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Chapter 8

BCM 475 Chapter Notes - Chapter 8: X-Ray Crystallography, Atp Hydrolysis, Enzyme Kinetics


Department
Biochemistry
Course Code
BCM 475
Professor
Welch
Chapter
8

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ENZYMES: BASIC CONCEPTS/KINETICS
Enzymes are special because of their catalytic power and specificity
Most enzymes are proteins
- Proteins are highly effective catalysts for a wide variety of chemical reactions because of their capcity to specifically bind a wide
range of molecules
Bring substrates (reactants) together in optimal orientation
Catalyze reactions by stabilizing transition states
o Ex. The enzyme carbonic anhydrase catalyzes CO2 hydration 107 times faster
Specificity of an enzyme is due to precise interaction of substrate with the enzyme
result of 3D structure of enzyme
- Ex: thrombin, trypsin catalyzes hydrolysis of certain peptide sequences only
Cofactors
- Small molecules that play a role in enzyme catalytic activity
- Able to execute chemical rxns that cannot be performed by the 20 aminos
Types:
1. Metals
2. Coenzymes (small organic molecules)
o Prosthetic groups - Tightly bound
remain bound for life of the protein
o Cosubstrates - Loosely bound
Bound and released with substrates
Enzymes Transform Energy | They play vital roles in energy transformation
Examples:
- ATP hydrolysis into gradient of Ca2+ across membrane
- Myosin converts ATP into muscle contraction
Enzymes Laws of Thermodynamics
Must consider
1) Free energy difference (G) between products/reactants
o Determines spontaneous or not
2) Energy required to initiate conversion of reactants products
o Determines rate of rxn
Rules of free-energy change
I. Rxn can occur spontaneously ONLY if G is negative exergonic
Endergonic = positive G, energy input required
II. System is a equilibrium and NO NET CHANGE can occur is G = 0
III. G depends only on free energy of products free energy of
reactants
(final initial)
mechanism/path of transformation is irrelevant
IV. G has nothing to do with rate of rxn
G depends on the nature of the reactants and their concentrations
G = 0 at equilibrium
- The criterion of spontaneity in a rxn is G, not G
- Reactions not spontaneous (based on G) can be made spontaneous by
adjusting reaction and product concentrations
Enzymes Alter Only Rxn Rate
Enzymes cannot alter the equilibrium of a chemical rxn
- Amount of product formed is same (whether enzyme is present or not) BUT: with enzyme, the process takes
seconds whereas without it, it might take hours/years.
o The equilibrium concentration of P is 100x greater than S despite enzyme presence
Enzymes accelerate the attainment of equilibria, but do not shift their positions.
o Equilibrium is a function ONLY of the free-energy difference between reactants and products
Apoenzyme + cofactor = holoenzyme
apoenzyme = enzyme without cofactor
holoenzyme = catalytically active enzyme
G = standard free energy change
R = gas constant
(8.315 x 10-3 kJ mol-1 deg-1)
T = absolute temp.
[A] [B] [C] [D] = molar conc. of rxts
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