A conjugated protein is a protein that is permanently joined to a non-protein component. The non-protein part is called a prosthetic group. The protein part on its own usually cannot do its job; the prosthetic group is essential for the protein's function.The best-known example is haemoglobin, the oxygen-carrying protein in red blood cells. Haemoglobin is a globular protein with a quaternary structure made of four polypeptide chains: two α-globin chains and two β-globin chains. Each chain is folded around a haem group, the prosthetic group. Every haem group contains an iron ion (Fe2+), and each iron ion can bind one oxygen molecule reversibly.Because there are four haem groups, one haemoglobin molecule can carry a maximum of four oxygen molecules (eight oxygen atoms).
Worked example: a sample contains 5000 haemoglobin molecules, all fully saturated. Each carries 4 O2, so the sample carries 5000 × 4 = 20 000 oxygen molecules.Other conjugated proteins are named after their prosthetic group:• glycoproteins have a carbohydrate group (for example many cell-surface receptors and the mucin in mucus)
• lipoproteins have a lipid group (they carry cholesterol and triglycerides in the blood)
• many enzymes have prosthetic groups; catalase, for instance, also contains haem.Without its haem groups, the globin protein could not transport oxygen, which shows why the prosthetic group matters.
Worked example: a sample contains 5000 haemoglobin molecules, all fully saturated. Each carries 4 O2, so the sample carries 5000 × 4 = 20 000 oxygen molecules.Other conjugated proteins are named after their prosthetic group:• glycoproteins have a carbohydrate group (for example many cell-surface receptors and the mucin in mucus)• lipoproteins have a lipid group (they carry cholesterol and triglycerides in the blood)
• many enzymes have prosthetic groups; catalase, for instance, also contains haem.Without its haem groups, the globin protein could not transport oxygen, which shows why the prosthetic group matters.