Amino Acids Constituents of Proteins

The a-amino acids in proteins are the most varied of the monomers found in biomacromolecules. These amino acids are distingiushed by the chemical properties of their side chains that, in turn, dictate the properties of proteins. Table 2.2 highlights some characteristics of a-amino acids found in proteins. 

The physicochemical properties of the side chains of amino acid residues and the amino acid sequence are dominant factors in determining the structure and function of proteins. A number of parameters are proposed to quantify the contribution of each amino 

TABLE 2.2 Standard a-Amino acids of proteins and their characteristics 

With no side chain hindrance, Gly can feed the main chain through tight places in the protein molecule. This gives the polypeptide backbone at Gly residues much greater conformational flexibility. 

The smallest nonpolar residue of Ala does not show preference with respect to the inside or the surface of a protein. 

---

The following structure is a representation of histidine, an amino acid constituent of proteins. Only the connections between atoms are shown multiple bonds are not indicated. Give the molecular formula of histidine, and complete the structure by showing where the multiple bonds and lone pairs are (red = O, gray = C, blue = N, ivory = H). 

Table 1 Common Amino Acid Constituents of Protein... 

The first experiments on the acid hydrolysis of proteins were performed by Braconnot in 1820. Over the next 100 years studies by other workers with a variety of hydrolytic agents led to major advances in our knowledge of proteins, including the identification of the amino acid constituents of proteins and the development of the polypeptide concept of protein structure. These studies are now, for the most part, of historical interest, and a detailed insight into protein hydrolysis has come only in the past 20 to 30 years. This is largely the result of three significant developments ... 

In peptide synthesis, only alanine and leucine out of the 20 amino acid constituents of proteins are free from characteristic side reactions, although not from racemization. In order to synthesize optically active peptides, it is important to understand the mechanism of racemization during peptide synthesis.0 1... 

Aspartic acid o- spar-tik- [ISV, irreg. fr. L asparagus] (1863) n COOHCH2CH(NH2) COOH. Amino acid constituent of proteins of both animal and vegetable origin. In particular it is present in casein. It is a dibasic acid. 

Carboxylic acids and primary amines react to form carboxylic acid amides (R-NH-CO-R ). The amino acid constituents of peptides and proteins are linked by carboxylic acid amide bonds, which are therefore also known as peptide bonds (see p. 66). 

The sequence and properties of the amino acid constituents determine protein structure, reactivity, and function. Each amino acid is composed of an amino group... 

Although proteins and amino acids in the food influence the metabolism of the fish, they do not alter the amino acid composition of proteins in the body. In contrast, the lipids in the body of the fish are greatly influenced by the dietary lipids. In particular, it is the triacyl-glycerols (the main constituents of reserve energy) which are influenced by diet (Lovem, 1937,1942,1964 Kelly et al., 1958 Brockerhoff et al., 1963, 1964 Ackman, 1964, 1967 Ananyev, 1965 Ackman and Eaton, 1966, 1976). Food lipids also influence the structural lipids of the fish. 

The coenzyme and the substrate are held in the correct orientation for reaction by no less than ten hydrogen bonds to the protein structure. The three-letter codes are those conventionally used for the amino acid constituents of the proteins and the subscript numbers indicate the position of that specific amino acid in the total amino acid sequence of the protein backbone. The range of numbers quoted and the gaps between them show how the active site is formed from amino acids from all along the peptide chain and that they are brought together by the folding in the tertiary structure of the protein. 

By combining the K-B theory with the RISM theory, we have derived the equation for calculating the partial molar volume of a polyatomic solute in solvent. We have calculated the VM-values of the 20 amino acids, constituents of natural proteins. The calculated values are always smaller than the corresponding experimental values. Moreover, the discrepancy becomes larger as the number of the atoms in the amino-... 

Table 2. Some amino acid constituents of microbial peptides (amino acids not found in proteins)...

The families of alkanes, amides and carboxylic acids contain the constituents which comprise most of the amino acid residues of protein chains, and indeed the major motivation for deriving a consistent force field for these families was its potential application for biopolymers. However, a comprehensive consistent force field for biological molecules should include more groups e.g. aromatic and heterocyclic rings, carbohydrate, phosphate and sulfur groups it should also extend considerably the consistent analysis of intramolecular energy parameters pertaining to polypeptide and polynucleotide chains. This could be done fully only if more experimental data could be accumulated to supply the specific needs of the consistent force field analysis. 

As constituents of proteins the amino-acids are important constituents of the food of animals. Certain amino-acids can be made in the body from ammonia and non-nitrogenous sources others can be made from other amino-acids, e.g. tyrosine from phenylalanine and cystine from methionine, but many are essential ingredients of the diet. The list of essential amino-acids depends partly on the species. See also peptides and proteins. 

Amino acids are the main components of proteins. Approximately twenty amino acids are common constituents of proteins (1) and are called protein amino acids, or primary protein amino acids because they are found in proteins as they emerge from the ribosome in the translation process of protein synthesis (2), or natural amino acids. In 1820 the simplest amino acid, glycine, was isolated from gelatin (3) the most recendy isolated, of nutritional importance, is L-threonine which was found (4) in 1935 to be a growth factor of rats. The history of the discoveries of the amino acids has been reviewed... 

Although all the chiial amino acids obtained from proteins have the l configuration at then a carbon, that should not be taken to mean that D-amino acids are unknown. In fact, quite a number of D-anino acids occur naturally. D-Alanine, for example, is a constituent of bacterial cell walls and D-seiine occurs in brain tissue. The point is that D-fflnino acids are not constituents of proteins. 

The predominance of L-amino acids in biological systems is one of life s most intriguing features. Prebiotic syntheses of amino acids would be expected to produce equal amounts of L- and D-enantiomers. Some kind of enantiomeric selection process must have intervened to select L-amino acids over their D-connterparts as the constituents of proteins. Was it random chance that chose L- over D-isomers ... 

Proteins are biopolymers of some 22 different amino acids. Because of the variation in physical-chemical properties, mainly polarity and electrical charge, between the constituent amino acids, protein molecules are am-pholytic (i.e., containing positively and negatively charged groups) and more or less amphiphilic (i.e. comprising polar and apolar domains). These properties, in turn, lead to the formation of complex three-dimensional (3D) structures. 

---