M2C: Enzyme Regulation 2

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L-B Plots

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Have inverse axes. Non-competitive inhibitors do not change the Km, but are rare. Competitive inhibitors DO change the Km of an enzyme. Curvature of the line in the plot signifies changing affinity (Km) of the enzyme for the substrate as the concentration of substrate increases. If enough activator present, line will become straight signifying that the enzyme now has a constant affinity as the activator is stabilizing the R conformation.

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K-type allosteric enzymes

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Experience a change in Km but Vmax remains the same. Much more common. Sometimes can have change in Vmax, but also similar change in Km.

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V-type allosteric enzymes

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Experience a large change in Vmax with almost no change in Km. Rare, but important in cases where enzyme acts as or is acted on by a regulatory switch. Like in blood clotting cascade.

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G-proteins

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Best studied example of V-type enzymes. Named for slow hydrolysis of a bound GTP. Normally have two or more active sites on different domains. Protein kinase binds to one active site changing it to active which allows binding of GTP to ITS binding site. Enzyme undergoes conformational change when GTP is bound. While bound, GTP can be hydrolyzed to GDP. This change turns the domain to T conformation, activating the neighboring domain, allowing it to bind its target protein. The target protein then has a set clock for function and then release and reset.

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Anabolic

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Also called biosynthetic. A sequence of enzymatic reactions that lead to the synthesis of an important molecule. Often involves the input of ATP energy.

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Catabolic

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Degradative. A sequence of enzymatic reactions that lead to the breakdown of an important molecule. Products of these pathways are often used to create energy in the form of ATP.

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Branch Point

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Point of divergence in an enzymatic pathway.

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Changing amount of enzyme as regulating enzyme activity.

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1. Genetically: Steroid hormones effect genes to allow more transcription of enzyme-coding genes.
2. Catabolism: Proteases break down enzymes creating a lower total enzyme concentration. Increased proteases can cause this.

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Metabolite Regulation as way of enzymatic regulation

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Allosteric regulation or product competition cause activation or inhibition.

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Blood Clotting

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Many factors remain in inactive state until stimuli activates. Clot forms when structural molecule fibrin cross-links and attaches to platelets to form an expandable and solid plug to prevent blood loss. Fibrin usually exists as zygomen fibrinogen. Fibrinogen is cleaved by thrombin to create fibrin. Thrombin also usually exists as pre-form prothrombin until cleaved to activate. In cascades like these, usually the pre-enzyme is cleaved and then must attach to some activator protein. This usually occurs at membrane and requires Calcium. Ca++ bound by two carboxyles (Vitamin K) to stabilize complex. In this complex, only prothrombin and fibrinogen are available in high concentrations.

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Concentrations of Enzymes in Blood Clotting

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Prothrombin and Fibrinogen are available at high levels because they are key "players' in the cascade. There must be enough so that blood will clot fast enough for a person not to bleed to death. All enzymes can't be present at high levels, however, because can't have clotting under normal conditions inside body.

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Natural Time Clocks

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Built-in mechanisms that control the lifetime or duration of some process.

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Telomeres and time clocks

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60-100 years. Determined by number of possible genome replications possible. Exonucleases degenerate telomeres and they can survive until telomerse creates stops.

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Circadian Clock

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24 hour natural cycle of kinetic mechanisms. Oscillation between min and max activity.

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Enzyme Time Clock

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Many minutes to hours. Time for a protein to hydrolyze a bound regulatory ligand allowing enzyme to maintain active conformation, do its thing, de-hydrolyze, and become inactive.

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RNA transcription time clock

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A few minutes. Limited lifetime for messenger RNA to be translated.

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Isozomes/isoenzymes

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In evolution, genes can be duplicated and inserted in genome. Because only one copy is necessary, other copies can mutate and have other, sometimes good functions for the organism.

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Hexokinases

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glucose (or other hexose) + ATP = glucose-6-P + ADP.

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Globins as O2 Carriers

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Hb, Mb, Nb, Cg, and HbF are all isozomes and have specific, although similar funcitons, that allow them to do exactly what the body needs.