Protein is the most abundant nitrogen-containing compound in the diet and in the body. It is one of the five classes of complex biomolecules present in cells and tissues, the others being DNA, RNA, polysaccha-rides, and lipids. The polymerization of L-α-amino acids through synthesis of peptide bonds contributes to the formation and structural framework of pro-teins. These may contain two or more polypeptide chains forming multimeric proteins, with the indi-vidual chains being termed subunits. Proteins are the workhorses in cells and organs and their building blocks are the amino acids, which are joined together according to a sequence directed by the base sequence of the DNA (the genome), and so they serve as the currency of protein nutrition and metabolism. The Human Genome Project completed in 2000 revealed that the human genome consists of only 30 000 genes, whereas there may be hundreds of thousands of pro-teins that are responsible for giving a human its par-ticular characteristics and uniqueness. A new field of nutrition research has now opened up and is referred to as “nutrigenomics,” which is the study of how nutrition and genomics interact to influence health. Proteins and amino acids fulfill numerous functions, many of which are summarized in Table 4.1. Some
amino acids, such as glutamine (Tables 4.2 and 4.3), play multiple roles. It is not surprising, therefore, that inappropriate intakes of proteins and/or of specific amino acids can have important consequences for tissue and organ function, and the maintenance of health and the well-being of the individual.