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Human methemoglobin was crystallized in a unique unit cell and its structure was solved by molecular replacement. The hexagonal unit cell has unit-cell parameters a = b = 54.6, c = 677.4 Å, with symmetry consistent with space group P6₁22. The unit cell has the second highest aspect ratio of all unit cells contained in the PDB. The 12 molecules in the unit cell describe a right-handed helical filament having no polarity, which is different from the filament composed of HbS fibers, which is the only other well characterized fiber of human hemoglobin. The filaments reported here can be related to canonical sickle-cell hemoglobin filaments and to an alternative sickle-cell filament deduced from fiber diffraction by slight modifications of intermolecular contacts.

Bovine pancreatic ribonuclease A (RNase A) was crystallized from a mixture of small molecules containing basic fuchsin, tobramycin and uridine 5'-monophosphate (U5P). Solution of the crystal structure revealed that the enzyme was selectively bound to U5P, with the pyrimidine ring of U5P residing in the pyrimidine-binding site at Thr45. The structure was refined to an R factor of 0.197 and an R(free) of 0.253.

Pig heart citrate synthase was crystallized from a small-molecule cocktail containing cystamine dihydrochloride, aspartame and benzamidine hydrochloride. The structure was refined to an R factor of 0.179 (R(free) = 0.222) using synchrotron data to a resolution of 1.78 A. The model includes the full-length protein, a chloride ion, a sulfate ion, 305 water molecules and an unexpected moiety attached through a disulfide linkage to Cys184, which was modeled as a half-cystamine molecule generated by disulfide exchange with the cystamine in the small-molecule cocktail.

Proteinase K, a subtilisin-like fungal protease, was crystallized from a cocktail of small molecules containing digalacturonic acid (DGA). The crystal structure was determined to 1.32 A resolution and refined to an R factor of 0.158. The final model contained, beside the protein, two calcium ions, 379 water molecules, a molecule of DGA and a partially occupied HEPES molecule. The DGA molecule has one sugar moiety disposed exactly on a crystallographic twofold axis; the second ring was not observed. The DGA molecule is bound to two protein molecules across the twofold axis through hydrogen-bonding networks involving Ser150 and water molecules. One of the calcium-ion sites has not been reported previously. This study further illustrates the involvement of small molecules in the crystallization of macromolecules through their ability to form intermolecular lattice interactions.