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Enzyme lab Concept 8.4 Enzymes speed up metabolic reactions. • Catalyst a chemical agent that changes the rate of a reaction without being consumed by the reaction. • Enzymes – biological catalysts Enzymes • Almost all are proteins. • Names end in “ase ” Name usually involves cytidine deaminase the substrate with which the enzyme reacts. • Example: helicase = breaks down helices. helicase 1 B. Substrate specificity of enzymes Substrate = reactant that binds to an enzyme . Fig 8-13 sucrase Enzyme: sucrase Substrate = sucrose (and only sucrose ) Active site of an enzyme – site at which substrate binds and catalysis occurs active site enzyme substrate enzyme-substrate complex Substrate must “fit” in the active site of the enzyme. Mutations can alter shape of the enzyme or the substrate, reducing or preventing its function. Enzyme substrate complexes substrates 2 Substrate binding >> induced fit Result: chemical groups in position for catalysis . C. Catalysis in the active site • Active site R-groups - interact with substrate(s). • Cofactors may assist in catalysis. Cofactors • metal ions e.g., zinc, iron, copper, etc from trace metals in diet Coenzymes: small organic molecules e.g., NAD, FADH from vitamins in diet 3 Catalytic cycle Fig 8.17 How does an enzyme catalyze reactions? • Making bonds: orients substrates for reaction. Breaking bonds: stresses bonds to be broken D. Effects of local conditions on enzyme activity 1. Optimal temperature Increased temperature rx rate WHY? Temperature too high rx rate WHY? 4 Optimal temperature Fig 8-18 2. Optimal pH pH 6 - 8 for most enzymes in most organisms 3. Presence or absence of enzyme inhibitors or activators Small molecules that disable or stimulate enzyme activity Enzyme inhibitors: Some are toxins, poisons Some are part of normal enzyme regulation 5 e.g., toxins: sarin, some snake venoms, malethione Enzyme: Acetyl cholinesterase Mode of action of inhibitors: competitive inhibitor • Inhibitor binds to same site as the substrate Mode of action of inhibitors: noncompetitive inhibitor • Inhibitor binds somewhere other than active site 6 Concept 8.5 - Regulation of enzyme activity helps control metabolism A. Allosteric regulation of enzymes Allosteric enzymes: oscillate between 2 conformations. able to unable to bind bind substrate substrate Multimeric: many shapes Conformations stabilized by allosteric activators or inhibitors noncompetitive inhibition Binding site = allosteric site Metabolic pathways Substrates can sometimes act as activators. Products can sometimes act as inhibitors. E1 E2 E3 A B C D 7 Example: Feedback Inhibition Pathway regulated by its end product. 8.

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On the Active Site Thiol of Y-Glutamylcysteine Synthetase
Proc. Natl. Acad. Sci. USA Vol. 85, pp. 2464-2468, April 1988 Biochemistry On the active site thiol of y-glutamylcysteine synthetase: Relationships to catalysis, inhibition, and regulation (glutathione/cystamine/Escherichia coli/kidney/enzyme inactivation) CHIN-SHIou HUANG, WILLIAM R. MOORE, AND ALTON MEISTER Cornell University Medical College, Department of Biochemistry, 1300 York Avenue, New York, NY 10021 Contributed by Alton Meister, December 4, 1987 ABSTRACT y-Glutamylcysteine synthetase (glutamate- dithiothreitol, suggesting that cystamine forms a mixed cysteine ligase; EC 6.3.2.2) was isolated from an Escherichia disulfide between cysteamine and an enzyme thiol (15). coli strain enriched in the gene for this enzyme by recombinant Inactivation of the enzyme by the L- and D-isomers of DNA techniques. The purified enzyme has a specific activity of 3-amino-1-chloro-2-pentanone, as well as that by cystamine, 1860 units/mg and a molecular weight of 56,000. Comparison is prevented by L-glutamate (14). Treatment of the enzyme of the E. coli enzyme with the well-characterized rat kidney with cystamine prevents its interaction with the sulfoxi- enzyme showed that these enzymes have similar catalytic prop- mines. Titration of the enzyme with 5,5'-dithiobis(2- erties (apparent Km values, substrate specificities, turnover nitrobenzoate) reveals that the enzyme has a single exposed numbers). Both enzymes are feedback-inhibited by glutathione thiol that reacts with this reagent without affecting activity but not by y-glutamyl-a-aminobutyrylglycine; the data indicate (16). 5,5'-Dithiobis(2-nitrobenzoate) does not interact with that glutathione binds not only at the glutamate binding site but the thiol that reacts with cystamine.
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