Multiple products, enzyme-catalysed

LOXs catalyse the addition of dioxygen to methyl-interrupted cis double bond [(Z,Z)-pentadiene] in a polyunsaturated fatty acid to produce a hydroperoxy fatty acid containing a Z,E conjugated double bond system. Where multiple pentadienes occur in a single molecule such as arachidonic acid, position-specific oxygenation can take place, resulting in the production of 5-, 12- or 15-hydroperoxyeicosa-tetraenoic acids. These hydroperoxy acids may be subsequently converted into other oxylipins by enzymic or non-enzymic reactions [3]. Table 1 shows some examples of LOX products from fungi. 

Studies on the regulation of the common pathway of aromatic biosynthesis in several micro-organisms have shown that control of the first reaction (Figure 1.2), the conversion of o-erythrose-4-phosphate (7) and phosphoenolpyruvate (8) to 3-deoxy-o-arabino-heptulosonic acid-7-phosphate (9, DAHP), catalysed by the enzyme DAHP synthetase (EC 4.1.2.15) is an important factor in the overall control of the pathway In a number of enteric bacteria this enzyme exists in multiple molecular forms each of which is under the feedback control of a specific end-product. Thus in Escherichia coli there are three DAHP synthetases (iso-enzymes), the activity and formation of which are controlled by the three aromatic amino acids The formation and activity of DAHP synthetase... 

Isoenzymes or isozymes are families of oligomeric enzymes which catalyse the same reaction but differences in their subunit composition modify the rate at which each molecular species transforms substrate. Isoenzymes may be divided into primary or secondary isoenzymes. Primary isoenzymes are the products of multiple gene loci which code for distinct protein molecules or are the products of... 

Further work has been reported - with Fe-Mo models for nitro-genase, and a molecular mechanism has been proposed for the action of molybdenum in enzymes. In all reactions catalysed by Mo enzymes, the product and substrate differ by two electrons and two protons (or some multiple thereof). The co-ordination chemistry of Mo suggests that there is a distinct relationship between acid-base and redox properties of Mo complexes, and that a coupled electron-proton transfer (to or from substrate) may be mediated by Mo in enzymes. Each of the molybdenum enzymes (nitrogenase, nitrate reductase, xanthine oxidase, aldehyde oxidase, and sulphite oxidase) is discussed and it is shown that a simple molecular mechanism embodying coupled proton-electron transfer can explain many key experimental observations. 

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