Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Amino acid chemistry

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describes how the subunits pack together to create the overall

structure of the protein.

Even small changes in the primary structure of a protein

may have a large effect on that protein’s properties. Even a

single misplaced amino acid can alter the protein’s function.

This situation occurs in certain genetic diseases such as sickle-

cell anemia. In that disease, a single glutamic acid molecule

has been replaced by a valine molecule in one of the chains of

the hemoglobin molecule, the protein that carries oxygen in

red blood cells and gives them their characteristic color. This

seemingly small error causes the hemoglobin molecule to be

misshapen and the red blood cells to be deformed. Such red

blood cells cannot distribute oxygen properly, do not live as

long as normal blood cells, and may cause blockages in small

blood vessels.

Enzymesare large protein molecules that catalyze a

broad spectrum of biochemical reactions. If even one amino

acid in the enzyme is changed, the enzyme may lose its cat-

alytic activity.

The amino acid sequence in a particular protein is deter-

mined by the protein’s genetic code. The genetic code resides

in specific lengths (called genes) of the polymer doxyribonu-

cleic acid (DNA), which is made up of from 3000 to several

million nucleotide units, including the nitrogeneous bases:

adenine, guanine, cytosine, and thymine. Although there are

only four nitrogenous bases in DNA, the order in which they

appear transmits a great deal of information. Starting at one

end of the gene, the genetic code is read three nucleotides at a

time. Each triplet set of nucleotides corresponds to a specific

amino acid.

Occasionally there an error, or mutation, may occur in

the genetic code. This mutation may correspond to the substi-

tution of one nucleotide for another or to the deletion of a

nucleotide. In the case of a substitution, the result may be that

the wrong amino acid is used to build the protein. Such a mis-

take, as demonstrated by sickle cell anemia, may have grave

consequences. In the case of a deletion, the protein may be

lose its functionality or may be completely missing.

The twenty most common amino acids.Illustrations reprinted by permission of Robert L. Wolke.

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