We now address polymers of amino acids, peptides and proteins. Biologically occurring polypeptides home in size from small to very large, consisting of two or three to thousands of linked aminoalkanoic acid residues. Our focus is on the elemental chemical properties of those polymers. Peptides Are Chains of Amino Acids Two aminoalkanoic acid molecules are often covalently joined through a substituted amide linkage, termed a peptide linkage, to yield a dipeptide. Such a linkage is made by the removal of the weather of water (dehydration) from the -carboxyl group of 1 aminoalkanoic acid and therefore the -amino group of another. Peptide linkage formation is an example of a condensation reaction, a standard class of reactions in living cells. Under standard biochemical conditions, the equilibrium for the reaction favours the amino acids over the dipeptide. to form the reaction thermodynamically more favourable, the carboxyl must be chemically modified or activated in order that the hydroxyl is often more readily eliminated.
Three amino acids are often joined by two peptide bonds to make a tripeptide; similarly, four amino acids are often linked to making a tetrapeptide, five to make a pentapeptide, then forth. When a couple of amino acids are joined during this fashion, the structure is named an oligopeptide. When many amino acids are joined, the merchandise has named a polypeptide. Proteins may have thousands of aminoalkanoic acid residues. Although the terms “protein” and “polypeptide” are sometimes used interchangeably, molecules mentioned as polypeptides generally have molecular weights below 10,000, and people called proteins have higher molecular weights.
Although hydrolysis of a peptide linkage is an exergonic reaction, it occurs only slowly because it's the high energy of activation (p. 27). As a result, the peptide bonds in proteins are quite stable, with a mean half-life (t1/2) of about 7 years under most intracellular conditions.
Peptides are often Distinguished by Their Ionization Behavior
Peptides contain just one free-amino group and one free-carboxyl group, at opposite ends of the chain. These groups ionize as they are doing in free amino acids, although the ionization constants are different because an oppositely charged group is not any longer linked to the carbon. The -amino and -carboxyl groups of all nonterminal amino acids are covalently joined within the peptide bonds, which don't ionize and thus don't contribute to the entire acid-base behaviour of peptides. However, the R groups of some amino acids can ionize, and during a peptide, these contribute to the general acid-base properties of the molecule. Thus the acid-base behaviour of a peptide is often predicted from its free -amino and -carboxyl groups combined with the character and number of its ionizable R groups.
No generalizations are often made about the molecular weights of biologically active peptides and proteins in reference to their functions. Present peptides home in length from two to several thousands of aminoalkanoic acid residues. Even the littlest peptides can have biologically important effects. Consider the commercially synthesized dipeptide L-aspartyl-L-phenylalanine Many small peptides exerts their effects at very low concentrations. For instance, a variety of vertebrate hormones are small peptides. These include oxytocin (nine aminoalkanoic acid residues), which is secreted by the posterior pituitary gland and stimulates uterine contractions, and thyrotropin-releasing hormone (three residues), which is made within the hypothalamus and stimulates the discharge of another hormone, thyrotropin, from the anterior pituitary gland. Some extremely toxic mushroom poisons, like amanitin, also are small peptides, as are many antibiotics.
Biologically Active Peptides and Polypeptides Occur during a Vast Range of Sizes and Compositions
Human cytochrome has 104 aminoalkanoic acid residues linked during a single chain; bovine chymotrypsinogen has 245 residues. At the acute is titin, a constituent of vertebrate muscle, which has nearly 27,000 aminoalkanoic acid residues and a relative molecular mass of about 3,000,000. The overwhelming majority of present proteins are much smaller than this, containing fewer than 2,000 aminoalkanoic acid residues. Some proteins contain one polypeptide chain, but others, called multi-subunit proteins, have two or more polypeptides associated noncovalently.
The individual polypeptide chains during a multi-subunit protein could also be identical or different. If a minimum of two is identical the protein is claimed to be oligomeric, and therefore the identical units (consisting of 1 or more polypeptide chains) are mentioned as protomers. Haemoglobin, for instance, has four polypeptide subunits: two identical chains and two identical chains, all four held together by noncovalent interactions. Each subunit is paired in a uniform way with a subunit within the structure of this multisubunit protein, in order that haemoglobin is often considered either a tetramer of 4 polypeptide subunits or a dimer of protomers.
A few proteins contain two or more polypeptide chains linked covalently. For instance, the 2 polypeptide chains of insulin are linked by disulfide bonds. In such cases, the individual polypeptides aren't considered subunits but are commonly mentioned simply as chains. The aminoalkanoic acid composition of proteins is additionally highly variable. The 20 common amino acids almost never occur in equal amounts during a protein. Some amino acids may occur just one occasion or not in the least during a given sort of protein; others may occur in large numbers. the aminoalkanoic acid composition of bovine cytochrome and chymotrypsinogen, the inactive precursor of the digestive enzyme chymotrypsin. These two proteins, with very different functions, also differ significantly within the relative numbers of every quite aminoalkanoic acid residue.
We can calculate the approximate number of aminoalkanoic acid residues during a protein containing no other chemical constituents by dividing its relative molecular mass by 110. Although the typical relative molecular mass of the 20 common amino acids is about 138, the smaller amino acids predominate in most proteins. If we take under consideration the proportions during which the varied amino alkanoic acids occur in a mean protein ( the averages are determined by surveying the amino acid compositions of quite 1,000 different proteins), the typical relative molecular mass of protein amino acids is nearer to 128. Because a molecule of water (Mr 18) is removed to make each peptide linkage, the typical relative molecular mass of an aminoalkanoic acid residue during a protein is about 128 - 18 =110.
References :
1. Principles of biochemistry by David L. Nelson and Michael M. Cox.
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