A Proton Nuclear Magnetic Resonance and Nuclear Overhauser Effect (NOE) Study of Human Plasma Prealbumin, Including the Development and Application to Spectral Assignment of a Combined Ring Current Shift and NOE Prediction Program

Human prealbumin, a homotetrameric protein with an Mr of approximately 55,000 has been characterised by proton magnetic resonance spectroscopy. The pH dependences of the chemical shifts of the 4 histidine residues/subunit are unremarkable; variable temperature studies show that the protein tertiary and quaternary structure is stable to at least 80 degrees C. Assignment of a number of residues was aided by the development of a computer program, PARSNIP (Programme to Analyse Ring current Shifts and nuclear Overhauser effects (NOEs) In Proteins), which employs high resolution x-ray crystal structure atomic co-ordinates (Blake, C.F., and Oatley, S.J. (1977) Nature 268, 115-120, in the case of prealbumin) and several different predictive formalisms to calculate structure-dependent ring current-induced shifts of aromatic and methyl group protons. The program combines these with distance measurements to give plots of expected NOE difference profiles resulting from irradiation at a discrete frequency. The routine should prove applicable to many proteins whose structures have been determined but which are too large for the application of sequential residue assignment techniques. Three well resolved high field shifted methyl groups resonating at 0.18, 0.28, and 0.34 ppm are assigned to Ala-120, Leu-111, and Val-122, respectively. NOEs between these signals and the aromatics, interaromatic effects, and effects involving some of the considerable number of downfield shifted alpha-protons, have been used to identify substituents on several other residues.