Subject: Why proteins are not branched?

The proteins are linear polymers of amino acids linked with each other by peptide bond, forming the so called primary structure. The primary structure can be presented as linear sequence of amino acid residues (from N- to C-terminus). That differs them from e.g. polisaccharides which often form 'branched' structure (e.g. glycogen). The principal condition of branching is the existence of more than two linking sites in the constituent of a polymer. However theoretically there is a possibility for proteins to be branched. There are amino acids possessing more than one carboxyl group (Asp, Glu) or amino group (Lys, Arg, ornitine) which makes possible the formation of additional direction of the polymerisation. The simplest example (although it is not a protein) is glutathione where peptide bond is formed by gamma-carboxyl group of glutamic acid.

Why then we do not observe proteins with branched primary structure, i.e. possessing more than one N-termini or C-termini? The restrictions resulting from translation process are not satisfactory explanations, since it could be possible during post-translational modifications. But the only inter-chain bonds that are present are disulfide bonds.

PS. I hope this time my question is clear enough.

Re: Why proteins are not branched?