An SRLS Study of2H Methyl-Moiety Relaxation and Related Conformational Entropy in Free and Peptide-Bound PLCγ1C SH2.
The two-body (protein and probe) coupled-rotator slowly relaxing local structure (SRLS) approach for NMR relaxation in proteins is extended to derive conformational entropy, Ŝ. This version of SRLS is applied to deuterium relaxation from the C-CDH2bonds of free and peptide-bound PLCγ1C SH2. Local C-CDH2motion is described by a correlation time for local diffusion, τ2, and a Maier-Saupe potential, u. On average, τ2, which largely fulfills τ2≪ τ1(τ1- correlation time for global tumbling), is 270 ± 41 ps and u is 2 ± 0.1 kBT. The PLCγ1C SH2 data were analyzed previously with the model-free (MF) method. SRLS is a generalization of MF, assumed so far to yield the latter for τ2≪ τ1and simple local geometry. Despite these conditions being fulfilled, we find here that τ2and u differ substantially from their MF counterparts. This is shown to stem from MF (a) disregarding mode-coupling of the first type (see below) and (b) parametrizing the methyl-moiety-related spectral density function (SDF). Our main interest lies in ΔŜ, the conformational entropy difference between the peptide-bound and free PLCγ1C SH2 forms. We find that ΔŜ is rendered inaccurate in MF because factors a and b above impair the accuracy of Saxis, the parameter on which the calculation of ΔŜ is based. Conformational entropy was obtained previously using various simple system-specific models. SRLS is unique in obtaining this important thermodynamic quantity based on a general physically well-defined local potential. It is also unique in its ability to extract the information inherent in2H relaxation parameters from methyl moieties in protein with accuracy commensurate with data sensitivity.
The Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University , Ramat-Gan 52900, Israel.
Oren
O
Tchaicheeyan
The Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University , Ramat-Gan 52900, Israel.
Eva
E
Meirovitch