Structural and functional consequences of replacement of His403 with Arg near the catalytic site of Anoxybacillus flavithermus cyclomaltodextrinase.

The hydrolytic activity of a thermophilic cyclomaltodextrinase (CMD) from Anoxybacillus flavithermus ZNU-NGA and a representative single mutant were investigated against soluble substrates including α-, ß- and γ-cyclomaltodestrines (CDs). Based on the occurrence of arginine (Arg) at position 403 in some homologue proteins, His403 in Wild-type (WT) CMD was replaced with Arg (H403R variant) with site-directed mutagenesis procedures. According to bioinformatics data, Arg403 in mutant protein is located near Glu357 as one of the catalytic residues in a manner that they are able to create a medium-range attractive electrostatistic interaction. Structural studies by Far UV-CD showed that this mutation is accompanied by conversion of a small fraction of α-helix to ß-form structure. Fluorescence data reveals that, the hydrophobic regions at the surface of protein, as the binding sites for ANS (8-Anilinonaphthalene-1-sulfonic acid) increase in mutant protein, demonstrating relative inflation of H403R variant compared with WT protein. However, the polarity of microenvironment around chromophores did not change upon mutation. Activity measurement in different ranges of pH and temperatures showed that the optimum values of pH and temperature in mutant enzyme is the same as WT enzyme, however; the activity at optimum points increased in H403R variant. It was also revealed that the H403R variant had slightly improved catalytic efficiency for γ-CD. The same value of activation parameters for both protein variants indicates that mutation does not alter the mechanism of catalysis during enzyme-substrate formation.

Thermodynamics of protein folding: methodology, data analysis and interpretation of data.

Thermodynamics of protein folding refers to the stability measurements where structural changes of a given protein in the presence of a denaturing agent are monitored by spectroscopic or calorimetric techniques. In macroscopic point of view, protein stability represents the ratio of the population of its unfolded state to that of folded one in equilibrium condition, while in microscopic point of view, the stability is actually a net value from a combination of favorable and unfavorable contributions that affect the structural integrity of a protein molecule. In this manuscript, the principles and methodological aspects of thermodynamic studies and methods of data analysis as well as interpretation of the results are presented.

Molecular mechanisms governing the evolutionary conservation of Glycine in the 6th position of loops ΙΙΙ and ΙV in photoprotein mnemiopsin 2.

Photoproteins in their functional form are complexed noncovalently with 2-hydroperoxycoelenterazine. A conformational change upon coordination of Ca+2 ions with their EF-hand loops leads to oxidation of substrate and emission of light. In all photoproteins, EF-hand loops Ι, ΙΙΙ and ΙV have standard sequence for binding to Ca+2 ion, however the second one is not able for Ca+2 coordination. Sequence analysis of Mnemiopsin 2 and other known photoproteins shows that Glutamate (Glu) is occurred in the 6th position of its first EF-hand loop, but this position in other loops of mnemiopsin 2 and all functional loops of other photoproteins is occupied by Glycine (Gly). Here we designed and made single and double mutants where Gly residue at the 6th positions of loops ΙΙΙ and ΙV of mnemiopsin 2 was replaced with Glu. According to the activity measurements, wild-type (WT) and G142E variants have more initial luminescence intensity than G176E and double mutants; while WT and G176E have higher values of half decay time when compared with G142E and double mutants. According to the isothermal denaturation experiments, all protein variants are structurally more stable than WT mnemiopsin 2 and that the stabilizing effects of single mutants are paired resulting in more stability of double mutant against urea denaturation. We concluded that simultaneous occurrence of Gly in the 6th position of loops ΙΙΙ and ΙV is essential for evolutionary adjustment of initial intensity and decay rate of luminescence emission via affecting the interaction of the core structure of photoprotein with coelenteramide and binding affinity of Ca+2 to the corresponding loops, respectively.

Investigating the structural and functional features of representative recombinants of chondroitinase ABC I.

Chondroitin Sulfate Proteoglycans (CSPGs) are the main inhibitors for axon regeneration after damaging of Central Nervous System (CNS). Chondroitinase ABC I (cABC I) can degrade CSPGs by removing chondroitin and dermatan sulfate side chains from proteoglycans. Hence, it may be considered as an attractive candidate in biomedicine. For practical applications of this enzyme, increasing the effective circulating level and reducing the number and volume of injections for patients is one of the main concerns which is directly related to conformational stability and catalytic efficiency of the enzyme. Structural examination of C-terminal domain of cABC I reveals that there are a few numbers of residues in helical conformation which are positioned at the context of a cohesive structural organization of ß-strands. In line with our previous studies on C-terminal domain of cABC I and regarding the residues in α-helix conformation; we designed and constructs some representative mutants including M889K, M889L, L679D/M889K and L679S/M889K. According to structural and functional characterization of protein variants and regarding the wide range of variability in determining parameters for ß-sheet conformation, we proposed a model in which the structural integrity of ß-strands at C-terminal domain can be manipulated and directed toward a new patterns of organization, some of them may have positive effects on the structural and functional features of the enzyme. Using this strategy it may be possible to improve functional and structural features of the enzyme by engineering the intra-molecular interactions in positions far from the active site of the enzyme.