118
UNIT ONE
ineffectual at high substrate concentrations, so it is termed a
rate-limiting enzyme.
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).
RECONNECT
To Chapter 1, Homeostasis, page 9.
Cofactors and Coenzymes
An enzyme may be inactive until it combines with a nonpro-
tein component called a
cofactor
which helps the active site
attain its appropriate shape or helps bind the enzyme to its
enzyme. Often sequences of enzyme-controlled reactions,
called
metabolic pathways,
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
x
-ase
added. ±or example, a lipid-
splitting enzyme is called a
lipase,
a protein-splitting enzyme
is a
protease,
and a starch (amylum)-splitting enzyme is an
amylase.
Similarly,
sucrase
is an enzyme that splits the sugar
sucrose,
maltase
splits the sugar maltose, and
lactase
splits
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
Product molecule
Unaltered
enzyme
molecule
Enzyme-substrate
complex
Active site
(a)
(b)
(c)
Substrate molecules
Enzyme
molecule
FIGURE 4.4
An enzyme-catalyzed reaction. (Many enzyme-catalyzed reactions, as depicted here, are reversible.) In the forward reaction (dark-
shaded arrows), (
a
) the shapes of the substrate molecules F
t the shape of the enzyme’s active site. (
b
) When the substrate molecules temporarily
combine with the enzyme, a chemical reaction occurs. (
c
) 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.
Inhibition
Substrate
1
Substrate
2
Enzyme B
Substrate
3
Enzyme C
Substrate
4
Enzyme D
Product
Rate-limiting
Enzyme A
FIGURE 4.6
A negative feedback mechanism may control a rate-limiting enzyme in a metabolic pathway. The product of the pathway inhibits the
enzyme.
Substrate
1
Enzyme A
Substrate
2
Enzyme B
Substrate
3
Enzyme C
Substrate
4
Enzyme D
Product
FIGURE 4.5
A metabolic pathway consists of a series of enzyme-controlled reactions leading to formation of a product.
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