determinant of the helix-forming tendency of residues in both peptide and protein helices.

Rules for alpha-helix termination by glycine

A predictive rule for protein folding is presented that involves two recurrent glycine-based motifs that cap the carboxyl termini of alpha helices. In proteins, helices that terminated in glycine residues were found predominantly in one of these two motifs. These glycine structures had a characteristic pattern of polar and apolar residues. Visual inspection of known helical sequences was sufficient to distinguish the two motifs from each other and from internal glycines that fail to terminate helices. These glycine motifs--in which the local sequence selects between available structures--represent an example of a stereochemical rule for protein folding.

Helix stop signals in proteins and peptides: the capping box

The alpha-helix [Pauling, L., Corey, R. B., & Branson, H. R. (1951) Proc. Natl. Acad. Sci. U.S.A. 37, 205-211] is a common motif in both proteins and peptides. Despite intense investigation, predictive understanding of helices is still lacking. A recent hypothesis [Presta, L. G., & Rose, G. D. (1988) Science 240, 1632-1641] proposed that the structural specificity of helices resides, in part, in those residues that flank helix termini. If so, then signals that arrest helix propagation--i.e., helix stop signals--should be found among these flanking residues. Evidence is presented for the existence of one such signal, a reciprocal backbone-side-chain hydrogen-bonding interaction, dubbed the capping box. In proteins, the capping box is found uniquely at helix N-termini. In peptides, the capping box can function as a helix stop signal, as shown in the work of Kallenbach and co-workers.

Effects of alanine substitutions in alpha-helices of sperm whale myoglobin on protein stability

The peptide backbones in folded native proteins contain distinctive secondary structures, alpha-helices, beta-sheets, and turns, with significant frequency. One question that arises in folding is how the stability of this secondary structure relates to that of the protein as a whole. To address this question, we substituted the alpha-helix-stabilizing alanine side chain at 16 selected sites in the sequence of sperm whale myoglobin, 12 at helical sites on the surface of the protein, and 4 at obviously internal sites. Substitution of alanine for bulky side chains at internal sites destabilizes the protein, as expected if packing interactions are disrupted. Alanine substitutions do not uniformly stabilize the protein, either in capping positions near the ends of helices or at mid-helical sites near the surface of myoglobin. When corrected for the extent of exposure of each side chain replaced by alanine at a mid-helix position, alanine replacement still has no clear effect in stabilizing the native structure. Thus linkage between the stabilization of secondary structure and tertiary structure in myoglobin cannot be demonstrated, probably because of the relatively small free energy differences between side chains in stabilizing isolated helix. By contrast, about 80% of the variance in free energy observed can be accounted for by the loss in buried surface area of the native residue substituted by alanine. The differential free energy of helix stabilization does not account for any additional variation.

Protein folding--what's the question?

The folding reactions of many small, globular proteins exhibit two-state kinetics, in which the folded and unfolded states interconvert readily without observable intermediates. Typically, the free energy difference, delta G, between the native and denatured states of such a protein is quite small, lying in the range of approximately -5 to -15 kcal/mol. We point out that, under these circumstances, a population of native-like molecules will persist, even in the presence of mutations sufficiently destabilizing to change the sign of delta G. Therefore, it is not energy per se that determines conformation. A corollary to this argument is that specificity--not stability--would be the more informative focus in future folding studies.

Hydrogen bonding in globular proteins

A global census of the hydrogen bonds in 42 X-ray-elucidated proteins was taken and the following demographic trends identified: (1) Most hydrogen bonds are local, i.e. between partners that are close in sequence, the primary exception being hydrogen-bonded ion pairs. (2) Most hydrogen bonds are between backbone atoms in the protein, an average of 68%. (3) All proteins studied have extensive hydrogen-bonded secondary structure, an average of 82%. (4) Almost all backbone hydrogen bonds are within single elements of secondary structure. An approximate rule of thirds applies: slightly more than one-third (37%) form i----i--3 hydrogen bonds, almost one-third (32%) form i----i--4 hydrogen bonds, and slightly less than one-third (26%) reside in paired strands of beta-sheet. The remaining 5% are not wholly within an individual helix, turn or sheet. (5) Side-chain to backbone hydrogen bonds are clustered at helix-capping positions. (6) An extensive network of hydrogen bonds is present in helices. (7) To a close approximation, the total number of hydrogen bonds is a simple function of a protein's helix and sheet content. (8) A unique quantity, termed the reduced number of hydrogen bonds, is defined as the maximum number of hydrogen bonds possible when every donor:acceptor pair is constrained to be 1:1. This quantity scales linearly with chain length, with 0.71 reduced hydrogen bond per residue. Implications of these results for pathways of protein folding are discussed.

