13.6 Introduction

Coenzymes are organic molecules commonly derived from vitamins. Vitamin is a substance that body can not normally synthesize, but they are required in small amount for normal body function. In this lecture we will cover niacin, vitamin B₁, vitamin B₂, vitamin B₆, vitamin B₁₂, vitamin H and vitamin K. For the details of the vitamin A, D and E, please see the lecture 28. 

13.7 Niacin

Nicotinamide adenine dinucleotide (NAD⁺) is a coenzyme that is used by enzyme as oxidizing agent for biological oxidations. Since niacin is the portion of coenzyme NAD⁺, it is to be added in the diet so that the body can synthesize NAD⁺ (Figure 1). Figure 2 illustrates an example for the enzyme catalyzed oxidation/reduction reactions using NAD⁺/NADH.

Figure 1

Figure 2

13.8 Vitamin B₂

Riboflavin (flavin plus ribitol) is known as vitamin B₂. It is required to synthesize coenzyme flavin adenine dinucleotide (FAD) which is used as oxidizing agent in enzyme catalyzed oxidation reactions. For example, FAD is used by succinate dehydrogenase to oxidize succinate to fumarate (Scheme 1).

Scheme 1

13.9 Vitamin B₁

Thiamine is known as vitamin B₁. It is required to form the coenzyme thiamine pyrophosphate (TPP). TPP is required by enzymes that catalyze the transfer of a two carbon fragment from one species to another. For example, pyruvate decarboxylase enzyme requires TPP for the decarboxylation of pyruvate and transfer the remaining two-carbon fragment to a proton to afford acetaldehyde (Scheme 2).

Scheme 2