Salt bridge alternatives for PEPCK polymorphic site Asp/Glu 503. Models are of the cytosol form in lid-closed conformation. Color codes as for Figure 6. a) allele SDI, no salt bridge; b) allele SDV, salt bridge between the carboxyl of Asp 503 and a nitrogen of the guanidino side chain of nearby Arg 272; c), allele SEI, salt bridge between Glu 503 carboxyl and the guanidino side chain of Arg 506, near Asp/Glu 503 in the same α-helix, as shown in coarser scale by Figure 5b. Arg 272's side chain can sometimes hydrogen-bond to the backbone carbonyl of Glu 503 (not shown).
Watt et al. BMC Evolutionary Biology 2013 13:9 doi:10.1186/1471-2148-13-9