Amino acids as metabolic precursors
|Phenylalanine metabolism|| -Phenylalanine converted to tyrosine by phenylalanine hydroxylase.|
-Tyrosine converted to fumarate and acetoacetate through a number of steps.
|Alkaptonuria||Defect in enzyme involved in tyrosine metabolism leads to build up of homogentisic acid, which is excreted in urine and becomes black when oxidized.|
|Why does PKU result in blue eyes and fair skin?||Tyrosine is converted by tyrosinase into pathway for melanin production by melanocytes in epidermis. PKU individuals are completely dependent on dietary tyrosine, since they can't make it from phenylalanine.|
|Thyroid hormone production||1. Follicular cells of thyroid stimulated by TSH to make thyroglobulin, which goes to the colloid. |
2. In colloid, tyrosine residues of thyroglobulin are iodinated and form mono- and diiodotyrosine, precursors of triiodothyronine and thyroxine (thyroid hormones).
3. Mature thyroglobulin molecules are endocytosed back into follicular cells, where the thyroid hormones are cleaved off and enter circulation.
|Catecholamine synthesis|| 1. Tyrosine is converted to dopa by tyrosine hydroxylase.|
2. Dopa is converted to dopamine by dopa decarboxylase, which requires pyridoxal phosphate (derivative of B6).
3. Dopamine is converted to norepinephrine.
4. In the adrenal medulla, phenylethanolamine-N-methyltransferase converts NE to epinephrine.
|Metabolic roles of tryptophan|| -Conversion of tryptophan to niacin (B3), requires B6.|
-Conversionto kynurinine metabolites, involved in immune regulation.
-Conversion to serotonin, requires pyridoxal phosphate (from B6).
|Production and action of NO|| -Produced by oxidation of nitrogen on arginine by NO synthase, producing NO and citrulline.|
-NO stimulates guanylate cyclase to convert GTP to cGMP, which stimulates PKG and leads to smooth muscle relaxation.