17.0: Prelude to the Organic Chemistry of Vitamins In this final chapter, we focus on the organic chemistry of folate, along with three other coenzymes: pyridoxal phosphate, thiamine diphosphate, and lipoamide. Humans can synthesize lipoamide, but we depend on dietary sources for the other three: pyridoxal phosphate is a form of vitamin B6, and thiamine diphosphate is a form of vitamin B1. 17.1: Pyridoxal Phosphate (Vitamin B6) The coenzyme pyridoxal phosphate (commonly abbreviated PLP) is the active form of vitamin B6 , or pyridoxine. PLP is required for over 100 different reactions in human metabolism, primarily in the various amino acid biosynthetic and degradation pathways. The essential function of PLP is to act as an 'electron sink', stabilizing a negative formal charge that develops on key reaction intermediates. 17.2: Thiamine Diphosphate (Vitamin B1) Thiamine diphosphate (ThDP, sometimes also abbreviated TPP or ThPP) is a coenzyme which, like PLP, acts as an electron sink to stabilize key carbanion intermediates. The most important part of the ThDP molecule from a catalytic standpoint is its thiazole ring. 17.3: Thiamine Diphosphate, Lipoamide and the Pyruvate Dehydrogenase Reaction The enzyme pyruvate dehydrogenase is one of the most central of all the enzymes of central metabolism: by converting pyruvate to acetyl-CoA, it links glycolysis (where glucose is broken down into pyruvate) to the citric acid cycle, into which carbons enter in the form of acetyl-CoA. 17.4: Folate Folate, or vitamin B9 , is essential for a variety of important reactions in nucleotide and amino acid metabolism. The reactive part of folate is the pterin ring system, shown in red below. The conventional atom numbering system for folate is also indicated. 17.E: The Oganic Chemistry of Vitamins (Exercises) 17.S: The Oganic Chemistry of Vitamins (Summary)