Medical brain notes 38

  Nutrition and Metabolism of Proteins and Amino Acids Introduction   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...

  A historical perspective The early history of protein metabolism and nutrition is closely tied to the discovery of nitrogen and its distribution in nature. The reason for this is that pro-teins, on average, contain about 16% nitrogen by weight (to convert nitrogen to protein it is necessary to multiply by 6.25). Daniel Rutherford, in Edinburgh, can be regarded as the discoverer of nitrogen, which he called “phlogisticated air” in his Doctorate in  Medicine thesis in 1792.   The first amino acid to be discovered was cystine, which was extracted from a urinary calculus by Wallaston in England in 1810. It was not until 1935 that threonine, the last of the so-called nutritionally indispensable (essential) amino acids for mammals, including man, was discovered by WC Rose at the University of Illinois. Finally, the term “protein” was invented by the Swedish chemist Jons Jakob Berzelius (1779–1848) and this was later accepted and promoted by the influential Dutch chemist Gerha...

  Structure and chemistry of amino acids With the exception of proline, the amino acids that make up peptides and proteins have the same central structure (Figure 4.1; the A in this figure and  Figure 4.1  Structures of some of the nutritionally important amino acids. All are components of proteins and they are coded by DNA.  (a) Nutritionally indispensable (essential) includes also tryptophan and histidine; (b) nutritionally conditionally indispensable; (c) nutritionally dispensable. The carboxylic acid and amino nitrogen groups are the components of the peptide bond that links the amino acids within the linear peptide structure, while the side-chains distinguish the physical and chemical properties of each chemical class of amino acid. In addition, some features of the amino acid side-chains are critical to the metabolic and physiological roles of free, as opposed to protein-bound, amino acids (Table 4.3; Figures 4.1 and 4.2). These roles are reflections of Figure ...

  Classification of amino acids “Indispensability” as a basis of classification For most of the past 65 years amino acids have been divided into two general, nutritional categories: indis-pensable (essential) and dispensable (nonessential). This categorization provided a convenient and gener-ally useful way of viewing amino acid nutrition at the time. The original definition of an indispensable amino acid was:   One which cannot be synthesized by the animal organism out of materials  ordinarily available  to the cells  at a speed  commensurate with the demands for  normal growth . There are three important phrases in this definition:  ordinarily available ,  at a speed  and for  normal growth .   The phrase “ordinarily available” is an important qualifier within this definition because a number of nutritionally essential amino acids, for example the branched-chain amino acids, phenylalanine and methionine, can be synthesized by...

  Biology of protein and amino acid requirements Body protein mass A major and fundamental quantitative function of the dietary  α -amino acid nitrogen and of the indis-pensable amino acids is to furnish substrate required for the support of organ protein synthesis and the maintenance of cell and organ protein content. Therefore, in the first instance the body protein mass is a factor that will influence the total daily require-ment for protein. Adult individuals of differing size but who are otherwise similar in age, body composi-tion, gender, and physiological state would be expected to require proportionately differing amounts of nitrogen and indispensable amino acids. Changes in the distribution and amount of body protein that occur during growth and development and later on during aging may be considered, therefore, as an initial approach for understanding the metabolic basis of the dietary protein and amino acid needs. Direct measures of total body protein cannot yet be ...