Conjugated proteins and their non-protein components
A conjugated protein is an amino acid chain joined to a non-protein part, and the type of that part names the protein.
Remember
A conjugated protein is named for its non-protein partner, and the prefix tells you which. Lipo- stands for lipid, glyco- for carbohydrate and phospho- for phosphate.
A conjugated protein is a protein with a non-protein component closely attached to its amino acid chain, and that component contributes to the protein's structure or function. When the partner is bound tightly, it is called a prosthetic group. A protein made of amino acids alone is a simple protein, even if it has several subunits (separate polypeptide chains).
- Lipoproteins carry lipid, held to the protein by non-covalent interactions (weaker attractions rather than shared electrons). LDL and HDL are examples.
- Glycoproteins carry carbohydrate chains, covalently linked to the protein (a covalent link is a shared pair of electrons). Immunoglobulins (antibodies) are examples.
- Phosphoproteins have a phosphate group covalently linked to the protein, as in casein in milk.
- Haemoglobin carries haem, an iron-containing prosthetic group.
A lipoprotein is a roughly spherical particle that carries fats through the blood, which is a watery fluid. Its core holds triglycerides (the main dietary fats) and cholesterol esters (cholesterol joined to a fatty acid). A surface layer of phospholipids surrounds the core and carries the apolipoproteins, which are the protein part. This water-friendly coat lets the particle circulate in plasma.
Key facts
- Conjugated protein
- a protein with a non-protein component closely attached to its amino acid chain
- Simple protein
- made of amino acids alone, even if it has several subunits
- Lipoproteins
- lipid held to the protein by non-covalent interactions, as in LDL and HDL
- Glycoproteins
- carbohydrate covalently linked to the protein, as in the immunoglobulins
- Phosphoproteins
- a phosphate group covalently linked to the protein, as in casein in milk
- Haemoglobin
- carries haem, an iron-containing prosthetic group
Exam traps
- Haemoglobin is a conjugated protein, but its prosthetic group is haem, an iron-containing group, and not carbohydrate, so it is not a glycoprotein.
- A protein made only of amino acids stays a simple protein even when several polypeptide chains come together, because having several subunits does not decide the category.
- Immunoglobulins do not carry haem, because they are glycoproteins that carry carbohydrate chains, while haem is the iron-containing group of haemoglobin.
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