Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 431	Biochemistry	Fall 2008
Lecture Notes: 31 October	© R. Paselk 2008
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Introduction to Vitamins and Cofactors, cont.

• Vitamin C
• 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.
• Excess vitamin A can cause exfoliation.

Note the relationships of the various cofactors to their vitamin precursors.

Nucleotide Functions: Most involve use of the nucleotide as a recognition molecule, e.g. ATP (text Figure 1-25)

Nicotinamide Adenine Dinucleotide (NAD⁺) (text Figure 8-41-2) uses ADP (bolded) as a recognition "handle." (Note the two nitrogenous bases each attached to a ribose and linked through a phosphoric acid anhydride linkage:

Similarly adenosine with a modified ribose (reduced to the alcohol - ribitol) is used in Flavin Adenine Dinucleotide = FAD (text Figure 8-41-3; not truly a dinucleotide since ribitol instead of ribose!):

Coenzyme A (text Figure 8-41-1) has an ADP attached to an arm of pantothenic acid, which in turn is attached to beta-mercaptoethylamine. Acetyl groups can be carried on the sulfhydryl group:

To summarize, all three of these cofactors have ADP recognition portions which enzymes etc. can bind. (text Figure 8-41)

Three vitamins give cofactors with long "arms" which enable the cofactors to shift an attached substrate between adjacent active sites on a single enzyme. (Note attachment of biotin and lipoate to lysine side-chain to give 10 atom arms.)

Introduction to Metabolism

Catabolism: degradation of molecules to provide energy.

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

Metabolic Pathways

(overhead - Interactions of Metabolic Pathways) sequences of consecutive enzyme catalysed reactions which are readily studied and traced. A more rational definition is that of Newsholme and Leach (Biochemistry for the Medical Sciences, Wiley, 1983: pg.42)

"[A] metabolic pathway is a series of enzyme-catalyzed reactions, initiated by a flux-generating step and ending with either the loss of products to the environment, to a stored product (a metabolic 'sink') or in a reaction that precedes another flux-generating step (that is, the beginning of the next pathway)." Where a flux generating step is a non-equilibrium reaction that generates the flux going through the pathway and to whose rate all other reactions of the pathway conform. Note that by this definition some pathways may be inter-organ while others may take place in single compartment. We will explore this definition/concept as we look at metabolism.

Pathway Diagrams

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Last modified 29 October 2008