1. 2. dehydrogenases:
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. These dehydrogenases often utilize common coenzymes (act as hydrogen carriers) e.g. NAD+ and sine the reactions are reversible this enable the reducing equivalents (H+) to be freely transferred within the cell . this type of reactions enable oxidative processes to occur in the absence of oxygen such as during the anaerobic phase of glycolysis.

2. 2. as components in respiratory chain of electron transport
2. as components in respiratory chain of electron transport . Note: dehydrogenases cannot use oxygen as a hydrogen acceptor.

3. Dehydrogenases require a supportive co enzymes to complete their reactions, these co enzymes could be either nicotin amide coenzymes or riboflavins co enzymes. a. dehydrogenases using nicotin amide coenzymes: these dehydrogenases use nicotin amide adenine dinucleotide (NAD+) or nicotinamide adenine dinucleotide phosphate (NADP+ ) or both of them.

4. These co enzymes are synthesized from the vitamin B3 (niacin)
These co enzymes are synthesized from the vitamin B3 (niacin). These co enzymes are reduced by a suitable electron acceptor. These co enzymes may freely and reversibly dissociate from their respective apoenzymes.

5. Generally, NAD-linked dehydrogenases catalyze oxidoreduction reactions in the oxidative pathways of metabolism particularly in glycolysis, in citric acid cycle and in respiratory chain of mitochondria. While, NADP-linked dehydrogenases are found mainly in reductive syntheses , like extra mitochondrial pathway of fatty acid synthesis, steroid synthesis and also in the pentose phosphate pathway.

6. b. dehydrogenases using flavin coenzymes:
the flavin groups associated with these dehydrogenases are similar to FMN and FAD occurring in oxidases . they are generally more tightly bound to their apoenzymes than are the nicotinamide coenzymes. Most of the riboflavin – linked dehydrogenases are concerned with electron transport in the respiratory chain. For example:

7. The cytochromes
The cytochromes are iron containing hemoproteins in which the iron atom oscillates between Fe+3 and Fe+2 during oxidation and reduction . they are classified as dehydrogenases (except for cytochrome oxidase and cytochrome p450).
These cytochromes are important for the transportation of electrons in the respiratory chain ,e.g cytochrome b, c1 and c.

8. 3. hydroperoxidase:
two types of enzymes found in animals and plant fall into this category : peroxidases and catalases. a. Peroxidases: Mainly found in leucocytes, platelets, and other tissues. In the reactions catalyzed by peroxidase , Hydrogen peroxide is reduced with the help of different substances such as ascorbate (vitamin C) and quinones .

9. The overall reaction is complex but can summarized as in the following:
e.g. glutathione peroxidase , containing selenium as prosthetic group.

10. This enzyme catalyses the destruction of H2O2 and lipid hydroperoxides through the conversion of reduced glutathione to its oxidized form , thus protecting membrane lipids and hemoglobin against oxidation by peroxides.

11. b. Catalases:
Is a hemo protein containing 4 heme groups. In addition to possessing peroxidase activity, it is also able to use one molecule of H2O2 as a substrate electron donor and another molecule of H2O2 as an oxidant or electron acceptor.

12. Catalase is found in blood, bone marrow, mucous membranes, kidneys and liver. It functions to destroy the H2O2 formed by the action of oxidase Note: peroxisomes are found in many tissues including the liver. They are rich in oxidases and in catalase. Thus . the enzymes that producing H2O2 are grouped with the enzyme that destroy it. however many other biochemichal reactions can produce H2O2 like xanthine oxidase which must be considered as additional sources of H2O2.

13. 4. Oxygenases
These are enzymes that are concerned with the synthesis or degradation of different types of metabolites, they catalyze the incorporation of oxygen into a substrate molecules. Oxygenases may be divided into two subgroups , dioxygenases and monooxygenases. a. dioxygenases act by incorporating both atoms of molecular oxygen into the substrate. A + O2 → AO2

14. For example:L-tryptophan dioxygenase (tryptophan pyrolase) and carotenase. b. monooxygenases they act by incorporating only one atom of molecular oxygen into the substrate and the other oxygen atom is reduced to water, an additional electron donor or cosubstrate (Z) being necessary for this purpose : AH + O2 + ZH2 → AOH + H2O + Z Cytochromes P450 are monoxygenases important for the detoxification of many drugs and hydroxylation of steroids.

15. Cytochrome p450(CYP) are a family of heme containing monooxygenases (more than 50 enzymes) , these enzymes are mainly located in the endoplasmic reticulum in the liver and intestine but are also found in the mitochondria in some tissues. The Cytochromes P450 of the endoplasmic reticulum of the liver have an important role in detoxification of variety of xenobiotics.

16. Mitochondrial Cytochromes P450 systems are found in steroidogenic tissues such as adrenal cortex , testis, ovary and placenta and are concerned with the biosynthesis of steroid hormones from cholesterol.