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Lecture 6

Biology Lecture 6 Outcomes

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Biology 1002B
Tom Haffie

Lecture 6 Role of an Enzyme (enzymes are proteins that proteins catalyst)  Speed up reactions but don’t get consumed  Can’t provide energy to a reaction  Can’t change direction of reaction  Can’t change delta G of reaction Energy Profile of Exergonic reaction  Delta G <0  Products have less free energy  Have to acquire transition state o where bonds are ready to break o gets energy from environment  Activation energy: amount of energy needed to get to the transition point o Represents a barrier, prevents reaction from going very fast  Ex. Propane thermodynamically unstable, but kinetically stable- doesn’t have a tendency to combust with out help Enzymes Lower activation energy  Lower Ea  Makes reactions faster o More molecules have energy to get to transition state  Doesn’t change delta G  Only changes the path along which reaction takes place Why does life need enzymes: we need to get reactions to go faster but heat destroys cells Enzyme Structure:  Enzyme is very big, substrate is very small  Catalysis takes place in activation site of enzyme  Active site: specific part of enzyme where substrates bind o Becomes apparent only after it fold to it native conformation  Fundamental: must be flexible, induced fit upon substrate binding o Induced fit  E + S   ES  E + P  First arrow = substrate binding  Second arrow= catalysis  Measuring rate of catalysis: measure rate at which product is made or rate at which reactants disappear o Often the products are a different colour and amount of colour is proportional to how much product is made But how do enzymes lower Ea  Precise orientation of two substrates o Increases probability that two substrates will interact in the correct orientation  Charge interactions o Presents a specific charge environment that reaction might need  Conformational strain o Maybe substrates need to be strained, to strain the bonds  Catalytic site mimics transition state without energy input Enzyme Kinetics  Enzyme concentration is constant  Reaction eventually saturates  Enzyme can only turn over substrate so fast, adding any more substrate won’t make a difference  Turnover rate: how fast an enzyme can process a substrate  Vmax: maximum rate of enzyme o Saturation rate: depends on Vmax  Km: substrate concentration that g
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