Oxidases classification

Boadle, M. C. and Blaschko, H., Cockroach amine oxidase classification and substrate specificity. Comp. Biochem. Physiol., 25, 129-138, 1968. 

These light-producing oxidations are quite different reactions in each of the three most thoroughly studied organisms, Cypridina, bacteria, and fireflies (382,566,567,569,709,761). The structures of the two luciferins, LHk and LHjf, and of their oxidation products (oxyluciferins , 568), and the fate of the long-chain fatty aldehyde which acts as cofactor in bacterial luminescent oxidation of FMNH, are all unknown. Each luminescent system requires molecular oxj en and a potential source of electrons, in common with mixed function oxidases, but the significance of these characteristics in terms of oxidase classification remains to be determined. 

Cytochrome c oxidase contains two, or possibly three, copper atoms referred to as Cua and Cub since they do not fit into the usual classification. The former (possibly a dimer) is situated outside the mitochondrial membrane, whereas the latter is associated with an iron atom within the membrane. Both have electron transfer functions but details are as yet unclear. 

The large number of cytochromes identified contain a variety of porphyrin ring systems. The classification of the cytochromes is complicated because they differ from one organism to the next the redox potential of a given cytochrome is tailored to the specific needs of the electron transfer sequences of the particular system. The cytochromes are one-electron carriers and the electron flow passes from one cytochrome type to another. The terminal member of the chain, cytochrome c oxidase, has the property of reacting directly with oxygen such that, on electron capture, water is formed ... 

These copper ion-dependent enzymes [EC 1.10.3.1] (also referred to as diphenol oxidases, O-diphenolase, phe-nolases, polyphenol oxidases, or tyrosinases) catalyze the reaction of two catechol molecules with dioxygen to produce two 1,2-benzoquinone and two water. A variety of substituted catechols can act as substrates. Many of the enzymes listed under this classification also catalyze a monophenol monooxygenase activity [/.c., EC 1.14.18.1]. See also Monophenol Monooxygenase Tyrosine Monooxygenase... 

This copper-dependent enzyme [EC 1.14.18.1] (also known as tyrosinase, phenolase, monophenol oxidase, and cresolase) catalyzes the reaction of L-tyrosine with L-dopa and dioxygen to produce L-dopa, dopaquinone, and water. This classification actually represents a set of copper proteins that also catalyze the reaction of catechol oxidase [EC 1.10.3.1] if only 1,2-benzenediols are available as substrates. 

Cos, P. et al., Structure-activity relationship and classification of flavonoids as inhibitors of xanthine oxidase and superoxide scavengers, J. Nat. Prod., 61, 71, 1998. 

Examples of this class of enzymes are glucose oxidase and D-amino acid oxidase The classification of the flavoproteins used here is that originally suggested which has been modified recently . In the author s own view the original classification has the advantage of being simple and yet quite useful whereas the new classification does not add to simplify and classify the rather complex picture of flavoprotein catalysis. Nevertheless, in flavoprotein oxidases, the 1,5-dihydroflavin is very reactive towards Oj. On the other hand, the two-electron reduced form of flavoprotein oxidases reacts slowly with pure one-electron acceptors, e.g. ferricyanide. That the two-electron transition is biologically favoured in these enzymes explains why they can react easily with sulfite... 

The copper centres in the multicopper blue oxidases have been classified into three groups. This classification may be extended to include other copper proteins. 

Amine oxidases catalyze the oxidation of amines, diamines, and polyamines. According to their ability to recognize one of those substrates preferentially, amine oxidases may be divided into monoamine oxidases, diamine oxidases, and polyamine oxidases, respectively. Several different enzymes fall into the amine oxidase class, and the classification of some of them still remains ambiguous. The term monoamine oxidase (flavin-containing, EC 1.4.3.4) was introduced to contrast with copper-containing amine oxidases (EC 1.4.3.6). 

Scientists were looking for drugs to treat different medical problems when their observations almost accidentally led them to the study of depression and its treatment. Many scientists continued in this new direction to the discovery of the current three classifications of antidepressant drugs used today monamine oxidase... 

This classification is perfectly valid for the metalloenzymes, even though most of them fall in group 1, such as the several dioxygenases, (mono)oxygen-ases, and oxidases (dehydrogenase). Many examples will be discussed in the later chapters of the book. The enzymes known and classified [7] so far very often involve a metal, either directly at the active center or indirectly at another place, for instance, in the electron transfer process [8],... 

Protein sequence homology suggests that sulfite oxidase and assimilatory nitrate reductase are members of the same molybdenum enzyme subfamily [31]. Consistent with this classification, the cofactors of sulfite oxidase and assimilatory nitrate reductase differ significantly from those in dmso reductase, aldehyde oxido-reductase, xanthine oxidase (see Section IV.E.), and even respiratory nitrate reductase (Section IV.D). The EXAFS of both sulfite oxidase [132-136] and assimilatory nitrate reductase [131,137,138] and x-ray studies of sulfite oxidase (chicken liver) [116] confirm that the molybdenum center is coordinated by two sulfur atoms from a single MPT ligand and by the sulfur atom of a cysteine side chain. The Movl state is bis(oxido) coordinated (Figure 14). 

Based on spectroscopic properties, mainly electron paramagnetic resonance (EPR), the active sites of copper proteins have been classified into three groups, types I, II, and III. This nomenclature was originally applied to blue oxidases to distinguish the four copper ions contained in these proteins. The original classification has been extended to the copper sites of other proteins. The recent increase in structural information on the copper sites in proteins has, however, revealed greater diversity in the type of copper site. For instance, the type III and type II sites in ascorbate oxidase are in close proximity, forming a trinuclear site, in which all three copper ions are essential for the reactivity. Some proteins, once believed to contain a copper site with normal spectroscopic properties, and thus referred as type II, have been shown to contain copper coordinated by an unusual side chain. Therefore, in this review, new nomenclature is used to classify the copper sites more precisely with respect to their structural features and spectroscopic properties. The definitions are as follows ... 

The classification introduced in this review (type I-type IV) should cover all structural types of copper sites known to date. For instance, based on this nomenclature, ascorbate oxidase contains type I and type IV, and nitrite reductase contains type I and type II (more precisely, type IIA). Galactose oxidase has a type IIB site. 

Enzymes are proteinaceous catalysts peculiar to living matter. Hundreds have been obtained in purified and crystalline form. Their catalytic efficiency is extremely high—one mole of a pure enzyme may catalyze the transformation of as many as 10,000 to 1,000,000 moles of substrate per minute. While some enzymes are highly specific for only one substrate, others can attack many related substrates. Avery broad classification of enzymes would include hydrolytic enzymes (esterases, proteases), phosphorylases, oxidoreductive enzymes (dehydrogenases, oxidases, peroxidases), transferring enzymes, decarboxylases and others. 

---