Determinants of Enzyme Thermostability Observed in the Molecular Structure ofThermusaquaticusd-Glyceraldehyde-3-phosphate Dehydrogenase at 2.5 Å Resolution,

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Abstract
The crystal structure of holo d-glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from the extreme thermophile Thermus aquaticus has been solved at 2.5 Å resolution. To study the determinants of thermostability, we compare our structure to four other GAPDHs. Salt links, hydrogen bonds, buried surface area, packing density, surface to volume ratio, and stabilization of α-helices and β-turns are analyzed. We find a strong correlation between thermostability and the number of hydrogen bonds between charged side chains and neutral partners. These charged-neutral hydrogen bonds provide electrostatic stabilization without the heavy desolvation penalty of salt links. The stability of thermophilic GAPDHs is also correlated with the number of intrasubunit salt links and total hydrogen bonds. Charged residues, therefore, play a dual role in stabilization by participating not only in salt links but also in hydrogen bonds with a neutral partner. Hydrophobic effects allow for discrimination between thermophiles and psychrophiles, but not within the GAPDH thermophiles. There is, however, an association between thermostability and decreasing enzyme surface to volume ratio. Finally, we describe several interactions present in both our GAPDH and a hyperthermophilic GAPDH that are absent in the less thermostable GAPDHs. These include a four-residue salt link network, a hydrogen bond near the active site, an intersubunit salt link, and several buried Ile residues.