Posttranslational Modification

Some amino acid residues may become chemically modified after they have been incorporated into polypeptide chains. They will thus be present when the protein is degraded but cannot be reutilized for protein synthesis.

Hydroxylation of proline to hydroxyproline is mainly associated with collagen. Hydroxylysine is also found in collagen.

The side chain nitrogen atoms of the dibasic amino acids (histidine, arginine, and lysine) can all be methylated. For example, NT-methylhistidine (3-methylhistidine) is found mainly in the contractile proteins actin and myosin so that detection of NT-methylhistidine in a food sample usually indicates the presence of meat. It has also been suggested that measurement of the urinary excretion of NT-methyl-histidine could provide an index of the rate of breakdown of myofibrillar proteins in skeletal muscle, although interpretation is complicated by the presence of N^-methylhistidine derived from other tissues.

The hydroxyl groups of serine, threonine, and tyrosine can all be phosphorylated. Phosphoserine residues bind calcium and are found in proteins such as casein. Another calcium-binding residue is 7-carboxyglutamic acid, which is found in prothrombin.

The hydroxyl groups of serine can also be glycosy-lated to form glycoproteins and proteoglycans. The amide group of asparagine can also be glycosylated.

The e-N of certain lysine residues can be oxidized by the copper-containing enzyme lysyl oxidase to form allysine. Four allysine residues in adjacent polypeptide chains may then condense to form desmosine (Figure 4). This covalent link gives considerable strength and elasticity to the connective tissue protein elastin.

The e-N of lysine residues is also susceptible to chemical reactions within food systems. It undergoes the Maillard reaction with carbonyl groups of carbohydrates to form a series of brown and slightly bitter products. This is an integral part of the baking process when producing bread, cakes, and biscuits, although there is evidence that large quantities of some Maillard products may be toxic or carcinogenic. On the other hand, since the lysine in Maillard products is not biologically available when the food is ingested, this can seriously reduce the protein quality of heat-treated animal feedstuffs.

Lysine residue

Allysine residue

H —N C

Desmosine

Figure 4 Formation of allysine and structure of desmosine.

Proteins within living systems can also be damaged by covalent binding to other molecules (usually reactive biochemicals) to form adducts, thereby rendering the protein inoperative or immunogenic. Adducts can be formed by the reaction of an aldehyde function with a receptive nucleophilic centre in the protein, particularly the e-amino groups on lysine residues but also the a-amino terminus, the thiol groups on cysteine residues, the imidazole groups on histidine residues, and the phenolic groups on tyrosine residues. The aldehydes that may be involved in adduct formation include malondialdehyde and 4-hydroxy-2-nonenal, which are produced by free radical damage to polyunsaturated fatty acids in cell membranes, and acetaldehyde, which is produced when alcohol is metabolized. Adduct formation may play a role in the pathological processes leading to diseases such as alcoholic cirrhosis and coronary heart disease.

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