Preface

Oxidation-reduction reactions in our body are catalyzed by a class of enzymes called oxidoreductase. The mechanism is based on the transfer of electrons from one molecule (the oxidant) to another molecule (the reductant). Oxidoreductases catalyze reactions similar to the following, A− + B → A + B− where A is the oxidant and B is the reductant. From a biochemistry point of view, oxidoreductase enzymes are a group of enzymes that catalyze the transfer of electrons from one molecule, the reductant, also called the electron donor, to another, the oxidant, also called the electron acceptor. Oxidoreductase enzymes utilize NADP+ or NAD+ as cofactors. Oxidoreductase enzymes include the following: oxidase, dehydrogenase, peroxidase, hydroxylase, oxygenase, and reductase. Most oxidoreductase enzymes are dehydrogenases. However, reductases are also common. The accepted nomenclature for dehydrogenases is "donor dehydrogenase", where the donor is the oxidized substrate.

Oxidases are enzymes involved when molecular oxygen acts as an acceptor of hydrogen or electrons. Whereas dehydrogenases are enzymes that oxidize a substrate by transferring hydrogen to an acceptor that is either NAD+/NADP+ or a Flavin enzyme. While the other oxidoreductases, peroxidases, are localized in peroxisomes and catalyze the reduction of hydrogen peroxide. Hydroxylases add hydroxyl groups to their substrates. Oxygenases incorporate oxygen from molecular oxygen into organic substrates. Reductases catalyze reductions and in most cases can act as oxidases.

Oxidation-reduction reactions are essential for the growth and survival of organisms. During the oxidation process of organic molecules, energy is produced. Energy-producing reactions can liberate high energy containing compounds as the synthesis of important energy molecules, such as ATP.

Oxidoreductase enzymes achieve an important role under aerobic and anaerobic metabolism. They play an important role in glycolysis, the tricarboxylic acid cycle, oxidative phosphorylation, and in amino acid metabolism. In glycolysis, the glyceraldehydes-3-phosphate dehydrogenase enzyme catalyzes the transfer of hydrogen to coenzyme NAD, leading to the reduction of NAD+ to NADH. In order to maintain the redox state of the cell, this NADH is converted to NAD+, which occurs in the oxidative phosphorylation pathway. The final pathways for complete oxidation of glucose are achieved via the TCA cycle. More NADH molecules are generated in the TCA cycle. Except for leucine and lysine, the rest of the amino acid metabolites enter the TCA cycle as intermediates of the cycle. This allows for the formation of oxaloacetate from carbon skeletons of the amino acids and subsequently into pyruvate.

Metabolic abnormalities disorders resulting from a deficiency (quantitative and qualitative) or from over-activity of oxidoreductase, which may contribute to the decreased normal performance of life, are becoming common.

> **Mahmoud Ahmed Mansour** Department of Pharmaceutical Sciences, College of Pharmacy, King Saud bin Abdulaziz University for Health Sciences, Saudi Arabia

> > Section 1

Biological Application

of Oxidoreductase

Section 1
