Cellular Structure, Proteins, and Metabolism
71
may be present in the same cell. The chemical specifi cities of the
phosphoprotein phosphatases are broader; a single enzyme can
dephosphorylate many different phosphorylated proteins.
An important interaction between allosteric and cova-
lent modulation results from the fact that protein kinases are
themselves allosteric proteins whose activity can be controlled
by modulator molecules. Thus, the process of covalent modu-
lation is itself indirectly regulated by allosteric mechanisms.
In addition, some allosteric proteins can also be modifi ed by
covalent modulation.
In Chapter 5 we will describe how cell activities can be
regulated in response to signals that alter the concentrations
of various modulator molecules. These modulator molecules,
in turn, alter specifi c protein activities via allosteric and cova-
lent modulations.
Table 3–4
summarizes the factors infl uenc-
ing protein function.
Table 3–4
Factors that Infl uence Protein Function
I
.
CHANGING
PROTEIN
SHAPE
a.
Allosteric modulation
b.
Covalent modulation
i. Protein kinase activity
ii. Phosphoprotein phosphatase activity
II
.
CHANGING
PROTEIN
CONCENTRATION
a.
Protein synthesis
b.
Protein degradation
SECTION D
Enzymes and Chemical Energy
Thus far, we have discussed the synthesis and regulation of
proteins. In this section, we describe some of the major func-
tions of proteins, specifi cally those that relate to facilitating
chemical reactions.
Thousands of chemical reactions occur each instant
throughout the body; this coordinated process of chemical
change is termed
metabolism
(Greek, change). Metabolism
involves the synthesis and breakdown of organic molecules
required for cell structure and function and the release of
chemical energy used for cell functions. The synthesis of
organic molecules by cells is called
anabolism,
and their
breakdown,
catabolism.
For example, the synthesis of a
triglyceride is an anabolic reaction, whereas the breakdown
of a triglyceride to glycerol and fatty acids is a catabolic
reaction.
The body’s organic molecules undergo continuous trans-
formation as some molecules are broken down while others of
the same type are being synthesized. Molecularly, no person
is the same at noon as at 8 o’clock in the morning because
during even this short period, some of the body’s structure
has been broken down and replaced with newly synthesized
molecules. In a healthy adult, the body’s composition is in a
Regulation of Binding Site Characteristics
I. Protein function in a cell can be controlled by regulating either
the shape of the protein or the amounts of protein synthesized
and degraded.
II. The binding of a modulator molecule to the regulatory site on
an allosteric protein alters the shape of the functional binding
site, thereby altering its binding characteristics and the activity
of the protein. The activity of allosteric proteins is regulated by
varying the concentrations of their modulator molecules.
III. Protein kinase enzymes catalyze the addition of a phosphate
group to the side chains of certain amino acids in a protein,
changing the shape of the protein’s functional binding site and
thus altering the protein’s activity by covalent modulation. A
second enzyme is required to remove the phosphate group,
returning the protein to its original state.
SECTION C KEY TERMS
affi nity
67
allosteric modulation
69
allosteric protein
69
binding site
66
chemical specifi city
66
competition
68
cooperativity
69
covalent modulation
70
functional site
69
ligand
66
modulator molecule
69
phosphoprotein
phosphatase
70
phosphorylation
70
protein kinase
70
regulatory site
69
saturation
67
SECTION C REVIEW QUESTIONS
1. List the four characteristics of a protein-binding site.
2. List the types of forces that hold a ligand on a protein surface.
3. What characteristics of a binding site determine its chemical
specifi city?
4. Under what conditions can a single binding site have a
chemical specifi city for more than one type of ligand?
5. What characteristics of a binding site determine its affi nity for
a ligand?
6. What two factors determine the percent saturation of a binding
site?
7. How is the activity of an allosteric protein modulated?
8. How does regulation of protein activity by covalent
modulation differ from that by allosteric modulation?
SECTION C SUMMARY
Binding Site Characteristics
I. Ligands bind to proteins at sites with shapes complementary to
the ligand shape.
II. Protein-binding sites have the properties of chemical
specifi city, affi nity, saturation, and competition.
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