7 terms

Amino acids as metabolic precursors

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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.