Proteins: Polypeptides of

Defined Sequence

WHAT ARE PROTEINS?

Proteins are not just

polypeptides,

they are polypeptides of defined

sequence.

- complex natural compound: a complex

natural substance that has a globular or

fibrous structure composed of linked amino

acids.

Proteins are essential to the structure and

function of all living cells and viruses.

Meaning, every protein has a

defined order of amino acid residues.

As with the nucleic acids, this

sequence is referred to as the primary

structure of the protein.

When a living cell makes protein, the

carboxyl group of one amino acid is linked to

the amino group of another to form a peptide

bond. The carboxyl group of the second

amino acid is similarly linked to the amino

group of a third, and so on, until a long chain

is produced.

This chainlike molecule, which

may contain from 50 to several

hundred amino acid subunits,

is called a polypeptide. A

protein may be formed of a

single polypeptide chain, or it

may consist of several such

chains held together by weak

molecular bonds.

Each protein is formed according to a precise

set of instructions contained within the

nucleic, which is the genetic material of the

cell. These instructions determine which of the

20 standard amino acids are to be

incorporated into the protein, and in what

sequence.

The R groups of the amino acid

subunits determine the final shape of

the protein and its chemical

properties; an extraordinary variety

of proteins can be produced from the

same 20 subunits.