Food Chemistry and Fish in Nutrition ( 2+1)

6.5.3. Anaerobic dehydrogenases

(a) They catalyze the removal of hydrogen from a substrate but not able to use oxygen as hydrogen acceptor.

(b) They transfer hydrogen from one substrate to another by oxidation-reduction reaction not involving a respiratory chain.

(c) They perform oxidation of metabolite utilizing several components of a respiratory chain

(i) Dehydrogenase dependent on Nicotinamide Coenzymes:

(a) They are linked as coenzymes either to NAD (Nicotinamide adenine dinucleotide) or to NADP (Nicotinamide adenine dinucleotide phosphate).

(b) The coenzymes are reduced by the particular substrate of the dehydrogenase and reoxidized by a suitable electron acceptor and synthesized from the vitamin niacin (nicotinic acid and nicotinamide).

(c) NAD-linked dehydrogenases catalyze oxidoreduction reactions in glycolysis, the citric acid cycle and in the respiratory chain of mitochondria.

(d) NADP-linked dehydrogenases are found in fatty acid and steroid synthesis in the extramitochondria. They are also found in hexose monophosphate shunt.

(e) Some nicotinamide coenzyme-dependent dehydrogenases contain zinc, particularly alcohol dehydrogenases from liver and glyceraldehydes-3-phosphate dehydrogenase from skeletal muscle. The zinc ions do not take part in the oxidation and reduction.

(ii) Dehydrogenases dependent on Riboflavin Prosthetic Groups:

(a) Most of the riboflavin-linked anaerobic dehydrogenases are concerned with electron transport in the respiratory chain.

(b) Succinate dehydrogenase, acyl-CoA dehydrogenase and mitochondrial glycerol-3-phosphate dehydrogenase transfer electrons directly from the substrate to the respiratory chain.

(c) In the dehydrogenation of reduced lipoate, and intermediate in the oxidative decarboxylation of pyruvate and a -Ketoglutarate, the flavoprotein(FAD) due to the low redox potential acts as a carrier of electrons from reduced lipoate of NAD+. The electron transferring flavoprotein in an intermediary carrier of electrons between acyl-CoA dehydrogenase and the respiratory chain.

(iii) The Cytochromes:

(a) The cytochromes excepting cytochrome oxidase are anaerobic dehydrogenases. They are involved as carriers of electrons from flavoproteins to cytochrome oxidase in the respiratory chain.

(b) They are iron-containing hemoproteins in which iron becomes Fe+++ and Fe++ oxidation and reduction. The cytochromes in the respiratory chain are b,c₁, c, a and a₃.

(c) Cytochromes are also found in the endoplasmic reticulum (cytochromes P-450 and b₅) plant cells, bacteria and yeast.