Globular and Fibrous Proteins

Globular Proteins

• Molecule forms a coiled shape (globule)
• Hydrophobic groups point into centre of molecule away from water
• Only hydrophilic groups are exposed outside the molecule so globular proteins are soluble
• Globular proteins have roles in metabolic reactions:
  • Enzymes - catalyse metabolic reactions
  • Haemoglobin - binds to oxygen to transport it around body

Haemoglobin

Haemoglobin has a quaternary structure made up of 4 separate polypeptide chains:

• 2 identical alpha -chains with 141 amino acids each
• 2 identical beta -chains with 146 amino acids each
• Each polypeptide chain is folded/coiled into a compact shape due to hydrophobic interactions between the (hydrophobic) R groups
• All 4 polypeptide chains are linked to form a roughly spherical haemoglobin molecule
• The precise 3D-shape of the Haemoglobin molecule is absolutely critical to it's Oxygen-carrying function
• The Hydrophilic R-groups are arranged around the outside of the molecule which allows Haemoglobin to mix with the watery medium inside red blood cells
• Attached to each polypeptide chain is a prosthetic HAEM group with an Fe2+ ion
• Each Fe2+ ion can combine with one O₂ molecule
• Human haemoglobin has four polypeptide chains and four haem groups and can therefore carry 4 x O₂ molecules
• When haemoglobin is bound to oxygen it is called oxyhaemoglobin and the colour changes from purplish red to bright red
• The 3D-shape of globular proteins is critical to their function – slight changes can have radical effects – eg in sickle cell anaemia one amino acid change causes a shape change in the molecule that in turns reduces the ability of haemoglobin to bind to oxygen and changes the shape of the whole red blood cell from a biconcave disk to a sickle shape. Severe sickle cell anaemia can be fatal.

Fibrous Proteins

• Polypeptides form long chains running parallel to each other
• These chains are linked by disulphide cross bridges – making the proteins very stable and strong
• Fibrous proteins have Structural functions:
  • Keratin in skin and hair
  • Collagen - found in bone, cartilage, tendons and ligaments for tensile strength

Collagen

Collagen is the most abundant protein in the animal kingdom. It is found in many diverse organisms and organs:

• in humans in tendons, the walls of blood vessels, cartilage, bone, gums
• sea anemones
• egg cases of dogfish

Collagen is strong but still flexible – this is important in tendons which cannot be rigid

The Primary structure of collagen

• a repeat sequence of 3 amino acids glycine-proline-X (any other amino acid)

The Secondary/Tertiary Structure of Collagen

• glycine is the smallest amino acid and this together with proline allow the polypeptide chain to be wound into a tightly coiled, straight and unbranched helix

The Quaternary Structure of Collagen

• 3 of these helical polypeptides are wound around each other and held by H-bonds.
• The triple stranded molecules run parallel to others and disulphide cross-linkages between the R-groups of the amino acid lysine holds the molecules together forming fibres
• The ends of the collagen molecules are staggered to avoid weak points – giving collagen ¼ the tensile strength of steel