Correlating interfacial water dynamics with protein-protein interaction in complex of GDF-5 and BMPRI receptors.

GDF-5 mediated signal transduction regulating chondrogenesis and skeletogenesis involves three different type-I receptors viz. Act-RI, BMPRIA and BMPRIB. BMPRIA and BMPRIB generally shows temporal and spatial co-expression but some spatially different expression pattern has also been observed. BMPRIA receptor is the key receptor implicated in BMP signalling during osteogenesis and is expressed in osteoblasts during the course of bone formation. However, BMPRIB appears to be primarily expressed in mesenchymal pre-cartilage condensations and also found in differentiated osteoblast and chondrocytes. The extracellular pH affects bone cell function and it is experimentally known that mineralization of bone is affected by shift of pH in cultured osteoblast. Here we report the effect of pH on dynamics of water present at the interface of GDF-5:BMPRIA and GDF-5:BMPRIB and binding interaction energy of these complexes. Water dynamics at different pH was analysed using residence time and hydrogen bond relaxation kinetics. pH influences the interaction energy between GDF-5 and BMPRIA and BMPRIB receptors indicating the electrostatic environment modulating the activity of two receptors. This pH dependence of interaction energy is further supported by similar behaviour of hydrogen bond existence of buried water molecules at the interface. In contrast to this the slow and fast exchanging water molecules do not show similar pH dependence of hydrogen bonding relaxation kinetics. Hence; we conclude that only buried water molecule at the interface influences the protein-protein interaction and the electrostatic environment of the extracellular fluid might decide the specificity of the two receptors.