Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 431	Biochemistry	Fall 2001
Lecture Notes:: 22 October	© R. Paselk 2001
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Introduction to Vitamins and Cofactors

Terms:

• Cofactor: a co-catalyst required for enzyme activity.
  • coenzyme - a dissociable cofactor.
  • prosthetic group - non-dissociable cofactor.
  • metal ion - a metal cofactor.
• Vitamin: a required micro-nutrient (organism cannot synthesize adequate quantities for normal health - may vary during life-cycle).
  • water soluble - not stored, generally no problem with overdose.
  • lipid soluble - stored, often toxic with overdose.

Let's start with a brief overview of the names, structures and physiological functions of the common vitamins as shown on the overhead: Niacin, Riboflavin [B₂], Thiamine, Pantothenic acid, Biotin, Pyridoxal [B₆], Folic acid, Lipoic acid, Cobalamin [B₁₂], L-Ascorbic acid [C], and the lipid soluble vitamins A, D, E, and K). Fun facts:

• Niacin deficiency gives rise to pellagra (swollen tongue, dermatitis, neurologic and gastro-intestinal disturbances).
• Thiamine deficiency gives rise to berberi (extensive damage to nervous and circulatory systems, muscle wasting and edema).
• Folate and Cobalamin deficiency both give rise to megaloblastic anemia, however cobalamin deficiency also results in irreversible CNS damage.
• In contrast to most water soluble vitamins, the lipid soluble vitamins can be taken in excess, with toxic results:
  • Excess vitamin D (actually a hormone) causes excess Ca²⁺ buildup with precipitation to form kidney stones and precipitates in tissues. Deficiency results in rickets.

Vitamins giving "ADP" containing coenzymes:

Niacin (nicotinamide):

NAD⁺: Note that we have the active group, nicotinamide, attached to a recognition group "ADP" (in bold). This is the major redox coenzyme in organisms. It participates in oxidation by picking up a Hydride ion on the number 3 carbon. The ADP portion seems to function with the adenine ring portion binding to the protein and holding the nicotinamide so that it is properly positioned for catalysis. This appears to be a common theme among coenzymes which we will see again. Substituting a phosphate for the 2' hydroxyl gives NADP⁺, the redox cofactor used in biosynthetic pathways.

Pantothenic acid:

Pantothenate serves as a "flexible-arm" between the "ADP" recognition/binding group and the active 2-mercaptoethylamine (acyl-carrier) group in Coenzyme A:

Riboflavin (Vitamin B₂):

is the base of the other major redox coenzymes FMN and FAD. Again in FAD we see an "ADP" recognition group.

Vitamins having "acyl" arms:

Two other vitamins have "acyl" arms which can link covalently to lysine (amide bond) to give prosthetic groups on flexible arms, allowing the active portion to swing between active sites:

Lipoic acid:

In lipoate the coenzyme functions in redox with the S-S bond reduced/oxidized in the reaction.

Biotin:

INTRODUCTION TO METABOLISM

Catabolism: degradation of molecules to provide energy.

Anabolism: reactions using energy to synthesize new molecules for growth etc.

Pathway Diagrams

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Last modified 22 October 2001