Pyridoxal phosphate acts as coenzyme for the following except

Ans. b (Transketolase) (Ref Harper 28th/475;175)# Transketolase transfers the two-carbon unit comprising carbons 1 and 2 of a ketose onto the aldehyde carbon of an aldose sugar. It therefore effects the conversion of a ketose sugar into an aldose with two carbons less and an aldose sugar into a ketose with two carbons more. The reaction requires Mg2+ and thiamin diphosphate as coenzyme.# Both cystathionine b-synthase and cystathionase are pyridoxal phosphate (vitamin B6).Vitamin B6 Has Several Roles in Metabolism: Functions# Pyridoxal phosphate is a coenzyme for many enzymes involved in amino acid metabolism, especially transamination and decarboxylation. It is also the cofactor of glycogen phosphorylase, where the phosphate group is catalytically important. In addition, B6 is important in steroid hormone action. Pyridoxal phosphate removes the hormone receptor complex from DNA binding, terminating the action of the hormones. In vitamin B6 deficiency, there is increased sensitivity to the actions of low concentrations of estrogens, androgens, cortisol, and vitamin D.# Coenzyme in many reactions, including:- Decarboxylation and transamination of amino acids,- Deamination of hydroxyamino acids and cysteine,- Conversion of tryptophan to niacin, and- Metabolism of fatty acids.# Consequently, the vitamin B6 group is important in blood, CNS, and skin metabolism.# Vitamin B6 is important in erythropoiesis because pyridoxal phosphate is needed in the formation of delta- aminolevulinic acid synthase (ALA synthase), the rate-limiting step in heme biosynthesis.This first reaction in heme biosynthesis takes place in the mitochondrion.# Thiamine pyrophosphate (TPP) is a cofactor for several dehydrogenase enzyme reactions: Pyruvate dehydrogenase (links glycolysis to TCA cycle); a-ketoglutarate dehydrogenase (TCA cycle); Transketolase (HMP shunt); Branched- chain ketoacid dehydrogenase.# Wernicke-Korsakoff syndrome and beriberi. Seen in malnutrition and alcoholism (2deg to malnutrition and malabsorption). Impaired glucose breakdown?ATP depletion worsened by glucose infusion; highly aerobic tissues (e.g., brain, heart) are affected first. Diagnosis made by | in RBC transketolase activity following vitamin B1 administration.