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.