(n) the structure and function of globular proteins including a conjugated…

The idea

Globular proteins fold so that hydrophobic residues cluster in the core and hydrophilic residues face the aqueous surroundings, giving a compact, roughly spherical shape that is soluble. The precise three-dimensional shape comes from the primary sequence and is held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions; this shape creates a specific active site or binding site, so function depends on structure. Examples include enzymes, haemoglobin and antibodies. A conjugated protein contains a non-protein prosthetic group. Haemoglobin is the standard example: four polypeptide chains each associated with a haem group containing an iron ion (Fe²⁺) that binds oxygen. The prosthetic group is essential to function, so removing it destroys oxygen transport.

What earns the marks

• Globular proteins have a compact, roughly spherical shape with hydrophobic residues in the core and hydrophilic residues on the surface, making them soluble.
• The tertiary structure is held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions, determined by the primary sequence.
• The specific folded shape creates a precise active site or binding site, so structure relates directly to function.
• A conjugated protein contains a non-protein prosthetic group permanently associated with the polypeptide.
• In haemoglobin each of the four polypeptide chains is associated with a haem group containing an Fe²⁺ ion that binds oxygen.
• The prosthetic group is required for function; without the haem group the protein cannot transport oxygen.

Common mistakes and how to put them right

• Describing globular proteins as insoluble and fibrous.

  Put it right: Globular proteins are compact and soluble because hydrophilic groups face the water.

• Treating the prosthetic group as a separate protein.

  Put it right: It is a non-protein group attached to the polypeptide, such as the haem group in haemoglobin.

• Saying shape is irrelevant to function.

  Put it right: The specific folded shape forms the active or binding site, so loss of shape (denaturation) loses function.

Exam tips

• Always link a named structural feature to a named function rather than listing bonds alone.
• For conjugated proteins, name the prosthetic group and state what it does, e.g. haem binds oxygen via Fe²⁺.
• Use haemoglobin as your worked example and be ready to explain why its quaternary structure and prosthetic groups matter.