Dynamic model of globular protein conformations based on NMR studies in solution

Abstract

BASIC PANCREATIC TRYPSIN INHIBITOR (BPTI) is a small globular protein of molecular weight 6,500 (refs ^1–17). It consists of one polypeptide chain with 58 amino acid residues, including three disulphide bonds and four phenylalanines and four tyrosines as the only aromatics¹⁸. The crystal structure at 1.5 Å resolution has been extensively refined¹⁹. In the ¹H- and ¹³C-NMR (nuclear magnetic resonance) spectra, a large number of resolved resonance lines have been assigned to particular residues in the amino acid sequence^6,8,10–16 and related to particular aspects of the molecular conformation^1–17. Hence, NMR spectral similarities and differences between BPTI and related proteins can on the basis of these earlier studies be interpreted in terms of structural features in well defined locations of the protein molecule. We report here the use of NMR observations to describe the globular BPTI conformation in terms of a dynamic ensemble of rapidly interconverting molecular structures. BPTI was a suitable compound for this study as its globular form is outstandingly stable in aqueous solution (Table 1)^1,9,20, leaving a large experimentally accessible range for studies of modified BPTI species with reduced stability. The chemically modified and homologous proteins used are characterised in the footnotes to Table 1.