Use of Milk Proteins in Other Foods

Milk proteins are a heterogeneous mixture that includes two main groups of protein, casein and whey or serum protein. Within these two classes of proteins are six major subclasses: a,-casein, aa2-casein, p-casein, K-casein, (J-lactoglobulin, and a-lactalbumin. In addition, a number of other proteins are present, such as serum albumin, immunoglobulins, and lactoferrin.

Casein is used in a number of food preparations. Casein may be precipitated from the whey by acidification to its isoelectric point, ~ pH 6.0. This phenomenon is easily demonstrated by adding lemon to tea or coffee that contains milk or cream. The serum protein or whey fractions remain in solution.

The separation of casein from whey is important in that both protein classes have novel properties. Casein in the presence of lipids can form micellar structures ranging in size from 30 to 300 nm. Micellar structures are responsible for the opaqueness of milk. The ability to form micelles is useful, because a number of hydrophobic substances can be incorporated into casein micelles. The whey fraction in milk is also used in the preparation of fabricated foods. Whey proteins are hydrophilic and may be added to beverages to increase the protein content. Furthermore, whey proteins are highly water binding because of their hydrophilicity, which makes them well suited for addition to soups, gravies; and salad dressings. The knowledge that one can separate these two important milk fractions by acid precipitation is very useful. The separated forms differ in solubility, viscosity, and utility as thickening agents. Whey is ideally suited for increasing viscosity and thickening, whereas casein is an excellent choice when emulsification is needed to add flavoring agents. Moreover, under certain processing conditions, whey or whey/casein mixtures can be formed into hydrophobic micelles. Hydrophobic micellar structures can be designed to have oily characteristics for use as fat substitutes. An example is Simpless.

Peptide Bond ch

r oh

Peptide Bond h2n ch

II

A.

n

h2o o

Retinal Hydrogen Bond H2o

Figure 2-5. Formation of a peptide bond (A). The a-amino group of one amino acid displaces the hydroxyl of the carboxyl group in another. Although amino acids are good nucleophiles, the hydroxyl group is a poor leaving group. Therefore, the reaction is endergonic with a free energy of change of about 20 kJ or ~80-90 kcal/mol. As shown in B, the peptide bond is capable of resonance and charge separation. As indicated in C, the oxygen and nitrogen of the amide bond lie in a plane, which contributes to peptide bond stabilization. The hydrogen of the amino group is usually trans to the oxygen in the peptide backbone. Note that the bond length between the oxygen and carbon originating from the carboxyl group is 0.124 nm, which is typical of a C=0 double bond. The nitrogen that is attached to the C=0 forming the peptide bond also has a relatively short bond length (0.132 nm), indicating some double bond character. This causes an electric dipole. Because each bond has some double bond character, there is also restricted rotation. However, the nitrogen to a-carbon bond is 0.146 nm, which is typical of a single bond. Around this bond rotation can occur, unless it is hindered by the presence of a large R group or the presence of a prolyl residue.

Figure 2-5. Formation of a peptide bond (A). The a-amino group of one amino acid displaces the hydroxyl of the carboxyl group in another. Although amino acids are good nucleophiles, the hydroxyl group is a poor leaving group. Therefore, the reaction is endergonic with a free energy of change of about 20 kJ or ~80-90 kcal/mol. As shown in B, the peptide bond is capable of resonance and charge separation. As indicated in C, the oxygen and nitrogen of the amide bond lie in a plane, which contributes to peptide bond stabilization. The hydrogen of the amino group is usually trans to the oxygen in the peptide backbone. Note that the bond length between the oxygen and carbon originating from the carboxyl group is 0.124 nm, which is typical of a C=0 double bond. The nitrogen that is attached to the C=0 forming the peptide bond also has a relatively short bond length (0.132 nm), indicating some double bond character. This causes an electric dipole. Because each bond has some double bond character, there is also restricted rotation. However, the nitrogen to a-carbon bond is 0.146 nm, which is typical of a single bond. Around this bond rotation can occur, unless it is hindered by the presence of a large R group or the presence of a prolyl residue.

lar masses are often reported as daltons [Da] or kilodaltons [kDa].)

Protein structure is complex. Consequently, it is described at a number of different levels (Branden and Tooze, 1991; Darby, 1993). The first level is the linear sequence of amino acids. The term primary structure is used to delineate this one-dimensional linear sequence of amino acids. The amino acid sequence of a peptide or protein can be determined by sequencing the amino acids along the linear polypeptide chain. Alternatively the sequence may be inferred from the corresponding nucleotide sequence of the protein's gene or messenger RNA (mRNA).

