The alphabet of intrinsic disorder: II. Various roles of glutamic acid in ordered and intrinsically disordered proteins

Uversky, Vladimir N

Although some amount of glutamic acid residues is crucial for the structure and function of ordered proteins/domains, when a protein or a peptide contains a large number of glutamic acid residues and, as a consequence, possesses a small number of hydrophobic residues, it is likely to be disordered at physiological pH due to strong charge-charge repulsion and weak hydrophobic attraction. An illustrative example of such charge-infused proteins is Glurich human prothymosin α, in which 64 out of 111 residues are charged (there are 19 Asp, 35 Glu, 2 Arg and 8 Lys residues), the overall content of hydrophobic residues (Leu, Ile and Val) is very low, and aromatic residues (Trp, Tyr, Phe and His) and cystein are absent. 133 Based on this amino acid composition, it was not a big surprise to find that prothymosin α behaved as a highly disordered coil-like chain, since one cannot expect that a highly charged polypeptide (that contains 60% of Glu+Asp residues) will have a strong tendency to fold under physiological conditions. 133, 134 The lack of stable structure also explains the extreme thermal and acid stability of prothymosin α, since one cannot break what is non-existent. 133 The peculiar amino acid composition of prothymosin α, this biologically active random coil, was one of the defining factors behind the charge-hydropathy plot (CH-plot) development. 5 In fact, based on the analysis of prothymosin α and of 90 other non-globular proteins that lacked almost any ordered secondary structure under physiological conditions in vitro, it was concluded that a combination of high net charge and low hydropathy represents the necessary and sufficient factor for a polypeptide to behave as a natively unfolded protein. 5 Strategically positioned glutamic acid residues can modulate conformational stability and function of ordered proteins too. In fact, the role of a glutamic/aspartic acid cluster located outside the Ca 2+ -binding site, and of the N-terminal Glu1 residue in destabilizing the structure and weakening the calcium-binding capabilities of α-lactabumin has been already discussed (see above). 121, 125 Therefore based on these observations, protein regions and whole proteins enriched in glutamic acids are expected to be substantially disordered.