Fibrous and Globular Proteins

Fibrous and Globular Proteins

Proteins are necessary for function of nearly all forms of life on

this earth. They consist of one or several long chains (polypeptides)

of amino acids linked in a characteristic sequence. This sequence is

called the primary structure of the protein. These polypeptides may

undergo coiling to for an alpha helix, or pleating to forma beta

pleated sheet, the nature and extent of which is described as the

secondary structure. The three-dimensional shape of the coiled or

pleated polypeptides is called the tertiary structure. Quaternary

structure specifies the structural relationship of the component

polypeptides.

Proteins may be broadly classified into two categories; globular

proteins and fibrous proteins. Globular proteins have compact rounded

molecules and are usually water-soluble. They are particularly

important when in the form of enzymes, as they catalyse most

biochemical reactions. Other globular proteins include the antibodies,

which combine with foreign substances in the body to prevent infection

and illness; the carrier proteins, such as haemoglobin; the storage

proteins (e.g. casein in milk and albumin in egg white), and certain

hormones (e.g. insulin).

Haemoglobin is one of a group of globular proteins which occurs widely

in animals as oxygen carriers in blood. Vertebrate haemoglobin

comprises two pairs of polypeptide chains, known as a-chains and

b-chains (forming the globin protein), with each chain folded to

provide a binding site for a haem group. Each of the four haem groups

binds one oxygen molecule to form oxyhaemoglobin. The haemoglobin

molecule is a tetramer consisting of 4 p...

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...re bound

together to form fibrils, which have great strength and limited

elasticity. Collagen accounts for over 30% of the total body protein

of mammals.

Collagen is first synthesized as a large precursor protein containing

over 1400 amino acids. Only about 1000 of these are present in a

mature collagen fibril. However, other parts of the protein are

essential for its secretion from the cell and for forming the triple

helix. A short "signal" sequence moves the protein across the

membranes that will form secretion vesicles. On either side of the

helix-forming region are two sections that interact with similar

regions of two other collagen chains to align the three chains for

formation of the triple helix. After the helix has

been secreted by the cell, these alignment sequences are removed from

the protein.