Amino Acids, Proteins and Porphyrins
All are true about oxygenase except (AIIMS Nov 2011)
|A||Incorporate one atom of O2|
|B||Incorporate both atoms of O2|
|C||Hydroxylation of steroids|
|D||Help in carboxylation of drugs|
a. In addition, molecular oxygen is incorporated into a variety of substrates by enzymes designated as oxygenases; many drugs, pollutants, and chemical carcinogens (xenobiotics) are metabolized by enzymes of this class, known as the cytochrome P450 system. Administration of oxygen can be lifesaving in the treatment of patients with respiratory or circulatory failure.
b. Oxygenases Catalyze the Direct Transfer & Incorporation of Oxygen into a Substrate Molecule.
i. oxygen is bound to the enzyme at the active site, and
ii. the bound oxygen is reduced or transferred to the substrate.
d. Oxygenases may be divided into two subgroups, as follows.
i. Dioxygenases Incorporate Both Atoms of Molecular Oxygen into the Substrate. The basic reaction is shown below:Examples include the liver enzymes, homogentisatedioxygenase (oxidase) and 3-hydroxyanthranilate dioxygenase (oxidase), that contain iron; and L-tryptophan dioxygenase (tryptophan pyrolase) that utilizes heme.
c. Cytochromes P450 are an important superfamily of heme-containing monooxgenases, and more than 1000 such enzymes are known. Both NADH and NADPH donate reducing equivalents for the reduction of these cytochromes, which in turn are oxidized by substrates in a series of enzymatic reactions collectively known as the hydroxylase cycle.
d. In liver microsomes, cytochromes P450 are found together with cytochrome b5 and have an important role in detoxification. Benzpyrene, aminopyrine, aniline, morphine, and benzphetamine are hydroxylated, increasing their solubility and aiding their excretion. Many drugs such as phenobarbital have the ability to induce the formation of microsomal enzymes and of cytochromes P450.