Chapter 6 Flashcards
With the exception of a few classes of catalytic RNA molecules
(Chapter 26), enzymes are
proteins. Their catalytic activity
depends on the integrity of their native protein conformation. If
an enzyme is denatured or dissociated into its subunits, catalytic
activity is usually lost. The catalytic activity of each enzyme is
intimately linked to its primary, secondary, tertiary, and
quaternary protein structure.
description of enzyme add on things- pg 732
kk
Finally, some enzyme proteins are
modified covalently by
phosphorylation, glycosylation, and other
processes. Many of these alterations are involved in the
regulation of enzyme activity
urease catalyzes hydrolysis of
urea
Life depends on
powerful and specific catalysts: enzymes.
Almost every biochemical reaction is catalyzed by an enzyme.
With the exception of a few catalytic RNAs, all known enzymes
are
proteins. Many require nonprotein coenzymes or cofactors
for their catalytic function.
Enzymes are classified according to
the type of reaction they
catalyze. All enzymes have formal E.C. numbers and names, and
most have trivial names
good pic on pg 766
kk
Enzymes are highly effective catalysts, commonly
enhancing
reaction rates by a factor of 10
5
to 10
17
. Enzyme-catalyzed
reactions are characterized by the formation of a complex
between substrate and enzyme (an ES complex). Substrate
binding occurs in a pocket on the enzyme called the active site.
The function of enzymes and other catalysts is to
lower the
activation energy, ΔG‡
, for a reaction and thereby enhance the
reaction rate. The equilibrium of a reaction is unaffected by the
enzym
Reaction equilibria are described by
equilibrium constants, Keq
, which are related to the biochemical standard free-energy
change ΔG′°. Reaction rates are described by rate constants, k,
which are related to the activation energy ΔG
The extraordinary rate accelerations provided by
enzymes are
due to noncovalent binding energy supplemented by covalent
interactions or metal ion catalysis.
Noncovalent binding energy, ΔGB, is maximized in
the
transition state of the catalyzed reaction. Enzyme–transition state
complementarity is a fundamental principle of enzymatic
catalysis. Noncovalent interactions may facilitate the path to the
transition state, offsetting the energy required for activation,
ΔG‡
, by lowering substrate entropy, causing substrate
desolvation, or causing a conformational change in the enzyme
(induced fit). Binding energy also accounts for the exquisite
specificity of enzymes for their substrates
General acid-base catalysis and covalent catalysis mechanisms
contribute to
the catalytic power of enzymes. Covalent
interactions between the substrate and the enzyme, group
transfers to and from the enzyme, and interactions with metal
ions can provide a new, lower-energy reaction path.
To understand the complete mechanism of action of a purified
enzyme, we need to identify all substrates, cofactors, products,
and regulators. We also need to know
(1) the temporal sequence
in which enzyme-bound reaction intermediates form, (2) the
structure of each intermediate and each transition state, (3) the
rates of interconversion between intermediates, (4) the structural
relationship of the enzyme to each intermediate, and (5) the
energy that all reacting and interacting groups contribute to the
intermediate complexes and transition states. There are still only
a few enzymes for which we have an understanding that meets all
these requirements
