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TRANSCRIPT
Enzymes, the
Catalysts of Life
Chapter 6
Becker’s The World of Cell
Activation Energy and the
Metastable State
While thermodynamics allows us to assess
the feasibility of a reaction, it says nothing
about the likelihood that the reaction will
actually occur at a reasonable rate in the
cell.
For a given chemical reaction to occur in
the cell, substrates must reach the
transition state, which has a higher free
energy than either the substrates or
products. Reaching the transition state
requires the input of activation energy.
Activation Energy and the
Metastable State
Because of this activation energy barrier,
most biological compounds exist in an
unreactive, metastable state. To ensure
that the activation energy requirement is
met and the transition state is achieved, a
catalyst is required, which is always an
enzyme in biological systems.
Activation Energy and the
Metastable State
Enzymes as Biological Catalysts
Catalysts, whether inorganic or organic,
act by forming transient complexes with
substrate molecules that lower the
activation energy barrier and rapidly
increase the rate of the particular
reaction.
Chemical reactions in cells are catalyzed
by enzymes, which in some cases require
organic or inorganic cofactors for activity.
The vast majority of enzymes are proteins,
but a few are composed of RNA and are
known as ribozymes.
Enzymes as Biological Catalysts
Enzymes are exquisitely specific, either for
a single specific substrate or for a class of
closely related compounds. This is
because the actual catalytic process
takes place at the active site—a pocket
or groove on the enzyme surface that
only the correct substrates will fit into.
Enzymes as Biological Catalysts
Binding of the appropriate substrate at the active site causes a change in the shape of the enzyme and substrate known as induced fit. This facilitates substrate activation, often by distorting one or more bonds in the substrate, by bringing necessary amino acid side chains into the active site, or by transferring protons and/or electrons between the enzyme and substrate.
Enzymes as Biological Catalysts
Enzyme Kinetics
■ An enzyme-catalyzed reaction
proceeds via an enzyme substrate
intermediate. Most reactions follow
Michaelis–Menten kinetics, characterized
by a hyperbolic relationship between the
initial reaction velocity v and the substrate
concentration [S].
■ The upper limit on velocity is called Vmax,
and the substrate concentration needed to
reach one-half of this maximum velocity is
termed the Michaelis constant, Km. The
hyperbolic relationship between v and [S]
can be linearized by a double-reciprocal
equation and plot, from which Vmax and
Km can be determined graphically.
Enzyme Kinetics
■ Enzyme activity is sensitive to
temperature, pH, and the ionic
environment. Enzyme activity is also
influenced by substrate availability,
products, alternative substrates, substrate
analogues, drugs, and toxins, most of which
have an inhibitory effect.
Enzyme Kinetics
■ Irreversible inhibition involves covalent
bonding of the inhibitor to the enzyme
surface, permanently disabling the enzyme.
A reversible inhibitor, on the other hand,
binds noncovalently to an enzyme in a
reversible manner, either at the active site
(competitive inhibition) or elsewhere on the
enzyme surface (noncompetitive inhibition).
Enzyme Kinetics
Enzyme Regulation
■ Enzymes must be regulated to adjust their
activity levels to cellular needs. Substrate-
level regulation involves the effects of
substrate and product concentrations on
the reaction rate. Additional control
mechanisms include allosteric regulation
and covalent modification.