amino acids

amino acids are the building blocks of proteins. They are so named because all have a basic amino group (-NH₂) and an acidic carboxyl group (-COOH). Peptides, polypeptides, and proteins are formed from strings of amino acids joined together by the formation of peptide bonds. All proteins are formed from combinations of only 20 different amino acids, whether the proteins derive from bacteria or from man.

Amino acids are described as essential or non-essential. The non-essential ones can be synthesized in the body but the essential amino acids are those which must be present in the diet (phenylalanine, valine, tryptophan, threonine, lysine, leucine, isoleucine, and methionine). If any one of these amino acids is missing from the diet then many proteins which include this essential component cannot be synthesized. Consequently many other amino acids cannot then be used; they are broken down (deaminated) and the nitrogen is excreted as urea and creatinine, leading to a negative nitrogen balance, as more nitrogen is excreted than is taken in as dietary protein.

The adult body cannot absorb whole proteins from the gut, although young babies are able to absorb antibodies, which are proteins, from mother's milk; this provides passive immunity for the first year or so of life. The digestive processes break down dietary protein to amino acids and small peptides (two or more linked amino acids). Carriers, specific for a single amino acid or a group of similar amino acids, are present in the cells lining the intestine and are responsible for the specific uptake into these cells. Some dipeptides (and maybe tripeptides) also have specialized carrier molecules for uptake in the intestine, and the final stage of their digestion to amino acids takes place in these epithelial cells themselves. Thence they move into the circulating blood; thus amino acids from the diet enter the body's amino acid pool, mixing with other amino acids derived from the breakdown of body proteins in the continual turnover associated with growth, repair, and renewal of tissues. Cells of the different tissues take up selectively from the blood whichever amino acids they need for synthesis of their own proteins. The circulating amino acids gained from digestion are in no great danger of excretion via the kidneys: they are filtered at the glomeruli but are mostly reabsorbed into the blood as they pass down the kidney tubules.

Finally, how is the dietary intake of protein linked to the need for amino acids, particularly the essential ones? The linkage need not be a strong one, as connections exist between the metabolism of amino acids and the metabolism of fats and carbohydrates. Further, there can be conversion of one amino acid to another, at least for the non-essential amino acids. These transamination reactions are common in tissues that have been damaged, as repair and resynthesis take place. Thus after a myocardial infarction the level of the relevant enzymes — transaminases — rises in the blood, and this measurement is used for diagnostic purposes. Excess amino acids are subject to oxidative deamination: the amino group is removed and excreted as nitrogen products and the residue converted either to a ketone body, called acetoacetic acid (one of the products also of fat metabolism), or to products readily converted to glucose. Amino acids are there-fore divided into ketogenic or gluconeogenic (conversion to glucose) types.

Nitrogen losses in the urine may be greater than the nitrogen intake in the diet (negative nitrogen balance) not only when the essential amino acids are missing, but also when the calorie intake is adequate but the overall protein content of the diet is too low; this occurs in kwashiorkor, common in poorly nourished children. If the diet is inadequate in calories as well as deficient in protein, body proteins are broken down to form glucose for energy. This can be prevented by giving glucose, which is thus said to be ‘protein-sparing’.

Alan W. Cuthbert

See also peptides; proteins.