Considerable information may be inferred from knowing the primary structure. Many sequences act as specific signals for certain protein modifications that impact biological regulation. Examples include: (1) the sequences asn-x-ser and asn-x-thr, which are commonly associated as sites for /V-linked gly-cosylation in proteins (i.e., the addition of sugars at specified sites along the polypeptide chain); (2) the sequence arg-gly-asp-ser, which corresponds to a cell surface-binding domain in certain proteins (e.g., fibronectin) that allows a protein to bind to a cell's surface; and (3) the sequence lys-asp-glu-leu, which is one of several signals important for the vec-

toral movement of soluble proteins within the endoplasmic reticulum (ER). The use of x in the first example indicates that virtually any amino acid can be substituted for x without affecting the functional significance of the sequence when it appears in a protein. When a given amino acid sequence is commonly associated with a function, the sequence is often designated as a consensus sequence, particularly if the sequence is found in a di verse array of animal, plant, or bacterial cells and is consistently associated with a specific property or function.

Additional information that may be obtained from a primary sequence is an indication of regions of hydrophobicity and hydro-philicity A hydropathy plot is given in Figure 2-6. Hydropathy plots are useful in defining regions in a polypeptide or clusters of amino acids that differ in their polar or non-polar p o p o

Amino Acid Sequence => 100

Amino Acid Sequence => 100

regions are likely to interact with aqueous or ionic environments. See Table 2-1 for hydropathic indices for given amino acids. A hydropathy plot (A) identifies regions of a polypeptide chain that contain amino acids that are predominantly hydrophobic (shaded) or, in contrast, hydrophilic in nature. The example shown could apply to a transmembrane protein (B). Transmembrane proteins often cross the lipid bilayers of cell membranes. The hydrophobic regions are associated with the interior, whereas the hydrophilic regions may extend into the cytoplasmic compartment or toward the exterior of the cell.

regions are likely to interact with aqueous or ionic environments. See Table 2-1 for hydropathic indices for given amino acids. A hydropathy plot (A) identifies regions of a polypeptide chain that contain amino acids that are predominantly hydrophobic (shaded) or, in contrast, hydrophilic in nature. The example shown could apply to a transmembrane protein (B). Transmembrane proteins often cross the lipid bilayers of cell membranes. The hydrophobic regions are associated with the interior, whereas the hydrophilic regions may extend into the cytoplasmic compartment or toward the exterior of the cell.

characteristics. Protein folding is dependent on the location of hydrophobic and hydro-philic amino acids within a given segment or domain in a polypeptide chain. Moreover, hydropathy characteristics are important for the insertion of protein polypeptide sequences into cell membranes or for creation of networks of proteins that function in transmembrane communication (Hamaguchi, 1992).

Secondary structure refers to the spatial arrangement of atoms around the backbone of a given polypeptide chain. Amino acid R groups can influence this type of orientation. For example, large, bulky groups prevent rotation around the backbone and fix given conformations into place. As noted previously in amino acids such as proline, rotation is prohibited because of an imine bond. In addition, electrostatic attractions and repulsions between amino acid residues, interactions between the amino acids at the ends of the helix, and the electric dipole inherent to the peptide backbone contribute to secondary beta structures. When polypeptide chains are arranged in long strands or sheets (e.g., p-sheets), the consequence is often a fibrous protein (Nesloney and Kelly 1996). In contrast, globular proteins are folded in spirals or globular shapes and coils. In the polypeptides that form fibrous proteins, a common feature is segmental repeats of amino acids whose side chains restrict free rotation around the polypeptide backbone. In contrast, in a-heli-cal structures, the polypeptide chains are enriched in amino acids with freer rotation around the polypeptide chain. However, many different types of segmental polypeptidyl structures may exist in a single protein. Indeed, most proteins contain segments that have the characteristics of both a-helical and (3-sheet structures (Fig. 2-7).

Tertiary structure is used to describe the three-dimensional arrangements of amino acids within the entire protein (Darby, 1993). Another term, quaternary structure, is used to describe complex arrangements of tertiary structures (e.g., arrangement of subunits). Tertiary structures may also accommodate specific prosthetic groups. The term prosthetic group is used to describe a unique moiety or defined chemical structure that confers a specific function or additional property to a given protein. Prosthetic groups may be cova-Iently or non-covalently linked to the proteins they serve. Examples of prosthetic groups include the heme group in hemoglobin and myoglobin and enzyme cofactors such as Iipoic acid and pyridoxal 5'-phosphate.

