BIOINORGANIC CHEMISTRY A Short Course Second Edition

44 BIOCHEMISTRY FUNDAMENTALS

with the substrate (substance acted upon) and bind it. Some enzymes require
nonprotein molecules or groups, called cofactors , for their activity. Cofactors
may be metal ions such as Fe 2+,3+ , Cu 1+,2+ , Ni 1+,2+,3+ , or Co 1+,2+,3+ , and so on. Cofac-
tors may also be organic molecules such as nicotinamide adenine dinucleotide
(NAD + ), shown in Figure 2.11. These organic molecules are also called coen-
zymes. An enzyme without its cofactor, called an apoenzyme , usually will be
inactive in its catalytic role or exhibit greatly reduced catalytic activity. An
active enzyme with its cofactor is called aholoenzyme. Enzymes that require
metal ions as cofactors are calledmetalloenzymes. Examples of metalloen-
zymes to be discussed in later chapters include metal cations from periodic
chart Groups 1 and 2, which are extremely important in biological systems.
These metal ions — sodium and potassium in Na + /K + - ATPases; magnesium in
kinases, ATP pumps, and chlorophyll; calcium in P - type ATPases; and enzymes
like troponin and calmodulin — will be discussed in Chapters 5 and 6. Enzymes
that contain Fe 2+ /Fe 3+ and Cu 2+ /Cu + ions — cytochromes, cytochrome c oxidase,
aconitase — will be discussed in Chapter 7. Essential cofactors and coenzymes
that must be ingested in the diet include water - soluble vitamins such as the
cobalt - containing vitamin B 12. Isozymes may be structurally different forms of
the same enzyme or oligomeric proteins with differing sets of subunits. Iso-
zymes may also be defi ned as multiple forms of an enzyme whose synthesis is
controlled by more than one gene. Usually, isozymes catalyze similar types of
reactions. Allosteric enzymes have their activity modulated by the binding of
a second molecule, whereas nonallosteric enzymes do not. Proteins maintain
biological structures; that is, the protein collagen constitutes part of fi brous
connective tissues in skin, bone, tendon, cartilage, blood vessels, and teeth.
Proteins may facilitate movement; that is, the proteins actin and myosin
mediate muscle contraction. Proteins serve as storage compounds; that is, the

Figure 2.11 Cofactor nicotinamide adenine dinucleotide (NAD + ).

N

N N

N

NH 2

O

OH OH

H H

H

CH 2

H

P

O

O O-

P

O

O O-

O

O

OH OH

H H

H H

CH 2

N

C

O

NH 2

+