Download CATALYSIS OF BIOCHEMICAL REACTIONS

CATALYSIS OF BIOCHEMICAL REACTIONS
Proteins control the metabolism of a cell by catalyzing
specific chemical reactions
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Energetics of a typical chemical reaction
Model reaction: A and B can be transformed into C
and D by rearranging electrons, thus covalent bonds,
upon collision.
A+B
------>
(reactants, substrates)
C+D
(products)
1. A and B together have some potential energy (in
chemical bonds) and kinetic energy (in motion).
2. A and B collide; collision distorts or stresses bonds
to the point where they can rearrange electrons;
generally, this requires more potential energy (since
without stress, one expects electrons to find low
energy, stable state): extra energy is “activation
energy”.
3.Complex decays to give different (or same)
products.
What is wrong with this figure (Fig. 6.8)?
Rates of uncatalyzed reactions:
Rate depends on probabilities of:
! Collision
! Correct orientation
! Activation energy
! Rearrangement
Rate will be slow for most uncatalyzed reactions,
because:
! Concentrations are low
! Temperature is low
Enzyme-catalyzed reaction
Enzyme: protein molecule with a three-dimensional
shape that specifically promotes the chemical reaction
of interest
1. A,B, and E (enzyme catalyst) have some potential
energy and kinetic energy.
2. A,B, and E all collide, substrates A and B fitting
into the active site of E: kinetic energy of collision (or
of later collisions by solvent molecules) provides
potential energy to stress bonds sufficiently for
rearranging.
3. Complex decays to products C and D and
unchanged (or regenerated) E.
Rates of enzyme-catalyzed reactions:
Rate still depends on probabilities of:
(a) Collision
(b) Correct orientation
(c) Activation energy
(d) Rearrangement
Enzyme promotes (a), (b), and/or (c).
(a) “cage effect”: enzyme forms “cage” around both
substrates, increasing collision time.
(b) enzyme correctly orients positions of substrates
as part of the binding process.
(c) enzyme lowers activation energy needed for
reaction by distorting bonds of substrates
! active site sterically strains the substrate,
and/or
! electric charges in active site relocate
electrons of substrates, and/or
! amino acid side chains in active site react
covalently with substrate.
Examples:
Orientation
Strain
Charge effects
An example
of an enzyme
that sterically
strains the
substrate:
Lysozyme
distorts the bonds
of one of the
sugars in the
polysaccharide
of a bacterial
cell wall
It also places a
partial charge on
the substrate,
making it react
more easily
with water
(hydrolysis).
Hydrolysis
breaks the
polysaccharide
chain and
weakens
the wall so
that the cell
lyses.
The enzyme hexokinase changes shape when it
binds to the substrate, glucose, increasing the "cage
effect" and bringing other effects into play.
For some enzymes, a material in the solution different
from the substrate can interact with an enzyme’s
active site (or other site) and inhibit the enzyme’s
activity.
Some forms of inhibition are irreversible: e.g., the
covalent attachment of the compound DIPF to an
essential amino acid in the active site of the enzyme
trypsin.
Summary
Enzyme proteins catalyze reactions in cells. Because
of the variety of possible proteins, it is possible for
different proteins with different active sites to catalyze
all the various metabolic reactions in the cell and thus
control the chemical activity in the cell.
If enzymes lower the activation energy of most
reactions in the cell, will they not reduce the stability
of the cell?
(a) yes, and it is a good thing (if cytoplasm is too
stable, it is as good as frozen)
(b) no, because enzyme activity can be turned on and
off as the cell requires
(c) yes and no: both (a) and (b) have germs of truth