CHE 350 Lecture Notes - Lecture 4: Stereospecificity, Carbonic Anhydrase, Conformational Change

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CH 11 – Enzymatic Catalysis
General Properties – Enzymes Differ from Ordinary Catalysts
1) Higher reaction rates
2) Milder reaction conditions
a. T below 100C, atm pressure, nearly neutral pH
b. Chemical catalysis = high T, extreme pH conditions
3) Greater reaction specificity
a. Vastly higher degree of specificity w/ respect to their substrates and products 
rarely have side products
4) Capacity for regulation
a. Catalytic activity of enzymes vary in response to concentrations other than their
substrates. Mechanisms = allosteric control, covalent mods of enzymes, variation
in amount of enzymes synthesized
Enzymes are Classified by the Type of Reaction they Catalyze
^
Lyases also do group eliminations to form double bonds
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Enzymes React on Specific Substrates
Substrates and other molecules bind to enzymes noncovalently
oNoncovalent forces = van der Waals, electrostatic, H-bonding, hydrophobic
interxns
Geometric complementarity = a substrate-binding site consists of an indentation or cleft
on the surface of an enzyme molecule that is complementary in shape to the substrate
Electronic complementarity = the amino acid residues that form the binding site are
arranged to specifically attract the substrate
Molecules that differ in shape or functional group distribution from the substrate cannot
productively bind to the enzyme
Induced fit = most of the substrate-binding sites of most enzymes are largely pre-formed
but undergo some conformational change on substrate binding
Enzymes are stereospecific = enzymes are highly specific both in binding CHIRAL
substrates and in catalyzing their rxns.
oThis stereospecificity arises because enzymes, by virtue of their inherent
chirality, form asymmetric binding-active sites
oRemember, proteins consist of only L-amino acids
Enzymes can distinguish b/w similar molecules because of their chirality  this gives way
to asymmetric binding
Nearly all enzymes that participate in chiral rxns are 100% stereospecific
A substance of the wrong chirality will not fit productively into an enzymatic binding site
Geometric specificity = most enzymes are quite selective about the identities of the
chemical groups on their substrates
oThis is a more stringent requirement than stereospecificity
oEnzymes vary considerably in their degree of geometric specificity
Most enzymes catalyze the rxns of a small range of related compounds
w/ different efficiencies
Ex: alcohol dehydrogenase
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Some Enzymes Require Cofactors
Functional groups of proteins are great for acid-base rxns, forming covalent bonds and
charge-charge interxn. Not so great at catalyzing oxidation-reduction rxns, and group-
transfer processes
Although enzymes catalyze such rxns, they can only do so in association w/ small
cofactors
oCofactors = act as the enzymes “chemical teeth”
1) Cofactors can be metal ions  Cu2+, Fe2+, Zn2+
a. Explains why we need trace amounts in diet, explains in part the toxic effect of
some heavy metals (Cd2+, Hg2+ can replace Zn2+, same periodic table group, in
the actual site of certain enzymes, rendering them inactive)
2) Can be organic molecules (= coenzymes)
a. Cofactors transiently associated with enzyme (cosubstrates) = NAD+, NADP+
(oxidizing agent in ADH)
b. Prosthetic groups = cofactors permanently associated with their protein, often
by covalent bonds
3)
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Document Summary

Mechanisms = allosteric control, covalent mods of enzymes, variation in amount of enzymes synthesized. Enzymes are classified by the type of reaction they catalyze. Lyases also do group eliminations to form double bonds. Substrates and other molecules bind to enzymes noncovalently: noncovalent forces = van der waals, electrostatic, h-bonding, hydrophobic interxns. Geometric complementarity = a substrate-binding site consists of an indentation or cleft on the surface of an enzyme molecule that is complementary in shape to the substrate. Electronic complementarity = the amino acid residues that form the binding site are arranged to specifically attract the substrate. Molecules that differ in shape or functional group distribution from the substrate cannot productively bind to the enzyme. Induced fit = most of the substrate-binding sites of most enzymes are largely pre-formed but undergo some conformational change on substrate binding. Enzymes can distinguish b/w similar molecules because of their chirality this gives way to asymmetric binding.

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