My reference book (Princeton Review for SAT Chemistry Subject Test) mentions that:
Proteins and carbohydrates are both polymers; however, only carbohydrates commonly form branched polymers. Glycogen and cellulose are both carbohydrate polymers made up of glucose monomers; glycogen is a highly branched polymer while cellulose is primarily straight-chained.
But I don't get why proteins cannot form branched polymers. Proteins have both an amino group and a carboxylic acid group, and I understand that the two groups join together with the elimination of a water molecule during polymerization.
Branched polymers are formed when a hydrogen atom (or any other substituent) is replaced by another monomer unit, and I don't see why this can't take place in the case of proteins. Could someone please explain?