Fumarase, function

The activity of many enzymes is pH-dependent because the enzyme may ionize in solution and the biological activity of unionized and ionized forms may be different. In this case, the rate of an enzyme-mediated reaction can be expected to depend on the acidity of the solution. If the enzyme can lose more than one proton as the pH increases (Figure 8.12), the rate of reaction as a function of pH may display a maximum if the forms of the enzyme in strongly acidic or strongly basic solution are inactive, but the intermediate, monoanion, is active. An example of this behavior is provided by fumarase (Figure 8.13). 

Why did nature use an Fe-S cluster to catalyze this reaction, when an enzyme such as fumarase can catalyze the same type of chemistry in the absence of any metals or other cofactors One speculation would be that since aconitase must catalyze both hydrations and dehydrations, and bind substrate in two orientations, Fe in the comer of a cubane cluster may provide the proper coordination geometry and electronics to do all of these reactions. Another possibility is that the cluster interconversion is utilized in vivo to regulate enzyme activity, and thus, help control cellular levels of citrate. A third, but less likely, explanation is that during evolution an ancestral Fe-S protein, whose primary function was electron transfer, gained the ability to catalyze the aconitase reaction through random mutation. 

Other enzymes in the aconitase family include isopropylmalate isomerase and homoaconitase enzymes functioning in the chain elongation pathways to leucine and lysine, both of which are pictured in Fig. 17-18.90 There are also iron-sulfur dehydratases, some of which may function by a mechanism similar to that of aconitase. Among these are the two fumarate hydratases, fumarases A and B, which are formed in place of fumarase C by cells of E. coli growing anaerobically.9192 Also related may be bacterial L-serine and L-threonine dehydratases. These function without the coenzyme pyridoxal phosphate (Chapter 14) but contain iron-sulfur centers.93-95 A lactyl-CoA... 

The role of the iron-sulfur clusters in many of the proteins that we have just considered is primarily one of single-electron transfer. The Fe-S cluster is a place for an electron to rest while waiting for a chance to react. There may sometimes be an associated proton pumping action. In a second group of enzymes, exemplified by aconitase (Fig. 13-4), an iron atom of a cluster functions as a Lewis acid in facilitating removal of an -OF group in an a,P dehydration of a carboxylic acid (Chapter 13). A substantial number of other bacterial dehydratases as well as an important plant dihydroxyacid dehydratase also apparently use Fe-S clusters in a catalytic fashion.317 Fumarases A and B from E. coli,317 L-serine dehydratase of a Pepto-streptococcus species,317-319 and the dihydroxyacid... 

Fumarase. See Fumarate hydratase Fumarase-aspartase family 685 Fumarate 481s, 516s, 683s Fumarate hydratase (fumarase) 526, 683,688 acid-base catalysis 471 concerted reaction 685 Fumarase A 688 Fumarase B 688 Fumarase C 683 mechanism 683 - 685 pH dependence 684 rates of substrate exchange 684 turnover number of 683 Fumarate reductase 785 Fumarylpyruvate 690s Function of state R 476 Fungal infections 20 Fungi 20... 

The apparent dissociation constant of the fumarase site-succinate complex to yield unoccupied site and succinate is represented by the following function of pH ... 

C-IO) Fumarase deficiency. There is a deficit in the transformation of fumarate to malate. The infant has developmental retardation, with abnormal neuromuscular function, lactic acidemia, and fumarate aciduria. The lactic acidosis may result from a backup of Krebs cycle function, all the way to lactate. Lactic acidosis may also be present in rare disorders of cytochrome oxidase activity. Diagnostically, there is a deficit in fumarase activity in assay of liver and skeletal muscle mitochondria. 

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