Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

PROTEIN FUNCTION
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Table 3–2 Many Vitamin Derivatives Are Critical Coenzymes for Human Cells
Vitamin Coenzyme Enzyme-catalyzed reactions requiring these coenzymes
Thiamine (vitamin B 1 ) Thiamine pyrophosphate Activation and transfer of aldehydes
Riboflavin (vitamin B 2 ) FADH Oxidation–reduction
Niacin NADH, NADPH Oxidation–reduction
Pantothenic acid Coenzyme A Acyl group activation and transfer
Pyridoxine Pyridoxal phosphate Amino acid activation; also glycogen phosphorylase
Biotin Biotin CO 2 activation and transfer
Lipoic acid Lipoamide Acyl group activation; oxidation–reduction
Folic acid Tetrahydrofolate Activation and transfer of single carbon groups
Vitamin B 12 Cobalamin coenzymes Isomerization and methyl group transfers
employ tools to enhance and extend the capabilities of their hands, enzymes and
other proteins often use small nonprotein molecules to perform functions that
would be difficult or impossible to do with amino acids alone. Thus, enzymes frequently
have a small molecule or metal atom tightly associated with their active
site that assists with their catalytic function. Carboxypeptidase, for example, an
enzyme that cuts polypeptide chains, carries a tightly bound zinc ion in its active
site. During the cleavage of a peptide bond by carboxypeptidase, the zinc ion forms
a transient bond with one of the substrate atoms, thereby assisting the hydrolysis
reaction. In other enzymes, a small organic molecule serves a similar purpose.
Such organic molecules are often referred to as coenzymes. An example is biotin,
which is found in enzymes that transfer a carboxylate group (–COO – ) from one
molecule to another (see Figure 2–40). Biotin participates in these reactions by
forming a transient covalent bond to the –COO – group to be transferred, being
better suited to this function than any of the amino acids used to make proteins.
Because it cannot be synthesized by humans, and must therefore be supplied in
small quantities in our diet, biotin is a vitamin. Many other coenzymes are either
vitamins or derivatives of vitamins (Table 3–2).
Other proteins also frequently require specific small-molecule adjuncts to
function properly. Thus, the signal receptor protein rhodopsin, which is made by
the photoreceptor cells in the retina, detects light by means of a small molecule,
retinal, embedded in the protein (Figure 3–53A). Retinal, which is derived from
vitamin A, changes its shape when it absorbs a photon of light, and this change
causes the protein to trigger a cascade of enzymatic reactions that eventually lead
to an electrical signal being carried to the brain.
COOH
CH 2
COOH
CH 2
H 3 C
(A)
CH 3
CH 3
CH 3
H 3 C
CHO
H 3 C
H
H 2 C C
(B)
CH 2
N + N
Fe
N + N
CH 3
CH 2
HC
CH 2
CH 3
CH 3
Figure 3–53 Retinal and heme. (A) The
structure of retinal, the light-sensitive
molecule attached to rhodopsin in the eye.
The structure shown isomerizes when it
absorbs light. (B) The structure of a heme
group. The carbon-containing heme ring
is red and the iron atom at its center is
orange. A heme group is tightly bound
to each of the four polypeptide chains in
hemoglobin, the oxygen-carrying protein
whose structure is shown in Figure 3–19.