Side-chain entropy opposes alpha-helix formation but rationalizes experimentally determined helix-forming propensities

In recent host-guest studies, the helix-forming tendencies of amino acid residues have been quantified by three groups, each obtaining similar results [Padmanabhan, S., Marqusee, S., Ridgeway, T., Laue, T. M. & Baldwin, R. L. (1990) Nature (London) 344, 268-270; O'Neil, K. T. & DeGrado, W. F. (1990) Science 250, 646-651; Lyu, P. C., Liff, M. I., Marky, L. A. & Kallenbach, N. R. (1990) Science 250, 669-673]. Here, we explore the hypothesis that these measured helix-forming propensities are due primarily to conformational restrictions imposed upon residue side chains by the helix itself. This proposition is tested by calculating the extent to which the bulky helix backbone "freezes out" available degrees of freedom in helix side chains. Specifically, for a series of apolar residues, the difference in configurational entropy, delta S, between each side chain in the unfolded state and in the alpha-helical state is obtained from a simple Monte Carlo calculation. These computed entropy differences are then compared with the experimentally determined values. Measured and calculated values are found to be in close agreement for naturally occurring amino acids and in total disagreement for non-natural amino acids. In the calculation, delta S(Ala) = 0. The rank order of entropy loss for the series of natural apolar side chains under consideration is Ala less than Leu less than Trp less than Met less than Phe less than Ile less than Tyr less than Val. Among these, none favor helix formation; Ala is neutral, and all remaining residues are unfavorable to varying degrees. Thus, applied to side chains, the term "helix preference" is a misnomer. While side chain-side chain interactions may modulate stability in some instances, our results indicate that the drive to form helices must originate in the backbone, consistent with Pauling's view of four decades ago [Pauling, L., Corey, R. B. & Branson, H. R. (1951) Proc. Natl. Acad. Sci. USA 37, 205-210].

The protein-folding problem: the native fold determines packing, but does packing determine the native fold?

A globular protein adopts its native three-dimensional structure spontaneously under physiological conditions. This structure is specified by a stereochemical code embedded within the amino acid sequence of that protein. Elucidation of this code is a major, unsolved challenge, known as the protein-folding problem. A critical aspect of the code is thought to involve molecular packing. Globular proteins have high packing densities, a consequence of the fact that residue side chains within the molecular interior fit together with an exquisite complementarity, like pieces of a three-dimensional jigsaw puzzle [Richards, F. M. (1977) Annu. Rev. Biophys. Bioeng. 6, 151]. Such packing interactions are widely viewed as the principal determinant of the native structure. To test this view, we analyzed proteins of known structure for the presence of preferred interactions, reasoning that if side-chain complementarity is an important source of structural specificity, then sets of residues that interact favorably should be apparent. Our analysis leads to the surprising conclusion that high packing densities--so characteristic of globular proteins--are readily attainable among clusters of the naturally occurring hydrophobic amino acid residues. It is anticipated that this realization will simplify approaches to the protein-folding problem.

Hydrophobicity of amino acid subgroups in proteins

Protein folding studies often utilize areas and volumes to assess the hydrophobic contribution to conformational free energy (Richards, F.M. Annu. Rev. Biophys. Bioeng. 6:151-176, 1977). We have calculated the mean area buried upon folding for every chemical group in each residue within a set of X-ray elucidated proteins. These measurements, together with a standard state cavity size for each group, are documented in a table. It is observed that, on average, each type of group buries a constant fraction of its standard state area. The mean area buried by most, though not all, groups can be closely approximated by summing contributions from three characteristic parameters corresponding to three atom types: (1) carbon or sulfur, which turn out to be 86% buried, on average; (2) neutral oxygen or nitrogen, which are 40% buried, on average; and (3) charged oxygen or nitrogen, which are 32% buried, on average.

