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ellie3
lesson3
| Question | Answer |
|---|---|
| what components besides amino acid residues are sometimes necessary for an enzyme to be active? | COFACTORS & COENZYMES. which are non-protein compounds that bind to protein. They are helper molecules. They are classified depending on how tightly they bind to an enzyme. loosely-bound cofactors=COENZYMES and tightly-bound cofactors=PROSTHETIC GROUP |
| what are the functions of coenzymes? Note that many are derivatives of vitamins? | they act as carriers for specific functional groups |
| why are catalysts necessary for reactions in living systems to proceed at a useful rate? | because they provide a specific environment needed for a rxn to occur at a useful rate. under normal conditions, uncatalyzed rxns tend to be slow and many chemical processes are unfavorable are unlikely to occur in their given environment |
| what is the difference b/w a transition state and a reaction intermediate? | transient state=the precise point where decay to either S or P is equally likely-a rock at the top of a hill could roll either way.Rxn intermediates= molecular forms of C that're produced from the preceding R & then react w/ another R to form a final P |
| what affects the reaction equilibrium between S and P? what affects the reaction rate of the conversion from S and P? Which aspect of a reaction can an enzyme alter? | the difference in free energies of their ground state affects the rxn of quilibrium b/w S & P. The activation/energy barrier affects the rxn rate. Enzymes can alter rate of rxn but they do not alter equilibria |
| what does a large, positive K'eq (K'eq=1000) for a reaction mean in terms of the final relative concentrations of product and reactants? what does a very small K'eq (k"eq = .001) mean? | Large positive K'eq means G is negative and it is a favorable rxn. A small K'eq means G is positive and the Rxn is not favorable to proceed in a forward direction. |
| what does K'eq mean in terms of the standard free energy change of the reaction? how does it relate to the speed at which equilibrium is reached? | the standard free energy change simply means is K'eq is positive, G will be negative. K'eq does not relate to the speed at which equilibrium is reached. The rate of determined by the concentration of the reactant and by a rate constant. |
| in qualitative terms, what is the relationship b/w the rate constant K and the activation energy for an enzymatic reaction? | if the energy of activation decreases, the reaction rate increases |
| what is the specific source of energy for lowering the activation energy barriers in energy catalyzed reactions? | 1- covalent bond interactions b/w enzymes & substrates lower the activation energy (therefore accelerating the rxn) by providing an alternative low energy pathway 2-noncovalent interactions b/w E & Substrate -weak interactions release small amts of energy |
| why is it important for an enzyme to be complementary to the reaction transition state rather then the substrate? | b/c optimal interactions b/w substrate and enzyme occur only in the transition state. |
| what is one reason that some enzymes are very large molecules? | 1- b/c they are required for multiple weak interactions to drive catalysis. |
| how is binding energy important? | binding energy can be used to lower substrate entropy or to cause a conformational change in the enzyme (induced fit). Binding energy also accounts for the precise specificity of enzymes for their substrates. |
| describe 4 physical and thermodynamic barriers to reaction, and explain how enzymatic catalysis overcome them. | 1-entropy of mol. in SOL. reduces possibility that they'll react together 2- shell of H-bonded water that helps to stabilize most biomolecules 3- distortion of substrates 4- need for proper alignment. BINDING ENERGY CAN BE USED TO OVERCOME ALL BARRIERES |
| explain the difference b/w the "lock and key" hypothesis and the "induced fit" mechanism. | Lock & key model=that the S must perfectly fit the E, & the E does not change. Induced fit model is different B/C when the substrate fits together w/ the enzyme, the enzyme itself will change to either join substrates together or break a substrate down |
| when is general (as opposed to specific) acid-base catalysis observed? | it only occurs when the unstable reaction intermediate breaks down to reactants faster than protons can be transferred to or from water |
| why must the covalent bond formed b/w enzyme and substrate in covalent catalysis be transient? | to provide a new, lower-energy reaction path |
| in what ways can metal ions participate in catalysis? | interaction b/w enzyme ans substrate can help orient the substrate for reaction or stabilize charged rxns 2- metals can mediate oxidation-reduction rxns by reversibl changes in the metal ions oxidation state |
