Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Proteins and enzymes

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The end of polypeptide synthesis is signalled by a ter-

mination codon contacting the A site. Three prokaryotic

release factors (RF) are known: RF–1 is specific for termina-

tion codons UAA and UAG, while RF–2 is specific for UAA

and UGA. RF–3 stimulates RF–1 and RF–2, but does not in

itself recognize the termination codons. RF–3 also has GTPase

activity and appears to accelerate the termination at the

expense of GTP. Only one eukaryotic release factor is known

and it has GTPase activity.

At any one time, there can be several ribosomes posi-

tioned along the mRNA and thus initiation, elongation and ter-

mination proceed simultaneously on the same length of

mRNA. The three dimensional structure of the final protein

begins to appear during protein synthesis before translation is

completed. In many cases, after the synthesis of the amino

acid chain, proteins are subjected to further reactions which

convert them to their biologically active forms, e.g., by the

attachment of chemical groups or by removal of certain amino

acids—a processes known as post-translational modification.

See also Deoxyribonucleic acid (DNA); Genetic code;

Molecular biology and molecular genetics; Ribonucleic

acid (RNA)

PProteins and enzymesROTEINS AND ENZYMES

The building blocks of proteins and enzymesare molecules

formed by carboxyl acids attached to amino groups (–NH 2 ),

known as amino acids. Most protein structures consist of com-

binations of only about twenty of the most commonly found

amino acids.

Amino acids bind to each other to form peptides and

proteins. Conventionally, the term protein is used to designate

chains of several peptides, known as polypeptides, with a

molecular weight higher than thousands of Daltons. Peptides

with a biological function go in length from dipeptides and

tripeptides, up to polymers with thousands of Daltons.

Most proteins have well-defined structures and their spe-

cific biological functions depend upon the correct conforma-

Computer image of a protein moleucle, showing regions of different three-dimensional shape.

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