Tertiary structural changes associated with iron binding and release in hen serum transferrin: a crystallographic and spectroscopic study.

The iron binding and release of serum transferrin are pH-dependent and accompanied by a conformational change between the iron-bound (holo-) and iron-free (apo-) forms. We have determined the crystal structure of apo-hen serum transferrin (hAST) at 3.5A resolution, which is the first reported structure to date of any full molecule of an apo-serum transferrin and studied its pH-dependent iron release by UV-vis absorption and near UV-CD spectroscopy. The crystal structure of hAST shows that both the lobes adopt an open conformation and the relative orientations of the domains are different from those of apo-human serum transferrin and human apolactoferrin but similar to that of hen apo-ovotransferrin. Spectroscopic analysis reveals that in hen serum transferrin, release of the first iron starts at a pH approximately 6.5 and continues over a broad pH range (6.5-5.2). The complete release of the iron, however, occurs at pH approximately 4.0. The near UV-CD spectra show alterations in the microenvironment of the aromatic residues surrounding the iron-binding sites.

Structural basis of the unusual stability and substrate specificity of ervatamin C, a plant cysteine protease from Ervatamia coronaria.

Ervatamin C is an unusually stable cysteine protease from the medicinal plant Ervatamia coronaria belonging to the papain family. Though it cleaves denatured natural proteins with high specific activity, its activity toward some small synthetic substrates is found to be insignificant. The three-dimensional structure and amino acid sequence of the protein have been determined from X-ray diffraction data at 1.9 A (R = 17.7% and R(free) = 19.0%). The overall structure of ervatamin C is similar to those of other homologous cysteine proteases of the family, folding into two distinct left and right domains separated by an active site cleft. However, substitution of a few amino acid residues, which are conserved in the other members of the family, has been observed in both the domains and also at the region of the interdomain cleft. Consequently, the number of intra- and interdomain hydrogen-bonding interactions is enhanced in the structure of ervatamin C. Moreover, a unique disulfide bond has been identified in the right domain of the structure, in addition to the three conserved disulfide bridges present in the papain family. All these factors contribute to an increase in the stability of ervatamin C. In this enzyme, the nature of the S2 subsite, which is the primary determinant of specificity of these proteases, is similar to that of papain, but at the S3 subsite, Ala67 replaces an aromatic residue, and has the effect of eliminating sufficient hydrophobic interactions required for S3-P3 stabilization. This provides the possible explanation for the lower activity of ervatamin C toward the small substrate/inhibitor. This substitution, however, does not affect the binding of denatured natural protein substrates to the enzyme significantly, as there exist a number of additional interactions at the enzyme-substrate interface outside the active site cleft.

Purification and preliminary X-ray studies on hen serotransferrin in apo- and holo-forms.

Serum transferrins are monomeric glycoproteins with a molecular mass of around 80 kDa, that transport iron to cells via receptor-mediated endocytosis. Although both serum transferrins (STfs) and ovotransferrins (OTfs) are derived from the same gene in aves, the ovotransferrins do not transport iron in vivo. Crystal structures of OTf have been solved, in contrast no three-dimensional structure of avian STf have been determined as yet. Here we report the purification, crystallization, and preliminary crystallographic studies of the hen STf both in apo- (iron free) and holo- (iron loaded) forms. The hen STf has been purified to homogeneity by hydrophobic interaction chromatography. Both the apo- and holo-forms were crystallized by hanging drop vapor diffusion method at 277 K. The apo-crystals diffract to a resolution of 3.0 A and belong to the space group P4(3)2(1)2 with unit cell parameters a=b=90.5 and c=177.9 A. The holo-crystals diffract to a resolution of 2.8 A and belong to space group P2(1) with a=72.8, b=59.6, c=88.2 A, and beta=95.7 degrees.