9-28-09_kinetics3A

9-28-09_kinetics3A - In all science error precedes the...

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In all science, error precedes the truth, and it is better it should go first than last. Hugh Walpole Not all chemicals are bad. Without chemicals such as hydrogen and oxygen, for example, there would be no way to make water, a vital ingredient in beer. Dave Barry
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Ways that enzymes promote passage through intermediate state Substrate alignment Alter local charge environment Mechanical stress/conformational changes Noncovalent bonds critical in enzyme-substrate interactions
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How do we quantify reaction rates? The study of reaction rates is called kinetics Can measure [product] as a function of time to determine empirically the reaction rate, or reaction velocity, v, = number of reactions per enzyme molecule per second Vmax is the maximum velocity at a saturating [substrate] The [substrate] at which v=Vmax/2 is called the Michaelis constant, Km
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Lineweaver-Burk plots
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Competitive inhibitors Bind in active site of enzyme Reversible and compete with substrate Often a slight variant of the actual substrate, so can still be recognized by enzyme Captopril is a competitive inhibitor of angiotensin converting enzyme, was adapted from pit viper venom, and lowers blood pressure Enzyme Competitive Inhibitor + Enzyme - Inhibitor Complex
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Non-competitive inhibitors Typically does not bind at the active site Alters the active site conformation to make ES → E+P less likely The substrate can still bind to produce ES (maybe shorter duration or weaker fit) Enzyme Noncompetitive Inhibitor + Enzyme - Inhibitor Complex Altered Active Site
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Kinetic effects of inhibitors Vmax Km Active site Competitive Same Increases Occupied Non-competitive Decreases Same Not occupied
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Determination of the IC50 for an inhibitor IC50 is the [inhibitor] at which 50% of enzymatic activity has been inhibited Often plotted on a logarithmic scale Often sigmoidal function such as that shown above. It is important to use enough inhibitor concentrations to provide well- defined top and bottom plateau values. http://www.ncgc.nih.gov/guidance/section4.html
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Allosteric enzymes: “It is through cooperation , rather than conflict, that your greatest successes will be derived: - Anonymous Michaelis-Menten kinetics assumes no cooperativity, meaning binding of a substrate to one binding site should have no effect on the activity of neighboring sites. This assumption is often not true. An allosteric protein is defined as a protein containing two or more topologically distinct binding sites which interact functionally with each other . If binding of substrate to one binding site increases the activity of neighboring sites, the term positive cooperativity is used. If binding of substrate to one binding site decreases the activity of neighboring sites, the term negative cooperativity is used.
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Mechanisms of Allostery The concerted, or symmetry hypothesis Based on the idea that an allosteric enzyme consists of identical
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