Question

Give any two peroxidase proteins and there mode of operation

Answer 1

Peroxidases is a family of enzymes which catalyzes that typically catalyze a reaction of the form:

RCOOR' + 2e^- (electron donor) + 2H⁺ ROH + R'OH (in presence of peroxidases)

Example 1. Cytochrome c peroxidase (CCP): it is a water soluble heme containing enzyme which contains reducing equivalents from cytochrome c and reduces hydrogen peroxide to water:

CCP + H₂O₂ + 2 ferrocytochrome c + 2H⁺ CCP + 2H₂O + 2 ferricytochrome c

The reaction of cytochrome c peroxidase proceeds through a three-step process, forming a Compound I and then a Compound II intermediate:

CCP + ROOH Compound I + ROH + H₂O

CCP-compound I + e⁻ + H⁺ Compound II

Compound II + e⁻ + H⁺ CCP

CCP in the resting state has a ferric heme, and, after the addition of two oxidizing equivalents from a hydroperoxide and becomes oxidised to a formal oxidation state of +5 (Fe^V, commonly referred to as ferryl heme). Low-temperature magnetic susceptibility and Mössbauer spectroscopy show that the Fe in Compound I of CCP is a +4 ferryl iron, with the second oxidising equivalent existing as a long-lived free-radical on the side-chain of the tryptophan residue (Trp-191). Compound I of CCP is fairly long-lived, decaying to CCP-compound II with a half-life around 1 h.

Example 2. Glutathione peroxidase: Its main role is to protect the organism from oxidative damage. Its biochemical function is to reduce lipidhydroperoxides to their corresponding alcohols and to reduce free hydrogen peroxide to water.The main reaction that glutathione peroxidase catalyzes is:

2GSH + H₂O₂ GS–SG + 2H₂O

where GSH represents reduced monomeric glutathione, and GS–SG represents glutathione disulfide.

The mechanism involves oxidation of the selenol of a selenocysteine residue by hydrogen peroxide. This process gives the derivative with a selenenic acid (RSeOH) when is then converted back to the selenol by a two step process that begins with reaction with GSH to form the GS-SeR and water. A second GSH molecule reduces the GS-SeR intermediate back to the selenol, releasing GS-SG as the by-product.

RSeH + H₂O₂ RSeOH + H₂O

RSeOH + GSH GS-SeR + H₂O

GS-SeR + GSH GS-SG + RSeH

Glutathione reductase then reduces the oxidized glutathione to complete the cycle:

GS–SG + NADPH + H⁺ 2 GSH + NADP⁺.