Cellular Structure, Proteins, and Metabolism
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
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-
summarizes the factors inﬂ uenc-
ing protein function.
Factors that Inﬂ uence Protein Function
i. Protein kinase activity
ii. Phosphoprotein phosphatase activity
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
Thousands of chemical reactions occur each instant
throughout the body; this coordinated process of chemical
change is termed
(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
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
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
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
chemical speciﬁ city
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
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
6. What two factors determine the percent saturation of a binding
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.