How Protein Conformations Are Stabilized

Most protein conformations are stabilized by relatively weak interactions. Some examples of the types of interactions that stabilize proteins are given in Table 2-2. The difference in free energy between folded and unfolded states in typical proteins is in the range of 20 to 70 kJ/mol (5 to 17 kcal/mol). To put the value of 20 to 70 kJ/mol into perspective, the amount of energy needed to break covalent bonds is -350 kJ/mol (83 kcal/mol) for C-C bonds and —410 kJ/mol (98 kcal/mol) for C-H bonds. Although many conformations are possible, a large number of weak interactions can result in a degree of stabilization.

The stability of a given conformation is dictated by the entropy term associated with the energy relationships important to the folding and unfolding of the polypeptide chain. When stabilization occurs, it is the result of the sum total of the hydrophobic and ionic interactions of the various amino acid side chains. In particular, hydrophobic interactions are important for protein-protein interactions and tertiary structure in large proteins. As proteins become smaller in size, it often becomes difficult to accommodate or appropriately place hydrophobic residues. It is for this reason that many small proteins are held together by covalent bonds, usually disulfide linkages. When such covalent bonds are formed, they impose restrictions on folding (Hamaguchi, 1992).

The process of folding and unfolding of proteins is dynamic. For most proteins, this process results from a sequential series of events that takes place in a programmed manner. When a protein is folded so that it is functional, the process is referred to as natu-ration (Branden and Tooze, 1991). When unfolding or inappropriate folding occurs and results in a dysfunctional state, the process is

••Met-Tyr-Lys-Gly-Gly-Pro-Leu-lle-Arg-Primary Structure a-Hélix

3.6 Amino

Acid Units +

3.6 Amino

Acid Units +

Example of a ß-Sheet

Intermolecular Hydrogen Bonds

Example of a ß-Sheet

Intermolecular Hydrogen Bonds

Collagen Intramolecular Hydrogen Bond
  • a) Skeletal Representation
  • b) Ball-and-Stick Model

Secondary Structure

Dysfunctional Stick Figures

Tertiary Structure

Quaternary Structure Protein

Quaternary Structure

Figure 2-7. Levels of structure in proteins. The primary structure consists of a sequence of amino acids with secondary structures that often exist as a -helices or as (3-sheets. These structures combine to make complex tertiary structures, which may serve as a subunit that eventually forms the quaternary structure of multimeric proteins. The example is a representation of the tertiary and quaternary structure of hemoglobin. The heme molecules are shown within the folds.

Quaternary Structure

Figure 2-7. Levels of structure in proteins. The primary structure consists of a sequence of amino acids with secondary structures that often exist as a -helices or as (3-sheets. These structures combine to make complex tertiary structures, which may serve as a subunit that eventually forms the quaternary structure of multimeric proteins. The example is a representation of the tertiary and quaternary structure of hemoglobin. The heme molecules are shown within the folds.

referred to as denaturation. Most commonly proteins are denatured by (1) substances that disrupt associated water structures, (2) heat, and/or (3) extremes in the acid or base balance. Disruption of water organization by a dénaturant (e.g., urea) influences the secondary or tertiary structure of proteins. Extremes of heat denature proteins, because vibrational energy within the protein molecule is altered. An increase in vibrational energy, when exces-

NUTRITION INSIGHT

Vitamin and Mineral Prosthetic Groups Give Color to Certain Holoproteins

When some prosthetic groups associate with proteins, they often lend color to the protein as a functional property. A good example is the addition of flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN) to the apoprotein subunits that form into flavoproteins. Flavoproteins appear yellow. Prosthetic groups that contain iron, such as heme, give proteins colors that range from brown to bright red, depending on the oxidation state of the iron-containing heme moiety. As an example, the amount of myoglobin in muscle tissue imparts to meat varying amounts of color. White poultry meat contains relatively small amounts of myoglobin and heme (0.1 to 0.4 mg/g), whereas dark poultry meat often contains five to six times that amount of myoglobin. Veal and pork contain 2 to 7 mg of myoglobin per gram, and beef usually contains 10 mg or more of myoglobin per gram. The globins of myoglobin and hemoglobin are colorless; yet when they are complexed with heme through coordinate and synergistic histidyl residue interactions, red to pink colors are imparted.

