MCAT Biochemistry Review

Chapter 12: Bioenergetics and Regulation of Metabolism

12.3 Biological Oxidation and Reduction

Many key enzymes in ATP synthesis and other biochemical pathways have oxidoreductase activity.

HALF-REACTIONS

Just as you practiced with general chemistry, an important skill in biochemistry is to be able to divide oxidation–reduction reactions into their half-reaction components to determine the number of electrons being transferred. For example, in lactic acid fermentation, pyruvate and NADH are converted to lactate and NAD+ by lactate dehydrogenase. This reaction can be broken down into half-reactions as follows:

Remember that spontaneous oxidation–reduction reactions have a negative value of ΔG and a positive value of E (electromotive force).

BRIDGE

Oxidation–reduction reactions, discussed in Chapter 11 of MCAT General Chemistry Review and Chapter 4 of MCAT Organic Chemistry Review, are a staple of general chemistry and are characteristic of oxidoreductase enzymes. Take a moment to identify the oxidizing and reducing agents in the reaction catalyzed by lactate dehydrogenase.

ELECTRON CARRIERS

In the cytoplasm, there are several molecules that act as high-energy electron carriers. These are all soluble and include NADH, NADPH, FADH2, ubiquinone, cytochromes, and glutathione. Some of these electron carriers are used by the mitochondrial electron transport chain, which leads to the oxidative phosphorylation of ADP to ATP. As electrons are passed down the electron transport chain, they give up their free energy to form the proton-motive force across the inner mitochondrial membrane. In addition to soluble electron carriers, there are membrane-bound electron carriers embedded within the inner mitochondrial membrane. One such carrier is flavin mononucleotide (FMN), which is bound to complex I of the electron transport chain and can also act as a soluble electron carrier. In general, proteins with prosthetic groups containing iron–sulfur clusters can act in the transport of electrons.

Flavoproteins

Flavoproteins contain a modified vitamin B2, or riboflavin. They are nucleic acid derivatives, generally either flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN). Flavoproteins are most notable for their presence in the mitochondria and chloroplasts as electron carriers. Flavoproteins are also involved in the modification of other B vitamins to active forms. Finally, flavoproteins function as cofactors for enzymes in the oxidation of fatty acids, the decarboxylation of pyruvate, and the reduction of glutathione.

REAL WORLD

Deficiency of riboflavin, a key component of flavoproteins, leads to a lack of growth, failure to thrive, and eventual death in experimental models. In humans, riboflavin deficiency is very rare, but may occur in severely malnourished individuals.

MCAT Concept Check 12.3:

Before you move on, assess your understanding of the material with these questions.

1.    What is an advantage of analyzing the half-reactions in biological oxidation and reduction reactions?

2.    Name three soluble electron carriers and their relevant metabolic pathways in the cell.

Electron Carrier

Metabolic Pathway(s)