ineffectual at high substrate concentrations, so it is termed a
Such an enzyme is often the F
rst enzyme in a series
(f g. 4.6)
. This position is important because an intermediate
product of the pathway might accumulate if an enzyme occu-
pying another position in the sequence were rate limiting.
Often the product of a metabolic pathway inhibits the rate-
limiting regulatory enzyme. This type of control is an example
of negative feedback. Accumulating product inhibits the path-
way, and synthesis of the product falls. When the concentra-
tion of product decreases, the inhibition lifts, and more product
is synthesized. In this way, a single enzyme can control a
whole pathway, stabilizing the rate of production (F g. 4.6).
To Chapter 1, Homeostasis, page 9.
Cofactors and Coenzymes
An enzyme may be inactive until it combines with a nonpro-
tein component called a
which helps the active site
attain its appropriate shape or helps bind the enzyme to its
enzyme. Often sequences of enzyme-controlled reactions,
lead to synthesis or breakdown
of particular biochemicals
(f g. 4.5)
. Hundreds of different
types of enzymes are present in every cell.
Enzyme names are often derived from the names of their
substrates, with the sufF
added. ±or example, a lipid-
splitting enzyme is called a
a protein-splitting enzyme
and a starch (amylum)-splitting enzyme is an
is an enzyme that splits the sugar
splits the sugar maltose, and
the sugar lactose.
Regulation of Metabolic Pathways
The rate at which a metabolic pathway functions is often
determined by a regulatory enzyme that catalyzes one of its
steps. The number of molecules of such a regulatory enzyme
is limited. Consequently, these enzymes can become satu-
rated when the substrate concentration exceeds a certain
level. Once this happens, increasing the substrate concentra-
tion no longer affects the reaction rate. The enzyme becomes
An enzyme-catalyzed reaction. (Many enzyme-catalyzed reactions, as depicted here, are reversible.) In the forward reaction (dark-
shaded arrows), (
) the shapes of the substrate molecules F
t the shape of the enzyme’s active site. (
) When the substrate molecules temporarily
combine with the enzyme, a chemical reaction occurs. (
) The result is a product molecule and an unaltered enzyme. The active site changes shape
somewhat as the substrate binds, such that formation of the enzyme-substrate complex is more like a hand F
tting into a glove, which has some
exibility, than a key F
tting into a lock.
A negative feedback mechanism may control a rate-limiting enzyme in a metabolic pathway. The product of the pathway inhibits the
A metabolic pathway consists of a series of enzyme-controlled reactions leading to formation of a product.