Protein

Oxymyoglobin is usually red-pink in color, whereas deoxygenated myoglobin is purple-red. Denaturation of meat protein by cooking can cause dissociation of heme and the formation of protein, iron, and imidazole (from histidine) complexes that are brown to tan in color. When a prosthetic group imparts color to a protein, it is referred to as a chromophore.

sive, disrupts tertiary structures. Extremes of acid-base balance or pH cause denaturation by interfering with the organization of water or by altering the redox state of proteins. Finally chemical modifications that change amino acid R groups can have dramatic effects on protein structure. For example, introducing a reducing agent such as mercaptoethanol (HS—CH2—CH2—OH) can disrupt disulfide bond formation, which in turn can cause the opening of associated polypeptide chains.

NUTRITIONAL INFLUENCES ON PROTEIN STRUCTURE AND ASSEMBLY FUNCTION

There are a number of co- and posttransla-tional steps that depend on optimal nutrition and where protein structure or assembly is influenced when nutrition is suboptimal (Rucker and McGee, 1993). In particular, nutritional deficiencies may affect steps important to the co- and posttranslational processing of

TABLE 2-2

Forces that Stabilize Protein Structures

Type of Bond or Force

Description

Ionic bond

Attractive Forces

Charged complexes that are capable of creating very strong interactions depending on the dielectric constant

NH3+

Hydrogen bond

Noncovalent bonds that result from the association of hydrogen with atoms that are electronegative y=0----H-N

Hydrophobic interactions h3c ch3

I H3C. CH3 ch2 ch

Hydrocarbons in an aqueous medium force associations between adjacent hydrocarbon moieties; the energy required for the process comes from the reorganization of the surrounding water structure

■van der Waals interactions Weak electrostatic attractions proteins, because many of the enzymatic proteins involved in protein modifications require metals or vitamin-derived cofactors to function properly A cotranslational event occurs coincident with the actual translation (synthesis) of a protein. A posttranslational modification occurs after the protein has been synthesized. Posttranslational modifications of proteins usually occur in the Golgi or post-Golgi sites associated with the smooth ER or secretory vesicles. It is important to appreciate that (1) posttranslational modifications extend the range of chemical properties of the common amino acids within protein and (2) posttranslational protein modifications are as important to protein production as are any of the transcriptional and translational events that initiate polypeptide synthesis. That is, a defect at any of the steps in the process of protein assembly or modification can result in a dysfunctional product.

Most protein modifications can be placed in one of three broad categories: (1) modifications that involve peptide bond cleavage and formation, (2) those that involve the amino- or carboxy-terminal amino acid, and (3) those that involve specific amino acid side chains. Modifications in the first category may already be familiar. Activation of many peptide hormones and conversion of zymogens to active enzymes are examples of the first category (Konig, 1993). For example, trypsino-gen and other protease zymogens are converted to active proteinases by hydrolysis of a specific peptide bond to release an N-terminal segment. Formation of the hormone insulin

Pro-opiomelanocortin 7-MSH a-MSH 7-LPH p-Endorphin

ACTH p-LPH

Figure 2-8. Examples of peptide hormone formation. Bioactive peptides can arise by several mechanisms. As an example, the production of a- (3-, and y-melanocyte-stimulating hormones IMSH), adrenocorticotropic hormone (ACTH), p-endorphin, and p- and y-lipotropin (LPH) are shown in A. The peptides arise from the cleavage of peptide bonds in the protein pro-opiomelanocortin, which acts as a precursor. Arrows indicate sites where enzyme proteinases cleave specific peptide bonds to give rise to the designated peptides.

from its precursor polypeptide, proinsulin, occurs by proteolytic excision of an internal segment, leaving two polypeptide chains attached by disulfide bonds. Other examples are shown in Figure 2-8. The precursor polypeptide pro-opiomelanocortin produced by the pituitary gland can be cleaved to yield seven different polypeptide hormones. The size of peptide hormones may be quite small

(e.g., thyrotropin-releasing factor is only a tri-peptide) or relatively large (e.g., glucagon contains 29 amino acids and insulin contains 51 amino acid residues).

The second major type of modification, N- or C-terminus modifications, are important in (1) directing proteins to specific compartments within the cell, (2) protecting the amino- and carboxy-terminal sequences from proteolysis, and (3) selective activation of enzymes and hormones. For example, amidation of the carboxy-termini of many hormones (e.g., calcitonin) is essential for activity

The third major type of modification, R group or side chain modifications, provide chemical features important for cellular com-partmentalization and trafficking, receptor binding, regulatory signaling, prosthetic group addition or formation, protein cross-linking, and the creation of novel chemical sites such as those important to metal binding. Table 2-3 lists some examples of chemical modifications of side chains of amino acid residues in proteins that are common targets for the modifications.

