Amino Acids

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AMINO ACID IMBALANCE The concept of amino acid balance is founded on a knowledge of the relationship between the amino acid composition of a protein and its biological value. A protein that provides amino acids in roughly the proportions in which they are required by the body is termed a balanced protein and has a high biological value : a protein that is low in one or more of the indispensable amino acids is termed an unbalanced protein and has a lower biological value. The more unbalanced a protein is, the lower the efficiency with which it is used and the greater the amount needed in a diet to satisfy the amino acid requirements (Block and Mitchell, '46-'47; Oser, '51; Almquist, '53; Mitchell, '54; Flodin, '53, '57). The term amino acid imbalance has arisen from studies in which adverse effects, beyond the expected fall in the efficiency of protein utilization, have been observed when the protein of a diet, usually one low in protein, has been thrown out of balance by the addition of amino acids or a quantity of an unbalanced protein. In order to reverse these adverse effects, such as retarded growth or an accumulation of liver fat, a supplement of the amino acid that is most limiting in the diet must be provided. Thus, an amino acid imbalance, There is even some difference of opinion about the use of the term amino acid imbalance (Harper, '58; Salmon, '58). For the present, an amino acid imbalance will be defined as any change in the proportions of the amino acids in a diet that result in an adverse effect which can be prevented by supplementing the diet with a relatively small amount of the most limiting amino acid or acids. This leaves open the question of whether there are different types of amino acid imbalances, i.e., whether imbalances caused by adding relatively small amounts of one or two amino acids to a diet (Hankes et al., '49; Deshpande et al., '58a) are identical with those produced by adding a relatively large quantity of a protein or of an amino acid mixture lacking a single amino acid (Salmon, '54; Deshpande et al., '55, '58a, b; Sauberlich, '56; Harper, '59). It excludes, however, those conditions described as antagonisms and toxicities (Harper, '58), in which adverse effects are caused by the addition of a fairly large excess of a single amino acid, and which are not known to be prevented by a relatively small supplement of the amino acid that is most limiting for growth. Similar to carbohydrates, proteins contain carbon (C), hydrogen (H), and oxygen (O). However, unlike carbohydrates (and lipids) proteins also contain nitrogen (N). Proteins are made up of smaller units called amino acids. This name amino acid signifies that each contains an amino (NH2) and carboxylic acid (COOH) group. The only structural difference in the 20 amino acids is the side group represented by the R below.

Fig 2.211 Structure of an amino acid To illustrate the differences in the side group we will consider glycine and alanine, the two simplest amino acids. For glycine the R group is hydrogen (H), while in alanine the R group is a methyl (CH3). The structures of these two amino acids are shown below.

Figure 2.212 Structure of glycine

Figure 2.213 Structure of alanine Individual amino acids are bonded together through a dehydration reaction (-H2O) forming a dipeptide (2 amino acids). This bond between amino acids is known as a peptide bond and is shown in the two figures below. The following video also shows the formation of a peptide bond.

Figure 2.214 Peptide bond formation

Figure 2.215 Peptide bond formation part 21 Check Yourself What are the three components of an amino acid? Which is structurally different among the amino acids? Check Yourself What are the three components of an amino acid? Which is structurally different among the amino acids? Amino acids can also come together to form tripeptides (three amino acids), oligopeptides (3-10 amino acids), and polypeptides (10 or more amino acids). A polypeptide is a chain of amino acids as shown below.

Figure 2.216 A polypeptide chain2 References & Links 1. http://en.wikipedia.org/wiki/File:Peptidformationball.svg 2. http://www.genome.gov/Pages/Hyperion/DIR/VIP/Glossary/Illustration /amino_acid.cfm?key=amino%20acids