Showing papers by "Michael Levitt published in 1981"

Incomplete absorption of the carbohydrate in all-purpose wheat flour

Abstract: MALABSORPTION of the carbohydrate lactose is a common cause of flatulence, abdominal discomfort, and diarrhea in otherwise healthy persons. The possibility that such symptoms could result from malabsorption of the other major dietary carbohydrates (sucrose or starch) has received little consideration since these carbohydrates are assumed to be completely absorbed in health. We measured breath hydrogen to assess the absorption of the carbohydrate in white, all-purpose wheat flour. These studies demonstrate that nearly all normal subjects fail to absorb an appreciable portion of this form of starch, possibly because of an interaction between the starch and protein moieties of wheat . . .

Molecular dynamics of hydrogen bonds in bovine pancreatic trypsin inhibitor protein

Abstract: Molecular dynamics simulations starting from the X-ray structure1 of bovine pancreatic trypsin inhibitor protein (BPTI) reveal that the different hydrogen bonds observed in the X-ray coordinates have different stabilities as indicated by the mean lengths and length fluctuations. The most stable hydrogen bonds involve the hydrogen atoms observed to exchange most slowly with solvent in NMR experiments2, and to have the shortest lengths in the X-ray structure. This agreement between calculation and experiment suggests that molecular dynamics simulations can complement X-ray studies by providing reliable information about the rates and pathways of conformational changes about the mean positions observed in protein crystals.

Hydrogen bond and internal solvent dynamics of bpti protein

Abstract: The powerful technique of x-ray crystallography has, over the past 20 years, revealed the atomic detail of over a hundred different protein molecules. While these studies do provide the time-averaged positions of the atoms in the crystal and can also indicate the amplitude of their fluctuations about these positions, they cannot provide information about the rates and pathways of conformational fluctuations. Fortunately, theoretical methods may be able to complement the x-ray studies by using the well-established methods of molecular dynamics to simulate the motion about the mean positions observed in protein crystals.'*' The results of molecular dynamics simulations depend on the molecular potential energy from which the forces acting on the atoms are calculated. Present calculations use an approximate empirical potential energy function and omit the thousands of solvent molecules that normally surround the A quantitative comparison of the results of the simulation with experimentally observed quantities is, therefore, essential. Two observed properties of globular proteins seem particularly well suited to such a comparison: (I) the amplitudes of atomic vibration available from the highest resolution x-ray studies can be checked against amplitudes computed from a dynamics run and (2) the different stabilities of peptide protons available from 'H nuclear magnetic resonance (nmr) studies can be checked against the computed fluctuations in the peptide hydrogen bond lengths. A preliminary attempt to correlate the observed vibration amplitudes of rubredoxin obtained at 1.2 b; resolution6 with those calculated from a 32 picosecond (ps) dynamic simulation failed due to changes in conformation that were not associated with fluctuations about an equilibrium structure.' Here we analyze the results of molecular dynamics simulations of BPTI [bovine pancreatic trypsin inhibitor] in terms of the fluctuations of peptide hydrogen bonds and the mobility of the four internal water molecules included in the calculation. We focused our attention on the following two questions.