Globular and Fibrous Proteins

Fibrous Proteins

• Little or no tertiary structure.

• Long parallel polypeptide chains.

• Cross linkages at intervals forming long fibres or sheets.

• Usually insoluble.

• Many have structural roles.

• E.g. keratin in hair and the outer layer of skin, collagen (a connective tissue).

Globular Proteins

• Have complex tertiary and sometimes quaternary structures.

• Folded into spherical (globular) shapes.

• Usually soluble as hydrophobic side chains in centre of structure.

• Roles in metabolic reactions.

• E.g. enzymes, haemoglobin in blood.

Haemoglobin

An example of a globular protein

• Reddish-purple oxygen carrying pigment found in red blood cells.

• Made up of 4 polypeptide chains.

• 2 identical alpha-chains and 2 identical beta-chains.

• Nearly spherical - hydrophobic side chains point inwards.

• Outward pointing hydrophilic side chains maintain solubility.

• Each polypeptide chain contains a haem group.

• Haem group is a prosthetic group (i.e. an important permanent part of a protein molecule which is not made from amino acids) - when combined with 4 polypeptide chains it forms a conjugated protein.

• Haem group has an iron ion (Fe²⁺) at its centre.

• The iron combines with oxygen at high oxygen concentrations and releases oxygen at low oxygen concentrations.

• One haemoglobin molecule can carry 4 oxygen molecules.

• Colour is bright red when combined with oxygen / purplish if not.

Collagen

An example of a fibrous protein

• Found in skin, tendons, cartilage, bones, teeth, walls of blood vessels.

• Structural protein in most animals.

• Collagen molecule made of 3 polypeptide chains, each in the shape of a helix (not as tightly wound as an alpha-helix).

• Each chain contains 1000 amino acids - every third amino acid is glycine (the smallest amino acid).

• 3 helical chains wind around each other to form a three-stranded 'rope'.

• Glycine allows the 3 polypeptides to lie close together to form a tight coil (any other amino acid would be too big).

• 3 strands held together by H bonds.

• Each 3 stranded molecule interacts with others next to it. Bonding causes fibres to form.

• The ends of parallel molecules are staggered, preventing weak areas from running across the collagen fibre.

• Very strong - one quarter the tensile strength of mild steel.

Source:
www.kscience.co.uk/.../proteins/globular_and_fibrous_proteins.doc