As signals for compartmentalization, typi-

TABI.K

Examples of Modifications Involving Amino Acid Side Chains

Selected Functions

Process or Example

Commonly Targeted Amino Acids in Proteins

Compartmentalization, receptor binding

Regulatory signaling

Prosthetic group additions or formation

Protein cross-linking

Other

Acylations

Acetylations

Glycosylations

Acetylations

Adenylylations

ADP-ribosylations

Methylations

Phosphorylations

Ubiquitin addition

Sulfations

Biotinylations

Flavin (FAD and FMN) additions Phosphopantetheine addition Pyridoxal phosphate addition Retinal addition

Allysine and dehydronorleucine formation Cystine formation Glutamyllysine formation

Carboxylations

Halogen addition (iodine)

Hydroxylations

Nonenzymatic glycosylations

Sulfoxide formation

Glutamylation

Lys Tyr

Arg, Tyr

Cys, His Ser Lys Lys

Lys Cys

Gin, Lys

Asp, Glu Tyr

Asp, Lys, Pro Arg, Lys Met Glu

NUTRITION INSIGHT

NUTRITION INSIGHT

Niacin and Polyribosylation of Histone Proteins

Pellagra is a well known deficiency disease that results from an insufficient intake of niacin and tryptophan (see Chapter 20). Niacin is a precursor for nicotinamide adenine dinucleotide (NAD) synthesis. The amount of NAD in cells is very much dependent on the amount of niacin, which is either made from tryptophan-related pathways or present in the diet as a vitamin. One of the functions of NAD is as a substrate for mono- and polyribosylation reactions.

, , Acceptor 1 Protein

C0NH2

C0NH2

Adenine

Ribose Ribose

Adenine Ribose Ribose'

^pxer

Adenine

Acceptor Protein

Ribose Ribose

Polyribosylations change the surface property and structural conformation of some proteins (e.g., histones that are found in the nucleus of cells). Ribosylation reactions are important in many of the complex steps in enzyme regulation and also for DNA repair. In this regard, it is important to note that the skin lesions associated with pellagra may be due to the inability to carry out normal DNA repair because of an inability to alter histone structure as influenced by polyribosylation reactions.

cai reactions involve acylations, acetylations, and glycosylations. Acylation, the attachment of a long-chain fatty acid, provides lipophilic handles that facilitate the vectorial movement of protein from one compartment to another and that create specific sites to enhance attachments and compartmentalization within cells as well. There are also enzymes in post-translational pathways that modify lysyl, cys-teinyl, and glutamyl residues in specific proteins that may be influenced by changes in nutritional status.

Moreover, the formation or incorporation of various prosthetic groups in proteins can occur as posttranslational protein modifications. The addition of a given prosthetic group may be essential to the creation of an enzymatic or functional active site (e.g., the addition of biotin, one of the flavocoenzymes, heme, or a metal such as iron or copper). See Chapters 20, 22, 31, and 32 for more discussion of these prosthetic groups and the proteins that contain them.

For stabilization of protein structures, it is often essential to cross-link specific polypeptide chains together (Fig. 2-9). For structural proteins such as collagen and elastin, the formation of interchain cross-links facilitates the

Lysyl Oxidase

A /

\ + nh3 + h202

nh2

h20

h-S)

