Quantifying Lipari-Szabo modelfree parameters from 13CO NMR relaxation experiments

Area of Science:

• Biophysics
• Structural Biology
• Nuclear Magnetic Resonance (NMR) Spectroscopy

Background:

• Protein dynamics are crucial for function, but characterizing them at the residue level is challenging.
• Lipari-Szabo order parameters and local correlation times provide insights into molecular motion.
• Existing methods often require complementary data or have limitations in capturing specific motional modes.

Purpose of the Study:

• To develop and validate a new method for obtaining effective Lipari-Szabo order parameters and local correlation times for protein (13)CO nuclei.
• To introduce the COMFORD (CO-Modelfree Fitting Of Relaxation Data) program for fitting (13)CO relaxation data.
• To investigate the complementarity of (13)CO and (15)N relaxation data for characterizing protein dynamics.

Main Methods:

• Utilizing (13)CO-R(1) auto relaxation experiments.
• Performing transverse (13)CO CSA/13CO-13Calpha CSA/dipolar and (13)CO CSA/(13)CO-(15)N CSA/dipolar cross-correlation experiments.
• Applying the COMFORD program to fit (13)CO relaxation data, determining effective order parameter (S2CO), local correlation time, and CSA tensor orientation.

Main Results:

• The effective S2CO parameter shows sensitivity to rotational fluctuations along specific molecular axes, complementary to (15)N relaxation order parameters.
• (13)CO order parameters reveal slower and potentially different motions compared to (15)N relaxation order parameters.
• Local (13)CO correlation times correlate well with the secondary structure of calmodulin.

Conclusions:

• The COMFORD program and (13)CO relaxation experiments provide valuable, complementary information on protein dynamics.
• This approach offers a new paradigm for detailed characterization of residue-specific protein motions.
• The findings highlight the utility of (13)CO dynamics in understanding protein structure-function relationships.