Formation of an unusually short hydrogen bond in photoactive yellow protein

The photoactive chromophore of photoactive yellow protein (PYP) is p-coumaric acid (pCA). In the ground state, the pCA chromophore exists as a phenolate anion, which is H-bonded by protonated Glu46 (O-Glu46-O-pCA= similar to 2.6 angstrom) and protonated Tyr42. On the other hand, the O-Glu46-O-pCA H-bond was unusually short (O-Glu46-O-pCA= 2.47 angstrom) in the intermediate pR(CW) state observed in time-resolved Laue diffraction studies. To understand how the existence of the unusually short H-bond is energetically possible, we analyzed the H-bond energetics adopting a quantum mechanical/molecular mechanical (QM/MM) approach based on the atomic coordinates of the PYP crystal structures. In QM/MM calculations, the O-Glu46-O-pCA bond is 2.60 angstrom in the ground state, where Tyr42 donates an H-bond top. In contrast when the hydroxyl group of Tyr42 is flipped away from pCA, the H-bond was significantly shortened to 2.49 angstrom in the ground state. The same H-bond pattern reproduced the unusually short H-bond in the pR(CW) structure (O-Glu46-O-pCA= 2.49 angstrom). Intriguingly, the potential-energy profile resembles that of a single-well H-bond, suggesting that the pK(a) values of the donor (Glu46) and acceptor (pCA) moieties are nearly equal. The present results indicate that the equal pK(a) requirement for formation of single-well or low-barrier H-bond (LBHB) is satisfied only when Tyr42 does not donate an H-bond to pCA, and argue against the possibility that the O-Glu46-O-pCA bond is an LBHB in the ground state, where Tyr42 donates an H-bond to pCA (C) 2012 Elsevier B.V. All rights reserved.