The anthrax protective antigen (PA)
is an 83 kDa protein that is
one of three protein components of the anthrax toxin, an AB toxin
secreted by Bacillus anthracis. PA is capable of
undergoing several structural changes, including oligomerization to
either a heptameric or octameric structure called the prepore, and
at acidic pH a major conformational change to form a membrane-spanning
pore. To follow these structural changes at a residue-specific level,
we have conducted initial studies in which we have biosynthetically
incorporated 5-fluorotryptophan (5-FTrp) into PA, and we have studied
the influence of 5-FTrp labeling on the structural stability of PA
and on binding to the host receptor capillary morphogenesis protein
2 (CMG2) using 19F nuclear magnetic resonance (NMR). There
are seven tryptophans in PA, but of the four domains in PA, only two
contain tryptophans: domain 1 (Trp65, -90, -136, -206, and -226) and
domain 2 (Trp346 and -477). Trp346 is of particular interest because
of its proximity to the CMG2 binding interface, and because it forms
part of the membrane-spanning pore. We show that the 19F resonance of Trp346 is sensitive to changes in pH, consistent with
crystallographic studies, and that receptor binding significantly
stabilizes Trp346 to both pH and temperature. In addition, we provide
evidence that suggests that resonances from tryptophans distant from
the binding interface are also stabilized by the receptor. Our studies
highlight the positive impact of receptor binding on protein stability
and the use of 19F NMR in gaining insight into structural
changes in a high-molecular weight protein.
We report a method to express the solvent accessibility of histidine imidazole groups in proteins. The method is based on measuring the rate of hydrogen exchange (HX) reaction of the imidazole Cε1-hydrogen. The rate profile of the HX reaction as a function of pH gives a sigmoidal curve, which reaches the maximum rate constant (kmax) on the alkaline side of the sigmoidal curve. To quantitatively describe the solvent accessibility of imidazole groups in proteins, it is necessary to compare the kmax of the imidazole groups with their intrinsic kmax (ikmax), the maximum rate constants for the given imidazole groups when they are fully exposed to the bulk solvent. However, the mechanism of HX reaction suggests that the ikmax of an imidazole group differs depending on its pKa, and no systematic study has been conducted to clarify how the ikmax is affected by pKa. We therefore investigated the relationship between ikmax and pKa using four imidazole derivatives at three different temperatures. The experimentally determined pKa-specific ikmax values allowed us to derive a general formula to estimate the ikmax value of any given imidazole group exhibiting a specific pKa at a specific temperature. Using the formula, the protection factors (PF), the ratio of ikmax to kmax, of five imidazole groups in dihydrofolate reductase were obtained and used to express the magnitude of their solvent accessibility. In this definition, the smaller the PF value, the higher the solvent accessibility, and a value of 1 indicates full exposure to the bulk solvent. The solvent accessibility expressed by the PF values agreed well with the solvent accessible surface areas (ASA) obtained from the X-ray diffraction data.
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