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spinqualitylabelsall models Structure of HMG-CoA reductase (PDB entry 1dq8) Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image Anything in this section will appear adjacent to the 3D structure and will be scrollable.

Enzyme Kinetics

Fumarase catalyzes the reversible reaction between S-malate and fumarate in the citric acid cycle for cellular metabolism. When it catalyzes the addition of water to S-malate in order to form fumarate, the Km and Vmax values are 0.30 mM and 129 s^-1, respectively. The reverse reaction (dehydration of fumarate to form S-malate) has a Km of 0.10 mM and Vmax of 60 s^-1 (Rose, Weaver 2004). Thus, the forward pathway (fumarate to S-malate) is favored because of its higher Km and Vmax values. According to Beechmans (1998), fumarase kinetics normally follows Michaelis-Menten kinetic plots with low concentrations of substrate, but high concentrations influence the enzyme’s activity. Enzyme kinetics studies with fumarase mutants differ from wild type fumarase kinetics when the mutations alter amino acid residues involved in binding at the active site and B site. Also, the Michaelis-Menten kinetics plots for fumarase mutants exhibit a sigmoidal curve which suggests the presence of cooperativity in the enzymes activity.

Regulation

Similar to numerous other enzymes involved in biological processes, fumarase can be regulated in several different ways. Allosteric effects commonly regulate fumarase activity via substrate and inhibitor binding to the active site. Fumarase activity is both positively and negatively regulated by substrate concentration. When the concentration of a substrate is five times the Km value, it activates fumarase’s activity; however, substrate concentrations above 0.1 M result in inhibition of fumarase function (Beeckmans 1998). The concentration of substrate influences cooperativity of fumarase, depending on the availability of substrate to bind to domains.

The regulation of fumarase via allosteric effects involves conformational changes that occur when a substrate binds to the active site. Studies of the active site and B site amino acid residue manipulation show that the B site helps regulate the binding affinity for the active site by allosteric effects (Rose & Weaver 2004, Beeckmans 1998). According to Weaver (2004), the active site and B site are located 12 Å apart which suggests that the conformational changes resulting from the B site binding influence the active site affinity to bind with the substrate. Inhibitors also regulate the activity of an enzyme via binding to the active site. Both citrate and succinate are known as competitive inhibitors of fumarase since they negatively influence the enzyme’s activity. They are competitive inhibitors because they have structural similarity to the substrate; therefore, the inhibitors compete with substrates to bind with the active site.

Other Interesting Information

Fumarase expression mainly occurs in skin, parathyroid, lymph, and colon tissues, and it is present throughout all life stages, from early development to mature adults. Fumarase comprises two specific classes which relate to the enzyme's: arrangement of subunits, metal ion requirement, and thermal stability. Class I fumarase isozymes can change their state, become inactive upon exposure to heat or radiation, are sensitive to superoxide anions, and Fe2+ dependent. Class II includes fumarase found in eukaryotes and prokaryotes, and they are iron-independent and thermally stable. Mutations in the gene that encodes fumarase can lead to a deficiency in fumarase enzyme in the citric acid cycle which is known to cause certain diseases. Autosomal recessive mutants can result in fumarase deficiency, a metabolic disorder distinguished by excess fumaric acid in the body (Remes 1992). Inheritance of this autosomal recessive mutation has serious effects on early neural and brain development and can be fatal. Also, heterozygous fumarase mutations play a role in cancerous tumor development; specifically, the mutant H153R has identified as a factor in three families of malignant tumor growths (Kokko 2006).