BCH 380 1st Edition Lecture 14 Outline of Last Lecture II. Inhibitorsa. Competitive b. Noncompetitive Outline of Current Lecture III. Inhibitors Cont.IV. Allosteric Enzymesa. And RegulatorsCurrent LectureAnother class of enzyme inhibitors are those that react with the enzyme to form a covalent bond. These kinds of inhibitors are irreversible- For example, reaction of the amino group of a lysine residue with an aldehyde. Reduction of the Schiff base with sodium borohydride (NaBH4) forms a stable substituted enzyme- This kind of chemistry can be used to find out if there is a critical lysine residue in the active site of the enzyme Reaction of diisopropyl fluorophosphates (DFP) with a single, highly nucleophilic serine residue (Ser-195) at the active site of chymotrypsin, producing inactive diisopropylphosphoryl-chymotrypsin- DFP inactivates serine proteases and serine esterases, including acetylcholine esterase, an important enzyme in neurotransmission Irreversible inhibition of triose phosphate isomerase- The 1-bromo analog of dihydroxyacetone phosphate reacts with the side chain of the active-site glutamate residue- Reduction of the carboxyl group of the substrate by NaBH4 The Michaelis-Menten and Linweaver-Burk equations are for the simplest enzyme reactions, S P. Nevertheless, they are extremely useful- We can also derive kinetic equation for more complicated enzyme catalyzed reactions. Notation of bisubstrate reactions:- In sequential reactions, all substrates are bound before a product is released- The binding of substrates may be either ordered or randomAn alternative to sequential reactions are the enzyme catalyzed “ping-pong” reactions- One substrate is bound and a product is released, leaving a substituted enzyme (F)- A second substrate is then bound and a second product is released, restoring the enzyme to its original form Allosteric Enzymes and Allosteric Regulators Allosteric Enzymes- enzymes whose properties are affected by changes in the structure of the enzyme- These changes are mediated by their interaction with small molecules, such as substrates- These enzymes do not obey Michaelis-Menten Kinetics- Binding of the first substrate molecule increases the affinity of the remaining enzyme subunits for additional substrate molecules Reaction catalyzed by phosphofructokiase-1- In some cases allosteric regulators stimulate the activity of the enzyme- In other cases, they can inhibit the enzyme, putting the breaks on the enzyme, thus lowering flux through that particular enzymatic pathway- As an example, consider the small molecules phosphoenolpyruvate and ADP/AMP, and the enzyme phosphofructokinase-1Role of cooperativity of binding in regulation- The activity of an allosteric enzyme with a sigmoidal binding curve can be altered markedly when either an activator or an inhibitor is bound to the enzymeImportant strategy for regulation:- phosphorylation-
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