Modulating functional loop movements: the role of highly conserved residues in the correlated loop motions

Loop flexibility in enzymes plays a vital role in correctly positioning catalytically important residues. This strong relationship between enzyme flexibility and function provides an opportunity to engineer new substrates and inhibitors. It further allows the design of site-directed mutagenesis experiments to explore enzymatic activity through the control of flexibility of a functional loop. Earlier, we described a novel mechanism in which a small loop triggers the motions of a functional loop in three enzymes (beta-1,4-galactosyltransferase, lipase, and enolase) unrelated in sequence, structure, or function. Here, we further address the question of how the interactions between various flexible loops modulate the movements of the functional loop. We examine beta-1,4-galactosyltransferase as a model system in which a Long loop undergoes a large conformational change (moves in space up to 20 A) upon substrate binding in addition to a small loop (Trp loop) that shows a considerably smaller conformational change. Our molecular-dynamics simulations carried out in implicit and explicit solvent show that, in addition to these two loops, two other neighboring loops are also highly flexible. These loops are in contact with either the Long loop or the Trp loop. Analysis of the covariance of the spatial displacement of the residues reveals that coupled motions occur only in one of these two loops. Sequence analysis indicates that loops correlated in their motions also have highly conserved residues involved in the loop-loop interactions. Further, analysis of crystal structures and simulations in explicit water open the possibility that the Trp loop that triggers the movement of the Long loop in the unbound conformation may also play the same role in the substrate-bound conformation through its contact with the conserved and correlated third loop. Our proposition is supported by the observation that four of the five conserved positions in the third loop are at the interface with the Trp loop. Evolution appears to select residues that drive the functional Long loop to a large conformational change. These observations suggest that altering selected loop-loop interactions might modulate the movements of the functional loop.