On how the conformation of biliverdins influences their reduction to bilirubins: A biological and molecular modeling study

The cyclic 2,18-bridged biliverdin ( 2) is excreted in rat bile without reduction to the corresponding bilirubin. Conformational analysis, employing an optimized Monte Carlo method and a mixed Monte Carlo/stochastic dynamics, reveals that biliverdin IXα ( 1) and the cyclic analogue 2 adopt ‘lock washer’ conformations, stabilized by the presence of intramolecular hydrogen bonds between N 23…H 22N and, to a lesser extent, between N 23…H 24N. Although 2 is very similar in overall shape to 1, the former adopts a ‘locked lock washer’ conformation unable to undergo fluctuations, thus possibly hampering a proper recognition by biliverdin reductase. Bridged biliverdin 2 is excreted in bile without reduction to a bilirubin. Monte Carlo and molecular dynamics simulations support the view that 2, although very similar in overall shape to biliverdin IX α, adopts a ‘locked lock washer’ conformation unable to undergo fluctuations necessary to fit into the active site of biliverdin reductase.