conjugated protein

Meaning: A conjugated protein is a polypeptide (the apoprotein) joined to a non-amino-acid part, the prosthetic group. The two together form the active holoprotein. The prosthetic group is usually bound tightly, often covalently or with very high affinity, and it supplies chemistry the 20 amino acid side chains cannot easily provide, such as binding O₂ or carrying out redox reactions.

Common classes, named by the attached group: Glycoproteins carry carbohydrates (cell-surface receptors, antibodies). Lipoproteins carry lipids (LDL, HDL). Metalloproteins contain metal ions (zinc in many enzymes). Hemoproteins contain heme (hemoglobin, myoglobin, cytochromes). Nucleoproteins are bound to nucleic acids (histones with DNA, ribosomes). Phosphoproteins carry phosphate groups.

On the MCAT: A passage may describe a protein that loses activity when a bound group is removed, or ask which protein type fits a given group. Hemoglobin is the classic example: four heme groups, each with Fe²⁺ that binds one O₂. Apoprotein without its group is inactive; holoprotein is active. Questions may also link glycosylation or lipid attachment to localization on membranes or secretion.

Common confusion: A prosthetic group is not the same as a loosely bound coenzyme or a substrate. Prosthetic groups stay attached through the reaction cycle, whereas many coenzymes (such as NAD⁺) bind transiently. Also, a conjugated protein is not the same as a protein with multiple subunits; a multimeric protein made only of amino acids is not conjugated.