Structural investigation of pig metmyoglobin by 129Xe NMR spectroscopy.

This research investigates the potential of xenon’s sensitivity to its local magnetic environment to understand structural differences between pig and horse metmyoglobin. The study explored xenon protein interactions using multiple techniques and discovered discrepancies in xenon’s affinity to the interior of pig metmyoglobin in comparison with that of the horse.

Context and Purpose of the Research

• The research aimed to utilize the magnetic sensitivity of xenon to probe the internal structural differences between pig and horse metmyoglobin (MMb).
• Xenon was chosen due to both its specific and non-specific interactions with proteins.
• Distinguishing these internal differences could provide valuable insights into the behaviour and characteristics of proteins.

Methodology

• The research methods involved the application of 129Xe NMR chemical shifts and relaxation rate in aqueous solutions of pig MMb as a function of both the xenon and protein concentrations.
• The results were then augmented with 129Xe NMR data of the isostructural carbonmonoxy myoglobin (COMb) and computational calculations to outline the structural differences between the cavities.
• A two-site model was utilized for a quantitative analysis of the 129Xe chemical shift NRM parameters.
• Additional tests were conducted using 1H NMR spectra to evaluate the reliance of the 1H chemical shift on the introduction of xenon.

Key Findings

• Xenon was found to form a 1:1 complex with the protein, where it quickly exchanges between a cavity within the protein and all other environments.
• There was a discrepancy in the equilibrium constant in the pig (74 M(-1)) and horse (146 M(-1)), which suggested differences in the affinity of xenon with the interior of pig MMb.
• A change in xenon binding in both pig and horse MMb was confirmed by other experimental results. For instance, the difference in the estimated delta(in), which shifted downwards in pig MMb and upwards in horse MMb, compared to 129Xe in buffer solution.
• The distance between xenon and iron was lengthier in pig (7.4 A) than in horse (5.3 A).

Significance of the Research

• The research offers a novel approach to analyze the structure of proteins using the magnetic properties of xenon.
• The findings highlight the noteworthy differences in the affinity of xenon to the interior of pig and horse metmyoglobin, thereby enhancing our understanding of the structure and behavior of proteins.