Extra-anatomic bypass grafting: a rational approach

To determine predictors of long-term patency in extra-anatomic bypass grafting, the authors studied retrospectively the charts of 134 patients who underwent bypass grafting (axillofemoral in 17, axillobifemoral in 32 and femorofemoral in 85). Of the study group, 64% were men; the mean age was 65 +/- 12 years (+/- SEM). The indications for grafting were limb salvage (102), claudication (27) and replacement of septic grafts (5), and for using the extra-anatomic route included high risk (83), sepsis (8) and unilateral disease (34). Operative mortality was 6% and the early graft occlusion rate 7.4%. The late death rate was 44%. At 3 years, the life-table patency rates for the various procedures were axillofemoral 52.5%, axillobifemoral 67.7% and crossfemoral 86.9%. Smoking significantly (p less than 0.05) decreased the patency rate, but diabetes did not. However, amputation was more frequent in diabetics. Indications for operation did not alter patency rates, but did affect operative mortality. The authors conclude that extra-anatomic bypass grafting is highly successful, but not as successful as anatomic bypass. When appropriate, the axillobifemoral graft is preferred to the axillounifemoral graft because of its increased patency. Crossfemoral grafts must be carefully monitored to ensure that no donor limb stenosis occurs and this procedure should not be attempted unless the disease is truly unilateral.

Helix signals in proteins

The alpha helix, first proposed by Pauling and co-workers, is a hallmark of protein structure, and much effort has been directed toward understanding which sequences can form helices. The helix hypothesis, introduced here, provides a tentative answer to this question. The hypothesis states that a necessary condition for helix formation is the presence of residues flanking the helix termini whose side chains can form hydrogen bonds with the initial four-helix greater than N-H groups and final four-helix greater than C-O groups; these eight groups would otherwise lack intrahelical partners. This simple hypothesis implies the existence of a stereochemical code in which certain sequences have the hydrogen-bonding capacity to function as helix boundaries and thereby enable the helix to form autonomously. The three-dimensional structure of a protein is a consequence of the genetic code, but the rules relating sequence to structure are still unknown. The ensuing analysis supports the idea that a stereochemical code for the alpha helix resides in its boundary residues.

The behavioral treatment of Raynaud's disease: a review

Raynaud's disease is a peripheral vascular system disorder characterized by episodes of vasoconstriction in the hands and feet resulting in a lowering of skin temperature and pain. Recent studies are reviewed that focus on the behavioral treatment of Raynaud's disease--in particular, biofeedback and autogenic training. Methodological problems and other difficulties include the measurement of skin temperature, schedules of reinforcement/feedback, and characteristics of the experimenter and subject. Studies in this area indicate some promise for certain behavioral interventions, especially finger temperature biofeedback under cold stress conditions. On the other hand, further research is needed to clarify the mechanisms, especially that of vasodilation, and the applications of temperature biofeedback, as well as the role of attitudinal, interpersonal, and cognitive factors.

Loops in globular proteins: a novel category of secondary structure

The protein loop, a novel category of nonregular secondary structure, is a segment of contiguous polypeptide chain that traces a "loop-shaped" path in three-dimensional space; the main chain of an idealized loop resembles a Greek omega (omega). A systematic study was made of 67 proteins of known structure revealing 270 omega loops. Although such loops are typically regarded as "random coil," they are, in fact, highly compact substructures and may also be independent folding units. Loops are almost invariably situated at the protein surface where they are poised to assume important roles in molecular function and biological recognition. They are often observed to be modules of evolutionary exchange and are also natural candidates for bioengineering studies.

Compact units in proteins

An explicit measure of geometric compactness called the coefficient of compactness is introduced. This single value figure of merit identifies those continuous segments of the polypeptide chain having the smallest solvent-accessible surface area for their volume. These segments are the most compact units of the protein, and the larger ones correspond to conventional protein domains. To demonstrate the plausibility of this approach as a method of identifying protein domains, the measure is applied to lysozyme and ribonuclease to discover their constituent compact units. These units are then compared with domains, subdomains, and modules found by other methods. To show the sensitivity of the method, the measure is used to successfully differentiate between native and deliberately misfolded proteins [Novotný, J., Bruccoleri, R., & Karplus, M. (1984) J. Mol. Biol. 177, 787-818]. Methods that utilize only backbone atoms to define domains cannot distinguish between authentic and misfolded molecules because their backbone conformations are virtually superimposable. Compact units identified by this method exhibit a hierarchic organization. Such an organization suggests possible folding pathways that can be tested experimentally.

Hydrophobicity of amino acid residues in globular proteins

During biosynthesis, a globular protein folds into a tight particle with an interior core that is shielded from the surrounding solvent. The hydrophobic effect is thought to play a key role in mediating this process: nonpolar residues expelled from water engender a molecular interior where they can be buried. Paradoxically, results of earlier quantitative analyses have suggested that the tendency for nonpolar residues to be buried within proteins is weak. However, such analyses merely classify residues as either "exposed" or "buried." In the experiment reported in this article proteins of known structure were used to measure the average area that each residue buries upon folding. This characteristic quantity, the average area buried, is correlated with residue hydrophobicity.