The interdigitating loop of the enolase superfamily as a specificity binding determinant or 'flying buttress'

Enzymes of the enolase superfamily (ENS) are mechanistically diverse, yet share a common partial reaction (abstraction of the α-proton from a carboxylate substrate). While the catalytic machinery responsible for the deprotonation reaction has been conserved, divergent evolution has led to numerous ENS members that catalyze different overall reactions. This rich functional diversity has made the ENS an excellent model system for developing the approaches necessary to validate enzyme function. However, enzymes of the ENS also share a common bidomain structure ((β/α)₇β-barrel domain and α+β capping domain) which makes validation of function from structural information challenging.

This review presents a comparative survey of the structural data obtained over the past decade for enzymes from all seven subgroups that comprise the ENS.

Of the seven ENS subgroups (enolase, mandelate racemase (MR), muconate lactonizing enzyme, β-methylaspartate ammonia lyase, d-glucarate dehydratase, d-mannonate dehydratase (ManD), and galactarate dehydratase 2), only enzymes of the MR and ManD subgroups exhibit an additional feature of structural complexity-an interdigitating loop. This loop emanates from one protomer of a homodimeric pair and penetrates into the adjacent, symmetry-related protomer to either contribute a binding determinant to the active site of the adjacent protomer, or act as a 'flying buttress' to support residues of the active site.

The analysis presented in this review suggests that the interdigitating loop is the only gross structural element that permits functional distinction between ENS subgroups at the tertiary level of protein structure.