Glycyl residues in proteins and peptides: An analysis

Glycyl residue, simplest of all the residues, is well known for its conformational freedom. An analysis of the conformational and structural aspects of this residue occurring in proteins and peptides has been made making use of the Ramachandran (ϕ, ψ) angles and their distribution. The chief observations are: (i) By and large, there is no bias for an amino-acid residue to precede or succeed Gly. (ii) The conformational points show clustering in the 'bridge' region. (iii) While in general, glycyl residue plays a passive role when it occurs in a helix and helps in helix propagation, it also acts as a helix breaker in some instances. (iv) It is a poor former of extended strands. (v) The conformational freedom is effectively used by Gly to prefer those positions in turns that are less favourable for non-glycyl residues. (vi) Analysis of doublet data reinforces the propagative tendency of glycyl residues. X-Gly doublet is a better turn former than Gly-Y doublet, (vii) Only one third of the glycyl residues are situated on the surface of the proteins. The results can be useful in modelling studies on proteins.