Dehydrogenases Cannot Use Oxygen As A Hydrogen Acceptor

There are a large number of enzymes in this class. They perform two main functions:

(1) Transfer of hydrogen from one substrate to another in a coupled oxidation-reduction reaction (Figure 11-3). These dehydrogenases are specific for their substrates but often utilize common coenzymes or hydrogen carriers, eg, NAD+. Since the reactions are re-

/2O2

/2O2

H2O2

H2O2

Figure 11-1. Oxidation of a metabolite catalyzed by an oxidase (A) forming H2O, (B) forming H2O2.

versible, these properties enable reducing equivalents to be freely transferred within the cell. This type of reaction, which enables one substrate to be oxidized at the expense of another, is particularly useful in enabling ox-idative processes to occur in the absence of oxygen, such as during the anaerobic phase of glycolysis (Figure 17-2).

(2) As components in the respiratory chain of electron transport from substrate to oxygen (Figure 12-3).

Many Dehydrogenases Depend on Nicotinamide Coenzymes

These dehydrogenases use nicotinamide adenine di-nucleotide (NAD+) or nicotinamide adenine dinu-cleotide phosphate (NADP+)—or both—and are formed in the body from the vitamin niacin (Chapter 45). The coenzymes are reduced by the specific substrate of the dehydrogenase and reoxidized by a suitable electron acceptor (Figure 11-4) .They may freely and reversibly dissociate from their respective apoenzymes.

Generally, NAD-linked dehydrogenases catalyze ox-idoreduction reactions in the oxidative pathways of metabolism, particularly in glycolysis, in the citric acid cycle, and in the respiratory chain of mitochondria. NADP-linked dehydrogenases are found characteristically in reductive syntheses, as in the extramitochon-drial pathway of fatty acid synthesis and steroid synthe-sis—and also in the pentose phosphate pathway.

Other Dehydrogenases Depend on Riboflavin

The flavin groups associated with these dehydrogenases are similar to FMN and FAD occurring in oxidases. They are generally more tightly bound to their apoen-zymes than are the nicotinamide coenzymes. Most of the riboflavin-linked dehydrogenases are concerned with electron transport in (or to) the respiratory chain (Chapter 12). NADH dehydrogenase acts as a carrier of electrons between NADH and the components of higher redox potential (Figure 12-3). Other dehydro-genases such as succinate dehydrogenase, acyl-CoA dehydrogenase, and mitochondrial glycerol-3-phos-phate dehydrogenase transfer reducing equivalents directly from the substrate to the respiratory chain (Figure 12-4). Another role of the flavin-dependent dehydrogenases is in the dehydrogenation (by dihy-drolipoyl dehydrogenase) of reduced lipoate, an intermediate in the oxidative decarboxylation of pyruvate and a-ketoglutarate (Figures 12-4 and 17-5). The electron-transferring flavoprotein is an intermediary carrier of electrons between acyl-CoA dehydrogenase and the respiratory chain (Figure 12-4).

Figure 11-2. Oxidoreduction of isoalloxazine ring in flavin nucleotides via a semi-quinone (free radical) intermediate (center).

Cytochromes May Also Be Regarded as Dehydrogenases

The cytochromes are iron-containing hemoproteins in which the iron atom oscillates between Fe3+ and Fe2+ during oxidation and reduction. Except for cytochrome oxidase (previously described), they are classified as dehydrogenases. In the respiratory chain, they are involved as carriers of electrons from flavoproteins on the one hand to cytochrome oxidase on the other (Figure 12-4). Several identifiable cytochromes occur in the respiratory chain, ie, cytochromes b, c1, c, a, and 1% (cytochrome oxidase). Cytochromes are also found in other locations, eg, the endoplasmic reticulum (cy-tochromes P450 and b5), and in plant cells, bacteria, and yeasts.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

Get My Free Ebook


Post a comment