Low-Barrier Hydrogen Bond
A Low-Barrier Hydrogen Bond ( LBHB , German: hydrogen bridge bond with low energy barrier ) is a special type of hydrogen bond . LBHB can occur when the p K a value of the two heteroatoms exactly matches, which allows the hydrogen atom between them to be used more evenly. This hydrogen exchange leads to the formation of particularly short, strong hydrogen bonds.
Standard hydrogen bonds are longer and the proton clearly belongs to one of the heteroatoms. If the p K a value of the heteroatoms is exactly the same, an LBHB at a shorter distance (≈ 2.55 Å) is possible. When the distance further decreases (<2.29 Å), the bond as is single-well - or short-strong hydrogen bond characterized.
Low-barrier hydrogen bonds occur in hydrophobic environments of proteins. Several amino acid residues work together in a charge transfer system (also called a proton relay) to control the p K a values of the residues involved. LBHB also occur on the surface of proteins, but are unstable due to their proximity to water and the conflicting requirements of strong salt bridges at protein-protein interfaces.
It has been suggested that low-barrier hydrogen bonds are relevant to enzyme catalysis in two circumstances. First, in a charge transfer system, an LBHB could activate a catalytic moiety within an active site (e.g., between an acid and base within a catalytic triad ). Second, the formation of an LBHB could form during catalysis to stabilize a transition state (e.g., with a substrate transition state in an oxyanion hole ). These two mechanisms are controversial, with theoretical and experimental evidence conflicting as to whether they occur at all.
Since the 2000s, there has been a general consensus that LBHB are not used by enzymes to aid catalysis. However, it was suggested in 2012 that LBHB is involved in the binding of phosphates to arsenates in the phosphate transport system (despite an arsenate-rich environment). This finding could indicate the possibility that, in very rare cases, LBHB may play a catalytic role in the selection of ion size.
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