Chapter 6: Amino Acids in Proteins

The chapter details the universal amino acid backbone consisting of an alpha-carbon bonded to an amino group, carboxyl group, hydrogen atom, and distinctive side chain, with emphasis on zwitterionic behavior at physiological pH and the role of pKa values in determining ionization states and chemical reactivity. Amino acids are systematically classified by their side chain properties: nonpolar hydrophobic residues including alanine, valine, leucine, isoleucine, and methionine; aromatic amino acids such as phenylalanine, tyrosine, and tryptophan; polar uncharged varieties like serine, threonine, asparagine, and glutamine; charged acidic residues including aspartate and glutamate; and positively charged basic amino acids such as lysine, arginine, and histidine. The hydropathic index quantifies hydrophobic character and predicts how amino acids distribute within protein structures, influencing solubility, hydrogen bonding capacity, ionic interactions, and disulfide bond formation between cysteine residues. Clinical applications demonstrate how amino acid substitutions compromise protein function, exemplified by sickle cell anemia where valine replaces glutamate in beta-globin, causing polymerization and vaso-occlusive crises, and cystinuria arising from defective renal transport of cystine and basic amino acids, leading to kidney stone formation. The chapter explores protein diversity through isoforms, isozymes, and polymorphisms, including hemoglobin variants and developmental switches between fetal and adult forms. Posttranslational modifications including phosphorylation, glycosylation, acetylation, hydroxylation, prenylation, and carboxylation regulate protein activity, localization, and stability, with clinical examples spanning histone modification, collagen cross-linking, and bacterial toxin mechanisms. Diagnostic applications highlight cardiac troponin and creatine kinase as myocardial infarction markers, while recombinant insulin analogs such as lispro represent advances in diabetes management by eliminating immunogenicity associated with animal-derived insulin preparations.