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Chapter 6

CHY 204 Chapter Notes - Chapter 6: Charles-Adolphe Wurtz, Weak Interaction, Non-Covalent Interactions


Department
Chemistry
Course Code
CHY 204
Professor
Mario Estable
Chapter
6

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CHAPTER 6: ENZYMES
-Enzymes are catalyctic, have a high degree of specificity for substrates, accelerate velocity
of rxn & function in aw solns along very mild conditions of temp & pH.
6.1 Intro to Enzymes
-with the exception of a small group of catalyctic RNA/ribozymes, all enzymes are pro.
Typically large w/ MW ranging from 12,000 to 1 million (12,000 / 110 =109 AA; 1 million / 110
= 9000 AA). Some enzymes only req aa residues for activity but other req additional
components such as cofactors—either one of more inorganic elements (Fe2+, Mg2+, Mn2+
& Zn2+) or coenzymes which are complex of organic/metalo-organic molecules. Coenzymes
act as transient carriers of specific func groups (5’-deozyadenosylcobalamin/coenzyme B12
transfers H atoms & akyl group; precursor is vit B12) mostly derived from vitamins & organic
nutrients req in small amts in diet. Some enzymes both req a coenzyme & another metal ions
for activity; a coenzyme/metal ion that is tightly bound to the enzyme pro is called prosthetic
group. A holoenzyme is a complete, catalytically active enzyme together w/ bound coenzyme
&/ metal ions; the pro part of it is called apoenzyme or apopro. Some enzymes are modified
by phosphorylation, glycosylation, etc & many of these processes are involved in regulation
of enzyme activity. Zn2+ is a cofactor for alcohol dehydrogenase.
-Enzymes are classified by the reactions they catalyze: Nomenclature involves adding –ase
to name of substrate or phrase describing activity, unrelated common names & EC (enzyme
commission). Most enzymes catalyze the transfer of e-s, atoms & func groups thus are
classified, give code #s & assigned names according to the type of transfer rxn, group donor
& group acceptor. The 4 digit # enzyme commission: 1) designates class, 2) subclass, 3)
specifics about rxn, 4) more specifics. Eg.
ATP + glucose ADP + D-glucose-6-phosphate
-Formal name is glucose 6 transferase. E.C.#2.7.1.1.: 2 designates class name
(transferases), 7 designates subclass (phosphotransferase), 1 designates specifics (OH
acceptor), 1 designates more specifics (P group acceptor D-glucose). Common name is
hexokinase.
6.2 How enzymes work
-Enzyme catalyzed rxns occur w/in confines of a pocket on enzyme called active site. The
substrate is the molecule bound in the active site & acted upon by the enzyme. Enzyme
substrate complex was 1st proposed by Charles Adolphe Wurtz.
-Enzymes inc rxn rates: E + S ESEP E + P –Transient transition state (E,S,P:
enzyme, substrate, product). Any rxn such as SP can be described by a reaction
coordinate diagram where free energy of system is plotted against progress of rxn. G is free
energy change, G° is std free energy change which is temp of 298K, partial pressure of
each gas 1 atm (101.3kPa), [ ] of each solute 1M, G’^0 is biochemical G° at pH 7, G is
activation energy & Gb is binding energy. The ground state is the starting pt for either
forward/reverse rxn. Enzymes do not affect equilibrium, only the rates of rxn; the equilibrium
b/w SP reflects the diff in free energies in their ground state (free energy of P at ground
state is lower than that of S thus G’^0 is negative & equilibria favours P. Enyzmes affect
rates of rxns by dec activation energy which is the diff b/w ground state & transition state (not
a chemical species but a fleeting molec moment in which events, eg. Bond breakage, bond
formation & charge development have proceeded to the precise pt at which decay to either
substrate/product is equally likely). The role of enzymes is to accelerate interconversion of
S&P w/o getting used up & w/o affecting equilibria but rxn reaches equilibria faster when
appropriate enzyme is present b/c rate of rxn is inc.
-Eg. C12H22O11 + 12 O2 12 CO2 + 11 H2O: Has a --G’^0, doesn’t occur w/o
enzymes to catalyze rxn by dec the a.e. Reaction intermediates is any species on the rxn
pathway that has finite chemical lifetime; these intermediates occupy valleys in the diagram
thus the interconversion of 2 sequential rxn intermediates is a rxn step. The rate limiting step
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