Lysine

Allysine

Addition Allysine And Lysine
Dehydrolysinorleucine

Desmosine

Hydroxypyridinoline

Desmosine

Lysinonorleucine

Hydroxypyridinoline

Lysinonorleucine

Lysyl Hydroxylase Desmosine

Figure 2-9. Amino acids that function to cross-link polypeptide chains. Examples include the ly-sine-derived cross-links (A), cystine formation from cysteine (B), and cross-links derived from the enzyme-catalyzed condensation of lysyl and glutaminyl residues (C). For the reactions shown in A, the first step is the oxidation and deamination of specific lysyl residues in proteins, such as elastin and collagen to form residues of allysine. The next steps occur nonenzymatically. Two examples are shown. An aldol condensation product is formed from condensation of two peptide-bound allysine residues. Peptidyl dehydrolysino-norleucine occurs as a product of the Schiff-base reaction. Peptidyl allysine on a polypeptide chain reacts with peptidyl lysine on an adjacent polypeptide chain to cause cross-linking of the two chains. These products may condense further to form even more complex cross-linking amino acids, such as desmosine, which is found in the structural protein elastin, or hydroxypyridinoline, which occurs in collagen. For hydroxypyridinoline, hydroxylysine residues Isee Fig. 2-3) serve as lysine-derived precursors. Note that the R depicted as a part of the hydroxypyridinoline structure can also be the site for glycosyla-tion. As shown in B, the formation of peptidyl cystine cross-links from peptidyl cysteine oxidation is also a common strategy for cross-linking or joining protein polypeptide chains together. C, The cross-linking amino acid residue, y-glutamyllysine. The formation of this cross-linking amino acid is catalyzed by a transaminase, and y-glutamyllysyl residues are present in the proteins fibrin and keratin.

NUTRITION INSIGHT

NUTRITION INSIGHT

Vitamin C and Connective Tissue Protein Synthesis

The nutritional deficiency disease scurvy has a dynamic and important impact on connective tissue and extracellular matrix integrity. With respect to extracellular matrix stability, ascorbic acid serves as a cofactor for lysyl and prolyl hydroxylases. A decrease in prolyl and lysyl hydroxylase activity causes a net decrease in the production of hydroxyprolyl and hydroxylysyl residues in collagen and related proteins. Collagen is a protein that constitutes one third of the total protein in the body and is the major protein in the extracellular matrix of the connective tissue. Under-hydroxylated collagen polypeptide chains do not associate properly and often are more susceptible to degradation than normal forms of collagen; this is an underlying factor in many of the lesions associated with scurvy. (Pro, proline; Hypro, hydroxyproline.)

Semi-dehydroascorbic Acid

Ascorbate

Ascorbate

Collagen a-Chains

Collagen a-Chain Association

Tropocollagen

Semi Dehydroascorbic
Semi-dehydroascorbic Acid Ascorbate

Under-hydroxylated Collagen a-Chains

Degraded Collagen formation of fibers and protein complexes, whose molecular masses can range into the millions of daltons (Reiser et al., 1992). The formation of interchain cross-links also renders collagen and elastin resistant to the action of many proteinases so that these proteins can exist , in a proteinase-enriched environment without significant damage or alteration.

As mechanisms for the involvement of nutrition in protein modifications, specific nutrient deficiencies can cause decreases in given enzymatic activities or catalytic functions, particularly if the nutrient serves as a cofactor for the activity or catalytic function. Caloric restrictions or excesses may also alter the synthesis or degradation of amino acids and other components important to amino acid, peptide, or protein modification.

Much of the regulation and metabolism discussed in subsequent chapters is largely the result of events that depend on posttran-scriptional/posttranslational modifications of proteins. Transcription, translation, and the movement of substances into, through, and out of given cells are all dependent on changes in proteins brought about by enzymatic and nonenzymatic chemical modifications.

In subsequent chapters, when the actions of specific nutrients are described, continually ask how the action or role of the nutrient may be related to some aspect of protein structure or amino acid modification.

REFERENCES

Branden, C. and Tooze, J. (eds.) (1991) Introduction to Protein Structure. Garland, New York

Darby, N. J. (1993) Protein Structure. IRL Press/Oxford University Press, Oxford.

Davies, J. S. (1985) Amino Acids and Peptides. Chapman and Hall, New York.

Hamaguchi, K. (1992) The Protein Molecule: Conformation, Stability, and Folding. Japan Scientific Societies Press, Tokyo; Springer-Verlag, New York.

Konig, W S. (1993) Peptide and Protein Hormones: Structure, Regulation, Activity. A Reference Manual. Weinheim VCH, New York.

Nesloney C. L. and Kelly, J. W (1996) Progress towards understanding beta-sheet structure. Bioorg Med Chem 4:739-766.

Reiser, K„ McCormick, R. J. and Rucker, R. B. (1992) Enzymatic and non-enzymatic cross-linking of collagen and elastin. FASEB J 6:2439-2449.

Rucker, R. B. and McGee, C. (1993) Chemical modifications of proteins in vivo: Selected examples important to cellular regulation. J Nutr 123:977-990.

Rucker, R. B. and Wold, F (1988) Cofactors in and as post-translational protein modifications. FASEB J 2:2252-2261.

RECOMMENDED READINGS

Branden, C. and Tooze, J. (eds.) (1991) Introduction to

Protein Structure. Garland, New York. Karplus, M. and McCammon, J. A. (1986) The dynamics of proteins. Sei Am 254:42-51.

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