Hemoglobin A(2) (alpha(2)delta(2)) is an important hemoglobin variant which is a minor component (2-3%) in the circulating red blood cells, and its elevated concentration in beta-thalassemia is a useful clinical diagnostic. In beta-thalassemia major, where there is beta-chain production failure, HbA(2) acts as the predominant oxygen deliverer. HbA(2) has two more important features. (1) It is more resistant to thermal denaturation than HbA, and (2) it inhibits the polymerization of deoxy sickle hemoglobin (HbS). Hemoglobin E (E26K(beta)), formed as a result of the splice site mutation on exon 1 of the beta-globin gene, is another important hemoglobin variant which is known to be unstable at high temperatures. Both heterozygous HbE (HbAE) and homozygous HbE (HbEE) are benign disorders, but when HbE combines with beta-thalassemia, it causes E/beta-thalassemia which has severe clinical consequences. In this paper, we present the crystal structures of HbA(2) and HbE at 2.20 and 1.74 A resolution, respectively, in their R2 states, which have been used here to provide the probable explanations of the thermal stability and instability of HbA(2) and HbE. Using the coordinates of R2 state HbA(2), we modeled the structure of T state HbA(2) which allowed us to address the structural basis of the antisickling property of HbA(2). Using the coordinates of the delta-chain of HbA(2) (R2 state), we also modeled the structure of hemoglobin homotetramer delta(4) that occurs in the case of rare HbH disease. From the differences in intersubunit contacts among beta(4), gamma(4), and delta(4), we formed a hypothesis regarding the possible tetramerization pathway of delta(4). The crystal structure of a ferrocyanide-bound HbA(2) at 1.88 A resolution is also presented here, which throws light on the location and the mode of binding of ferrocyanide anion with hemoglobin, predominantly using the residues involved in DPG binding. The pH dependence of ferrocyanide binding with hemoglobin has also been investigated.

Papain is a plant cysteine protease of industrial importance having a two-domain structure with its catalytic cleft located at the domain interface. A structure-based rational design approach has been used to improve the thermostability of papain, without perturbing its enzymatic activity, by introducing three mutations at its interdomain region. A thermostable homologue in papain family, Ervatamin C, has been used as a template for this purpose. A single (K174R), a double (K174RV32S) and a triple (K174RV32SG36S) mutant of papain have been generated, of which the triple mutant shows maximum thermostability with the half-life (t(1/2)) extended by 94 min at 60 degrees C and 45 min at 65 degrees C compared to the wild type (WT). The temperature of maximum enzymatic activity (T(max)) and 50% maximal activity (T(50)) for the triple mutant increased by 15 and 4 degrees C, respectively. Moreover, the triple mutant exhibits a faster inactivation rate beyond T(max) which may be a desirable feature for an industrial enzyme. The values of t(1/2) and T(max) for the double mutant lie between those of the WT and the triple mutant. The single mutant however turns out to be unstable for biochemical characterization. These results have been substantiated by molecular modeling studies which also indicate highest stability for the triple mutant based on higher number of interdomain H-bonds/salt-bridges, less interdomain flexibility and lower stability free-energy compared to the WT. In silico studies also explain the unstable behavior of the single mutant.

Papain is the archetype of a broad class of cysteine proteases (clan C1A) that contain a pro-peptide in the zymogen form which is required for correct folding and spatio-temporal regulation of proteolytic activity in the initial stages after expression. This study reports the X-ray structure of the zymogen of a thermostable mutant of papain at 2.6 Å resolution. The overall structure, in particular that of the mature part of the protease, is similar to those of other members of the family. The structure provides an explanation for the molecular basis of the maintenance of latency of the proteolytic activity of the zymogen by its pro-segment at neutral pH. The structural analysis, together with biochemical and biophysical studies, demonstrated that the pro-segment of the zymogen undergoes a rearrangement in the form of a structural loosening at acidic pH which triggers the proteolytic activation cascade. This study further explains the bimolecular stepwise autocatalytic activation mechanism by limited proteolysis of the zymogen of papain at the molecular level. The possible factors responsible for the higher thermal stability of the papain mutant have also been analyzed.

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Hen serum transferrin in its diferric form (hST) has been isolated, purified and the three-dimensional structure determined by X-ray crystallography at 2.8 A resolution. The final refined structure of hST, comprising 5232 protein atoms, two Fe(3+) cations, two CO(3)(2-) anions, 54 water molecules and one fucose moiety, has an R factor of 21.5% and an R(free) of 26.9% for all data. The structure has been compared with the three-dimensional structure of hen ovotransferrin (hOT) and also with structures of some other transferrins, viz. rabbit serum transferrin (rST) and human lactoferrin (hLF). The overall conformation of the hST molecule is essentially the same as that of other transferrins. However, the relative orientation of the two lobes, which is related to the species-specific receptor-recognition property of transferrins, has been found to be different in hST from that in hOT, rST and hLF. On the basis of superposition of the N lobes, rotations of 5.8, 16.9 and 11.3 degrees are required to bring the C lobes of hOT, rST and hLF, respectively, into coincidence with that of hST. A number of additional hydrogen bonds between the two domains in the N and C lobes have been identified in the structure of hST compared with that of hOT, which indicate a greater compactness of the lobes of hST than those of hOT. Being products of the same gene, hST and hOT have 100% sequence identity and differ only in the attached carbohydrate moiety. On the other hand, despite having similar functions, hST and rST have only 51% sequence similarity. However, the nature of the interdomain interactions of hST are closer to rST than to hOT. A putative carbohydrate-binding site has been identified in the N lobe of hST at Asn52 and a fucose molecule could be modelled at the site. The variations in interdomain and interlobe interactions in hST, together with altered lobe orientation with respect to hOT, rST and hLF, which are the representatives of the other subfamily of transferrins, are discussed.

Papain-like proteases contain an N-terminal pro-peptide in their zymogen form that is important for correct folding and spatio-temporal regulation of the proteolytic activity of these proteases. Catalytic removal of the pro-peptide is required for the protease to become active. In this study, we have generated three different mutants of papain (I86F, I86L and I86A) by replacing the residue I86 in its pro-peptide region, which blocks the specificity determining S2-subsite of the catalytic cleft of the protease in its zymogen form with a view to investigate the effect of mutation on the catalytic activity of the protease. Steady-state enzyme kinetic analyses of the corresponding mutant proteases with specific peptide substrates show significant alteration of substrate specificity—I86F and I86L have 2.7 and 29.1 times higher kcat/Km values compared to the wild-type against substrates having Phe and Leu at P2 position, respectively, while I86A shows lower catalytic activity against majority of the substrates tested. Far-UV CD scan and molecular mass analyses of the mature form of the mutant proteases reveal similar CD spectra and intact masses to that of the wild-type. Crystal structures of zymogens of I86F and I86L mutants suggest that subtle reorganization of active site residues, including water, upon binding of the pro-peptide may allow the enzyme to achieve discriminatory substrate selectivity and catalytic efficiency. However, accurate and reliable predictions on alteration of substrate specificity require atomic resolution structure of the catalytic domain after zymogen activation, which remains a challenging task. In this study we demonstrate that through single amino acid substitution in pro-peptide, it is possible to modify the substrate specificity of papain and hence the pro-peptide of a protease can also be a useful target for altering its catalytic activity/specificity.