A prosthetic group is the non-protein component of a conjugated protein, bound tightly to the polypeptide chain and essential to the protein’s function. Unlike a loosely associated cofactor, it stays attached through the protein’s working life. The haem group of haemoglobin and myoglobin is the classic example: an iron-containing porphyrin ring that carries oxygen while the surrounding protein provides the precise pocket it needs. Other examples include the lipid portions of lipoproteins, the carbohydrate chains of glycoproteins, and flavin groups in certain redox enzymes. In nutrition the concept matters because several B vitamins act as precursors of prosthetic groups or coenzymes, so deficiency in the vitamin directly disables the enzymes that depend on the group.