Amino Acids

Amino acids:


    • The genetic code specifies 20L amino acids which are the building blocks of protein. There are 20 alpha-amino acids that combine to make 50,000 to 100,000 different proteins in the body.
    • Organic compounds that can contain an amino carboxyl (-COOH) and a (-NH2) group.
    • There are both one-and three-letter abbreviations for each amino acid that can be used to represent the amino acids in peptides. Some proteins contain additional amino acids that arise by modification of an amino acid already present in a peptide.
    • An example is the conversion of peptidyl proline and lysine to 4-hydroxyproline and 5-hydroxylysine; converting peptidyl glutamate to γ-carboxyglutamate; and the methylation, formylation, acetylation, prenylation, and phosphorylation of certain aminoacyl residues.
    • By altering protein solubility, stability, and interaction with other proteins it creates biological diversity.
    • Both D-amino acids and non-α-amino acids occur in nature, L--Amino Acids only Occur in Proteins with the exception of glycine, the α-carbon of amino acids is chiral.
    • All amino acids possess at least two weakly acidic functional groups, RNH3 + and RCOOH. Many also possess additional weakly acidic functional groups such as OH, SH, guanidino, or imidazole groups.
    • The pKa values of all functional groups of an amino acid dictate its net charge at a given pH. pI is the pH at which an amino acid bears no net charge and thus does not move in a direct current electrical field.
    • Of the biochemical reactions of amino acids, the most important is the formation of peptide bonds.
    • The R groups of amino acids determine their unique biochemical functions. Amino acids are classified as basic, acidic, aromatic, aliphatic, or sulfur-containing based on the properties of their R groups.
    • Peptides are named for the number of amino acid residues present and as derivatives of the carboxyl-terminal residue. The primary structure of a peptide is its amino acid sequence, starting from the amino-terminal residue.
    • The partial double-bond character of the bond that links the carbonyl carbon and the nitrogen of a peptide renders four atoms of the peptide bond coplanar and restricts the number of possible peptide conformations.
    • All vertebrates can form certain amino acids from amphibolic intermediates or from other dietary amino acids. The intermediates and the amino acids to which they give rise are α-ketoglutarate (Glu, Gln, Pro, Hyp), oxaloacetate (Asp, Asn) and 3-phosphoglycerate (Ser, Gly).
    • Cysteine, tyrosine, and hydroxylysine are formed from nutritionally essential amino acids. Serine provides the carbon skeleton and homocysteine the sulfur for cysteine biosynthesis. Phenylalanine hydroxylase converts phenylalanine to tyrosine.
    • Neither dietary hydroxyproline nor hydroxylysine is incorporated into proteins because no codon or tRNA dictates their insertion into peptides.
    • Peptidyl hydroxyproline and hydroxylysine are formed by hydroxylation of peptidyl proline or lysine in reactions catalyzed by mixed-function oxidases that require vitamin C as a cofactor. The nutritional disease scurvy reflects impaired hydroxylation due to a deficiency of vitamin C.
    • Selenocysteine, an essential active site residue in several mammalian enzymes, arises by co-translational insertion of a previously modified tRNA.
    • Human subjects degrade 1–2% of their body protein daily at rates that vary widely between proteins and with a physiologic state. Key regulatory enzymes often have short half-lives.
    • Proteins are degraded by both ATP-dependent and ATP-independent pathways. Ubiquitin targets many intracellular proteins for degradation.
    • Liver cell surface receptors bind and internalize circulating a sialoglycoprotein destined for lysosomal degradation.
    • Ammonia (NH3) is highly toxic. Fish excrete NH3 directly; birds convert NH3 to uric acid. Higher vertebrates convert NH3 to urea.
    • Transamination channels α-amino acid nitrogen into glutamate. L-Glutamate dehydrogenase (GDH) occupies a central position in nitrogen metabolism.
    • Glutamine synthase converts NH3 to nontoxic glutamine. Glutaminase releases NH3 for use in urea synthesis.
    • NH3, CO2, and the amide nitrogen of aspartate provide the atoms of urea.
    • Hepatic urea synthesis takes place in part in the mitochondrial matrix and in part in the cytosol.
    • Inborn errors of metabolism are associated with each reaction of the urea cycle.
    • Changes in enzyme levels and allosteric regulation of carbamoyl phosphate synthase by N-acetylglutamate regulate urea biosynthesis.
    • In addition to their roles in proteins and polypeptides, amino acids participate in a wide variety of additional biosynthetic processes.
    • Glycine participates in the biosynthesis of heme, purines, and creatine and is conjugated to bile acids and to the urinary metabolites of many drugs.
    • In addition to its roles in phospholipid and sphingosine biosynthesis, serine provides carbons 2 and 8 of purines and the methyl group of thymine.
    • S-Adenosylmethionine, the methyl group donor for many biosynthetic processes, also participates directly in spermine and spermidine biosynthesis.
    • Glutamate and ornithine form the neurotransmitter γ-aminobutyrate (GABA).
    • The thioethanolamine of coenzyme A and the taurine of taurocholic acid arise from cysteine.
    • Decarboxylation of histidine forms histamine and several dipeptides are derived from histidine and β-alanine.
    • Arginine serves as the formamidine donor for creatine biosynthesis, participates in polyamine biosynthesis, and provides the nitrogen of nitric oxide (NO).
    • Important tryptophan metabolites include serotonin, melanin, and melatonin.
    • Tyrosine forms both epinephrine and norepinephrine, and its iodination forms thyroid hormone.