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So the rate of a reaction can be limited by the

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Unformatted text preview: tion potential of Fe++ Ferredoxin- an electron carrier Note how enzymes resemble mineral surfaces, in a new matrix. Not all enzymes require cofactors! Enzymes R-groups can participate in chemical reactions with the substrate Enzymes can simply orient/bend substrates R-groups’ charge can facilitate a reaction Many of the minerals and cofactors critical for catalysis may also be clues to how catalysis arose -Perhaps originally simple minerals were catalysts Some cofactors may have been adaptors that enhance or restrict this catalytic activity Amino acids (and later proteins) could be new developments that enhance specificity for particular substrates. Without a catalystRate (v) is rate constant (k) x [S] = k[S] Enzymes add another step to the reaction: S + E <-> ES <-> E + P The substrate has to bind to the catalytic surface. So the rate of a reaction can be limited by the availability of active sites. (note this axis is not [P]!!!) Rather than S <-> P [S] (note this axis is not ;me!!!) Note also, in these discussions we’ re talking about a specific experimental setup We measure initial reaction rate in a reaction that has only substrate, not product. How would you measure dependence of reaction rate on [S]? Add some enzyme to a little tube with a certain amount of S, Measure how much product is made in 1st minute of the reaction Toss the tube. Try again with a little higher concentration of S. Why does the enzyme-catalyzed r x reach a maximum rate? Initial reaction rate Why does the catalyzed reac5on rate fla[en out at high [S]? a) The back reac5on starts up b) The reac5on can only go so fast ­ that’s the top speed c) The enzyme is too busy to cope with addi5onal substrate kf When [S] >> [E], the velocity simply reflects the catalytic rate of the enzymeincreasing substrate concentration doesn’ t increase velocity, because the substrate’ s stuck waiting for an available enzyme… Reaction without enzyme: S (substrate) -> P (product) With a req...
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