Microbiology and Immunology

Amino acid chemistry WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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ities, which he argued should be centered on basic research

rather than regulation; and finally his belief that the large pub-

lic expenditures incurred by the regulatory process hurt

American economic competitiveness.

Ames and his colleague Gold have also argued that the

use of bioassays (animal tests) of chemicals to predict their

carcinogenic potential in humans should be abandoned. In a

typical bioassay, rats are given a maximum tolerated dosage

(MTD) of a particular chemical daily for a period of time

(such as a year). The maximum tolerated dosage is as much as

the animal can be given without immediately becoming ill or

dying. At the end of the time period, the number of animals

that have developed cancers is tabulated as an indicator of the

cancer causing potential of the chemical being tested. Ames

suggested that it is often the large dosage itself, rather than the

nature of the particular chemical that induces the rat cancers.

He argued that, since humans are not normally exposed to

such large doses, the assays were not valid for predicting

human cancers.

Ames’s arguments have some support both within and

outside scientific communities. However, he also has numer-

ous critics. Those taking issue with his positions have noted

that pollution control, for example, involves far more than just

carcinogenicity. These critics suggest that Ames has not

offered a substitute for animal assays (the Ames test has not

proved to be such a substitute), and that neither he nor they

have a good idea of what goes on at low dosages. Some argue

that Ames has an over-simplified view of the regulatory

process, which is based on a consideration of animal assays

but also on other factors. It has also been argued that the dis-

covery that many naturally occurring chemicals have a high

mutagenic rate (just as synthetic chemicals) should not lead to

the conclusion that synthetic chemicals pose less risk than was

previously supposed. Such an assumption places too much

emphasis on mutagenic rate as a sole indicator of carcino-

genicity, ignoring the complex, multi-stage developmental

process of the disease.

Yet the disagreements between Ames and his critics are

based on several points of commonality—that cancer is a

complex multi-stage process that is not fully understood; that

there is no perfect test or group of tests that can fully predict

the potential carcinogenicity of many substances in humans;

and that public regulatory and environmental policies must be

made and carried out in spite of this deficiency of knowledge.

As for Ames, he has described his public-policy activism as a

hobby, and he has noted that his recent scientific work

includes studies in the biochemistry of aging.

Elected to the National Academy of Sciences in 1972,

Ames has received many awards, including the Eli Lilly

Award of the American Chemical Society (1964), the Mott

Prize of the General Motors Cancer Research Foundation

(1983), and the Gold Medal of the American Institute of

Chemists (1991). He is the author or coauthor of more than

250 scientific articles.

See also Chemical mutagenesis; Molecular biology and

molecular genetics

AAmino acid chemistryMINO ACID CHEMISTRY

Amino acids are the building blocks of proteins and serve

many other functions in living organisms. The prime function

of DNAis to carry the information needed to direct the proper

sequential insertion of amino acids into protein chain during

protein synthesis(translation).

An amino acid is a molecule that contains a terminal

acidic carboxyl group (COOH) and a terminal basic amino

group (NH 2 ). The approximately 20 amino acids (plus a few

derivatives) that have been identified as protein constituents

are alpha-amino acids in which the -NH 2 group is attached to

the alpha-carbon next to the -COOH group. Thus, their basic

structure is NH 2 CHRCOOH, where R is a side chain. This side

chain, which uniquely characterizes each alpha-amino acid,

determines the molecules overall size, shape, chemical reac-

tivity, and charge. There are hundreds of alpha-amino acids,

both natural and synthetic.

The amino acids that receive the most attention are the

alpha-amino acids that genes are codes for, and that are used

to construct proteins. These amino acids include glycine

NH 2 CH 2 COOH, alanine CH 3 CH (NH 2 ) COOH, valine

(CH 3 )2CHCH (NH 2 )COOH, leucine (CH 3 ) 2 CHCH 2 CH(NH 2 )

COOH, isoleucine CH 3 CH 2 CH(CH 3 )CH(NH 2 )COOH, methi-

onine CH 3 SCH 2 CH 2 CH(NH 2 )COOH, phenylalanine C 6 H 5 CH 2

CH(CH 2 )COOH, proline C 4 H 8 NCOOH, serine HOCH 2 CH

(NH 2 )COOH, threonine CH 3 CH(OH)CH(NH 2 )COOH, cys-

teine HSCH 2 CH(NH 2 )COOH, asparagine, glutamine H 2 NC

(O)(CH 2 )2CH(NH 2 )COOH, tyrosine C 6 H 4 OHCH 2 CHNH 2

COOH, tryptophan C 8 H 6 NCH 2 CHNH 2 COOH, aspartate

COOHCH 2 CH(NH 2 )COOH, glutamate COOH(CH 2 )2CH

(NH 2 )COOH, histidine HOOCCH(NH 2 )CH 2 C 3 H 3 H 2 , lysine

NH 2 (CH 2 ) 4 CH(NH 2 )COOH, and arginine (NH 2 )C(NH)

HNCH 2 CH 2 CH 2 CH(NH 2 )COOH.

Proteins are one of the most common types of mole-

cules in living matter. There are countless members of this

class of molecules. They have many functions from compos-

ing cell structure to enabling cell-to-cell communication. One

thing that all proteins have in common is that they are com-

posed of amino acids.

Proteins consist of long chains of amino acids con-

nected by peptide linkages (-CO.NH-). A protein’s primary

structure refers to the sequence of amino acids in the mole-

cule. The protein’s secondary structure is the fixed arrange-

ment of amino acids that results from interactions of amide

linkages that are close to each other in the protein chain. The

secondary structure is strongly influenced by the nature of the

side chains, which tend to force the protein molecule into spe-

cific twists and kinks. Side chains also contribute to the pro-

tein’s tertiary structure, i.e., the way the protein chain is

twisted and folded. The twists and folds in the protein chain

result from the attractive forces between amino acid side

chains that are widely separated from each other within the

chain. Some proteins are composed of two of more chains of

amino acids. In these cases, each chain is referred to as a sub-

unit. The subunits can be structurally the same, but in many

cases differ. The protein’s quaternary structure refers to the

spatial arrangement of the subunits of the protein, and

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