d) Classify proteins into three broad groups, with one example from each group.
ANS- Sometimes also called heteroproteins, they contain in their structure a non-protein portion. Three examples are glycoproteins, chromoproteins, and phosphoproteins.
Glycoproteins--They are proteins that covalently bind one or more carbohydrate units to the polypeptide backbone.
Typically, the branches consist of not more than 15-20 carbohydrate units, where you can find arabinose, fucose (6-deoxygalactose), galactose, glucose, mannose, N-acetylglucosamine (GlcNAc, or NAG), and N-acetylneuraminic acid (Neu5Ac or NANA).
Examples of glycoproteins are:
glycophorin, the best known among erythrocyte membrane glycoproteins;
fibronectin, that anchors cells to the extracellular matrix through interactions on one side with collagen or other fibrous proteins, while on the other side with cell membranes;
all blood plasma proteins, except albumin;
immunoglobulins or antibodies.
Chromoproteins--They are proteins that contain colored prosthetic groups.
Typical examples are:
hemoglobin and myoglobin, which bind, respectively, one and four heme groups;
chlorophylls, which bind a porphyrin ring with a magnesium atom at its centre;
rhodopsins, which bind retinal.
Phosphoproteins--They are proteins that bind phosphoric acid to serine and threonine residues.
Generally, they have a structural function, such as tooth dentin, or reserve function, such as milk caseins (alpha, beta, gamma and delta), and egg yolk phosvitin.
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