15 Enzymes
K. WILSON
15.1 Characteristics and nomenclature
15.2 Enzyme steady-state kinetics
15.3 Analytical methods for the study of enzyme reactions
15.4 Enzyme active sites and catalytic mechanisms
15.5 Control of enzyme activity
15.6 Suggestions for further reading
15.1 CHARACTERISTICS AND NOMENCLATURE
15.1.1 Specificity and nomenclature
Enzymes are nature’s biological catalystspossessing the ability to promote specific
chemical reactions under the mild conditions that prevail in most living organisms.
They are all proteins but range widely in their size from as few as 60–70 amino acid
residues as in RNase to as many as several thousand. Generally they are much larger than
their substrates and bind with them by means ofactive sitescreated by the specific
three-dimensional folding of the protein.Interaction of specific functional groups in a
small number of amino acid residues lining the active site with the substrate results in the
formation of atransition statefor which the activation energy barrier is significantly
reduced relative to the non-enzyme-catalysed reaction. As a result, the reaction rate is
increased by a factor of many millions relative to the uncatalysed reaction. Enzymes do
not alter the position of equilibrium ofreversible reactions that they catalyse but they
do accelerate the establishment of the position of equilibrium for the reaction.
Many enzymes are members of coordinated metabolic or signalling pathways that
collectively are responsible for maintaining a cell’s metabolic needs under varying
physiological conditions (Sections 15.5 and 17.4.5). The over- or under-expression of
an enzyme can lead to cell dysfunction which we may recognise as a particular disease
state. Enzyme inhibitors are widely used as therapeutic agents for the treatment of
such conditions (Sections 15.2 and 18.1). Organ damage, for example heart muscle as
a result of deprivation of oxygen following a heart attack, or the liver as a result of
chemical damage as in alcoholic cirrhosis, results in the release of cellular enzymes
into extracellular fluids and eventually into the blood. Such release can be clinically
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