Food Chemistry and Fish in Nutrition ( 2+1)

6.5.4 Oxidases and carriers of reducing equivalents

6.4.2.1. Oxidases and carriers of reducing equivalents

Oxidases are enzymes that catalyze the removal of hydrogen from a substrate but use only oxygen as a hydrogen acceptor to form water as a reaction product (with the exception of uricase and monoamine oxidase which form H₂O₂).  They are conjugated proteins containing copper as prosthetic groups.

(i) Cytochrome oxidase: Cytochrome oxidase is a hemoprotein widely distributed in plants and animal tissues. It is the terminal component of respiratory chain found in mitochondria.  Cytochromes a and a₃ are combined with the same protein and the complex is known as cytochrome aa_3. Cytochrome aa₃ contains 2 molecules of heme A, each having one Fe atom.. 2 atoms of Cu are also present which are associated with the cytochrome oxidase activity. The terminal cytochrome aa₃ is responsible for the final combinant of reducing equivalents with molecular oxygen. It has a high affinity for oxygen. It is the only one in the chain which signifies the irreversible reaction.. It gives direction to the movement of reducing equivalents in the respiratory chain and to the production of ATP, to which it is coupled.

(ii) Phenolase (tyrosinase, polyphenol oxidase, catechol oxidase):   It is a copper containing enzyme. It converts monophenol to O-quinones.

3. Hydroperoxidases

They utilize hydrogen peroxide as a substrate. Two enzymes fall into this category (i) Peroxidase, (ii) Catalase.

(i) Peroxidase: It is found in milk and leukocytes and the prosthetic group is protoheme.  It catalyzes the reduction of hydrogen peroxide by the help of ascorbic acid, quinines and cytochrome C which act as electron acceptors.

(ii) Catalase: It is hemoprotein and found in blood and liver. It uses one molecule of H₂O₂ as a substrate electron donor and another molecule as electron acceptor.  Its function is to destroy H₂O₂ formed by the action of aerobic dehydrogenases.