| what assumption concerning substrate concentration is made in the discussion of enzyme kinetics? why is this important? | At low S concentrations, V0 increases linearly with an increase in S. At high S conc., V0 increases by small amounts in response to increases in S |
| explain the effect of saturating levels of substrate on enzyme catalyzed reactions. | Vmax is when the enzyme is saturated with its substrate, so that further increases in S have no effect on rate. this happens when all the free enzyme has been converted to the ES form. AKA saturation kinetics |
| why is there no k-2 in the equation describing the reaction from E+S to E+P? | b/c early in the rxn, the conc. of the P is not important. So we make the assumption that the reverse reaction can be ignored. it simplifies the task at hand |
| what is the steady state assumption? | that the initial rate of rxn reflects a steady state in which ES is constant. The rate of formation of ES is equal to the rate of its breakdown |
| what does the michealis-menten equation describe? | it is a rate equation. It is written as Km. it is a quantitative relationship b/w the initial velocity (V0), the maximum velocity (Vmax) and the initial S concentration |
| when enzyme concentration is held constant, what is the relationship b/w V0 and [S] at low [S]? At high [S]? How does the michealis-menten equation illustrate these relationships mathematically? | at low S, where Km is greater then the S, the S becomes insignificant. V0=Vmax[S]/Km and V0 shows a linear dependence on S. At high S, where the S is greater then the Km, the Km becomes insignificant V0=Vmax & plateau is observed. |
| what are the 2 definitions for Km? what are the units? | Km is the rate constant or the michaelis constant. the units are in concentration. |
| under what conditions does Km represent a measure of affinity of the enzyme for the substrate? | when k2 is rate-limiting. k2 is less then k1 and Km reduces to k-1/k1. |
| what does the high Kcat value mean? What does a high Km value mean? | a high Kcat value means a high turnover number. it is the number of substrate molecules converted to product in a given unit of time on a single enzyme molecule when the enzyme is saturated with substrate. |
| why is Kcat/Km, the specificity constant, the most useful parameter for discussing catalytic efficiency? | b/c it allows us to evaluate the kinetic efficiency of enzymes. |
| what kind of information can be obtained by the study of pre-steady state kinetics that cannot be obtained by steady state kinetics alone? | the rate of many reaction steps can be measured independently and events during reaction of a single substrate molecule can be observed. we can gain a complete and quantitative picture of the energy changes during the rxn |
| why are inhibitors necessary as enzymatic control mechanisms in biological systems? | because they interfere with catalysis, slowing or halting enxymatic rxns. |
| explain how the different types of enzyme inhibition affect the apparent Km and Vmax. | |
| how would it be possible to distinguish experimentally b/w a competitive enzyme inhibitor and a mixed inhibitor? | |
| would you expect to find irreversible inhibitors as normal enzymatic control mechanisms in biological systems? | |
| explain how alterations in the surrounding pH can affect enzyme activity. | enzymes have an optimum pH at which activity of maximal. At higher or lower pH, activity recreases |
| why is regulatory enzyme very often the first enzyme in a multiple reaction sequence? | ss |
| what is the most significant difference in how the two major classes of regulatory enzymes are controlled? How are these enzymes similar? | ss |
| how are allosteric enzymes different from other enzymes? | ss |
| what are the general characteristics of feedback inhibition, as illustrated by the regulation of the conversion of L-threonine to L-isoleucoine? | dd |
| why is the term K0.5 (or S0.5), rather then Km, used to describe the S conc. that produces a half-maximal velocity in an allosteric enzyme catalyzed reaction? | dd |
| what functional groups other than -Po24- can be added to or removed from enzymes to turn them on or off? | dd |
| in what ways can phosphoryl groups interact with other groups on an enzyme? how can this affect catalysis? | dd |
| is glycogen phosphorylase made more or less active by the addition of phosphoryl groups? | dd |
| compare enzymes regulated by allosteric modulation, by reversible covalent modification, and by proteolytic cleavage with respect to how they are activated and unactivated. | dd |
| why is control of catalysis just as important as the fact of catalysis itself? |
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