Oxidases catalyze the removal of hydrogen from a substrate using oxygen as a hydrogen acceptor.( The term “oxidase” is sometimes used collectively to denote all enzymes that catalyze reactions involving molecular oxygen.) They form water or hydrogen peroxide as a reaction product (Figure 1).

Fig1. Oxidation of a metabolite catalyzed by an oxidase (A) forming H₂O and (B) forming H₂O₂ .

 Cytochrome Oxidase Is a Hemoprotein

 Cytochrome oxidase is a hemoprotein widely distributed in many tissues, having the typical heme prosthetic group present in myoglobin, hemoglobin, and other cytochromes (see Chapter 6). It is the terminal component of the chain of respiratory carriers found in mitochondria (see Chapter 13) and it functions to transfer electrons resulting from the oxidation of substrate molecules by dehydrogenases to their final acceptor, oxygen. The action of the enzyme is blocked by carbon monoxide, cyanide, and hydrogen sulfide, and this causes poisoning by preventing cellular respiration. The cytochrome oxidase enzyme complex comprises heme a3 combined with another heme, heme a₃, in a single protein and so is also termed cytochrome aa₃ . It contains two molecules of heme, each having one Fe atom that oscillates between Fe³⁺ and Fe²⁺ during oxidation and reduction. Furthermore, two atoms of copper are present, one associated with each heme unit. Other Oxidases Are Flavoproteins Flavoprotein enzymes contain flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) as prosthetic groups. FMN and FAD are formed in the body from the vitamin riboflavin (see Chapter 44). FMN and FAD are usually tightly— but not covalently—bound to their respective apoenzyme proteins. Metallo flavoproteins contain one or more metals as essential cofactors. Examples of flavoprotein oxidases include L-amino acid oxidase, an enzyme found in kidney with general specificity for the oxidative deamination of the naturally occurring l-amino acids; xanthine oxidase, a molybdenum containing enzyme which plays an important role in the con version of purine bases to uric acid (see Chapter 33), and is of particular significance in uricotelic animals (see Chapter 28); and aldehyde dehydrogenase, an FAD-linked enzyme present in mammalian livers, which contains molybdenum and nonheme iron and acts on aldehydes and N-heterocyclic substrates. The mechanisms of oxidation and reduction of these enzymes are complex. Evidence suggests a two-step reaction as shown in Figure 2.

Fig2. Oxidoreduction of isoalloxazine ring in flavin nucleotides via a semiquinone intermediate. In oxidation reactions, the flavin (eg, FAD) accepts two electrons and two H⁺ in two steps, forming the semiquinone intermediate followed by the reduced flavin (eg, FADH₂ ) and the substrate is oxidized. In the reverse (reduction) reaction, the reduced flavin gives up two electrons and two H⁺ so that it becomes oxidized (eg, to FAD) and the substrate is reduced.

مواضيع ذات صلة

اضطرابات استقلاب النوكليوتيدات البيريميدينية

اضطرابات استقلاب النكليوتيدات البورينية

تخليق النوكليوتيدات منقوصة الاكسجين ( dNTPs )

تقويض النوكليوتيدات البيريميدينية

Dehydrogenases Perform Two Main Functions

تخليق النكليوتيدات البيريميدينية

تقويض النوكليوتيدات البورينية

تنظيم استحداث البورينات

التخليق الحيوي للنوكليوتيدات البورينية

سبيل كيناز جانوس ومُنبِّغات الإشعار ومنشطات الانتساخ Jak/STATS Pathway

آلية عمل الأنسولين (وغيره من الهرمونات التي تحوي فعالية كيناز التيروزين الداخلية)

آلية عمل هرمونات المجموعة II-C (المرسال الثاني هو الكالسيوم ومشتقات فسفاتيديل الإينوزيتول)