Allosterically regulated enzyme

The hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate (Eigure 23.7), like all phosphate ester hydrolyses, is a thermodynamically favorable (exergonic) reaction under standard-state conditions (AG° = —16.7 kj/mol). Under physiological conditions in the liver, the reaction is also exergonic (AG = —8.6 kJ/mol). Fructose-1,6-bisphosphatase is an allosterically regulated enzyme. Citrate stimulates bisphosphatase activity, hut fructose-2,6-bisphosphate is a potent allosteric inhibitor. / MP also inhibits the bisphosphatase the inhibition by / MP is enhanced by fructose-2,6-bisphosphate. 

The substrate concentration required for half-maximal activity (S0 0 of an allosterically regulated enzyme will depend on the cumulative effects of allosteric activators and/or inhibitors also present. Hence S,)5 may be decreased with allosteric activators and may be increased with allosteric inhibitors. 

Allosterically Regulated Enzymes Do Not Follow Michaelis-Menten Kinetics... 

The inhibitory ability of CTP is remarkable because CTP is structurally quite different from the substrates of the reaction (see Figure 10.1). Thus CTP must bind to a site distinct from the active site where substrate binds. Such sites are called allosteric or regulatory sites. CTP is an example of an allosteric ibitoY. In ATCase (but not all allosterically regulated enzymes), the catalytic sites and the regulatory sites are on separate polypeptide chains. 

Fig. 2.28. The effect of substrate concentration on the catalytic reaction rate, a Enzyme obeying Michaelis-Menten kinetics b allosterically regulated enzyme with positive cooperativity c allosterically regulated enzyme with negative cooperativity...

Allosteric regulation acts to modulate enzymes situated at key steps in metabolic pathways. Consider as an illustration the following pathway, where A is the precursor for formation of an end product, F, in a sequence of five enzyme-catalyzed reactions ... 

Metabolic regulation is achieved via regulating enzyme activity in three prominent ways allosteric regulation, covalent modi-... 

Citrate synthase is the first step in this metabolic pathway, and as stated the reaction has a large negative AG°. As might be expected, it is a highly regulated enzyme. NADH, a product of the TCA cycle, is an allosteric inhibitor of citrate synthase, as is succinyl-CoA, the product of the fifth step in the cycle (and an acetyl-CoA analog). 

To refer to the kinetics of allosteric inhibition as competitive or noncompetitive with substrate carries misleading mechanistic implications. We refer instead to two classes of regulated enzymes K-series and V-se-ries enzymes. For K-series allosteric enzymes, the substrate saturation kinetics are competitive in the sense that is raised without an effect on V. For V-series allosteric enzymes, the allosteric inhibitor lowers... 

This reaction is followed by another phosphorylation with ATP catalyzed by the enzyme phosphofructoki-nase (phosphofructokinase-1), forming fructose 1,6-bisphosphate. The phosphofructokinase reaction may be considered to be functionally irreversible under physiologic conditions it is both inducible and subject to allosteric regulation and has a major role in regulating the rate of glycolysis. Fructose 1,6-bisphosphate is cleaved by aldolase (fructose 1,6-bisphosphate aldolase) into two triose phosphates, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are inter-converted by the enzyme phosphotriose isomerase. 

Changes in enzyme levels and allosteric regulation of carbamoyl phosphate synthase by A -acetylglutamate regulate urea biosynthesis. 

Synthetic nonhydrolyzable analogs of nucleoside triphosphates (Figure 33-13) allow investigators to distinguish the effects of nucleotides due to phosphoryl transfer from effects mediated by occupancy of allosteric nucleotide-binding sites on regulated enzymes. 

Aiming at a computer-based description of cellular metabolism, we briefly summarize some characteristic rate equations associated with competitive and allosteric regulation. Starting with irreversible Michaelis Menten kinetics, the most common types of feedback inhibition are depicted in Fig. 9. Allowing all possible associations between the enzyme and the inhibitor shown in Fig. 9, the total enzyme concentration Er can be expressed as... 

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