411U2 - ENZYMES: PROTEINS THAT ARE CATALYSTS I. Catalytic...

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ENZYMES: PROTEINS THAT ARE CATALYSTS I. Catalytic Power- ability to increase reaction rates A. Most reactions in biological systems occur slowly in the absence of catalysts; for practical purposes the rate is zero. Reactions are not at equilibrium except as enzymes move them toward EQ. B. Fast enzyme? catalase: each molecule catalyzes 10,000,000 times per second C. Rate may be > 10 7 X as fast as uncatalyzed.
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Specificity toward substrates ( reactants ) • A given enzyme will catalyze a single reaction (or, in some cases, a single type of reaction). To be so specific, it must "recognize" its substrates and not interact strongly with the other molecules it encounters. The specificity is determined by complimentary 3-D shape (or “fit”: square peg can't fit round hole) AND reversible interactions. (fig 13-1, p460)
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Substrate Specificity Examples: 1. There are a variety of proteolytic enzymes, which catalyze hydrolysis of peptide bonds of other proteins (as in digestion of food). Some of these will cleave bonds only on a particular side of a particular type of AA residue, others between two particular residues. 2. Most enzymes that act on AAs are specific for the L isomer. Beyond this, enzymes are often stereospecific in that they will specifically remove one of 2 "equivalent" groups from a symmetric molecule.
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Regulation: Control of Enzyme Activity If all molecules of a given enzyme are inactivated, reaction rate approaches zero. If all are activated (or not deactivated), reaction proceeds toward EQ at high rate. We shall see that usually control of one reaction in a pathway results in control of all of them. Types: 1. Allosteric : a. as with Hb: small indicator molecules b. Regulatory proteins are themselves regulated and then act to control other proteins. 2. Covalent modification : often a hydroxyl containing residue (ser, thr, tyr) will be phosphorylated at its –OH group. This results in stabilization of one form of the protein, as in CO 2 effect on Hb. 3. Proteolytic activation: the enzyme is synthesized in inactive form, becomes active after a segment of its polypeptide chain is removed. (common for food digesting proteases)
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• Enzymes interconvert forms of energy. • Living things are "chemical engines". • Foods are energy fuels; the energy released when they are metabolized is used as mechanical (muscle contraction), osmotic (and nutrient transport), chemical (biosynthesis), etc. energy.
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Energy: Thermodynamics A. Second Law: For all spontaneous processes, the entropy of the universe increases. In biosynthetic processes the entropy decreases, but this is offset by a larger increase in the entropy of surroundings. (ΔS (universe) = ΔS (system) + ΔS (surroundings) ) B. The above statement of the Second Law is difficult to use practically: 1. It can be converted mathematically to the following result: a reaction occurs spontaneously only if ΔG < 0. It is at equilibrium if ΔG = 0
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411U2 - ENZYMES: PROTEINS THAT ARE CATALYSTS I. Catalytic...

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