NAD+-dependent formate dehydrogenase (FDH, EC 1.2.1.2) widely occurs in nature. FDH consists of two identical subunits and contains neither prosthetic groups nor metal ions. This type of FDH was found in different microorganisms (including pathogenic ones), such as bacteria, yeasts, fungi, and plants. As opposed to microbiological FDHs functioning in cytoplasm, plant FDHs localize in mitochondria. Formate dehydrogenase activity was first discovered as early as in 1921 in plant; however, until the past decade FDHs from plants had been considerably less studied than the enzymes from microorganisms. This review summarizes the recent results on studying the physiological role, properties, structure, and protein engineering of plant formate dehydrogenases.

It has been shown by an X-ray structural analysis that the amino acid residues Ala198, which are located in the coenzyme-binding domain of NAD+-dependent formate dehydrogenases (EC 1.2.1.2., FDH) from bacteria Pseudomonas sp.101 and Moraxella sp. C-1 (PseFDH and MorFDH, respectively), have non-optimal values of the angles ψ and φ. These residues were replaced with Gly by site-directed mutagenesis. The mutants PseFDH A198G and MorFDH A198G were expressed in E.coli cells and obtained in active and soluble forms with more than 95% purity. The study of thermal inactivation kinetics showed that the mutation A198G results in a 2.5- fold increase in stability compared to one for the wild-type enzymes. Kinetic experiments indicate that A198G replacement reduces the KMNAD+ value from 60 to 35 and from 80 to 45 μM for PseFDH and MorFDH, respectively, while the KMHCOO- value remains practically unchanged. Amino acid replacement A198G was also added to the mutant PseFDH D221S with the coenzyme specificity changed from NAD+ to NADP+. In this case, an increase in thermal stability was also observed, but the influence of the mutation on the kinetic parameters was opposite: KM increased from 190 to 280 μM and from 43 to 89 mM for NADP+ and formate, respectively. According to the data obtained, inference could be drawn that earlier formate dehydrogenase from bacterium Pseudomonas sp. 101 was specific to NADP+, but not to NAD+.

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The analysis of the 3D model structure of the ternary complex of recombinant formate dehydrogenase from soya Glycine max (EC 1.2.1.2., SoyFDH) with bound NAD+ and an inhibitor azide ion revealed the presence of hydrophobic Phe290 in the coenzyme-binding domain. This residue should shield the enzyme active site from solvent. On the basis of the alignment of plant FDHs sequences, Asp, Asn and Ser were selected as candidates to substitute Phe290. Computer modeling indicated the formation of two (Ser and Asn) or three (Asp) new hydrogen bonds in such mutants. The mutant SoyFDHs were expressed in Escherichia coli, purified and characterized. All amino acid substitutions increased K(м)(HCOO-) from 1.5 to 4.1-5.0 mM, whereas the K(м)(NAD+) values remained almost unchanged in the range from 9.1 to 14.0 μM, which is close to wt-SoyFDH (13.3 μM). The catalytic constants for F290N, F290D and F290S mutants of SoyFDH equaled 2.8, 5.1 and 4.1 s⁻¹, respectively; while that of the wild-type enzyme was 2.9 s⁻¹. The thermal stability of all mutant SoyFDHs was much higher compared with the wild-type enzyme. The differential scanning calorimetry data were in agreement with the results of thermal inactivation kinetics. The mutations F290S, F290N and F290D introduced into SoyFDH increased the T(m) values by 2.9°C, 4.3°C and 7.8°C, respectively. The best mutant F290D exhibited thermal stability similar to that of FDH from the plant Arabidopsis thaliana and exceeded that of the enzymes from the yeast Candida boidinii and the bacterium Moraxella sp. C1.

Recombinant formate dehydrogenase (FDH, EC 1.2.1.2) from soy Glycine max (SoyFDH) has the lowest values of Michaelis constants for formate and NAD+ among all studied formate dehydrogenases from different sources. Nevertheless, it also has the lower thermal stability compared to enzymes from bacteria and yeasts. The alignment of full sequences of FDHs from different sources as well as structure of apo- and holo-forms of SoyFDH has been analyzed. Ten mutant forms of SoyFDH were obtained by site-directed mutagenesis. All of them were purified to homogeneity and their thermal stability and substrate specificity were studied. Thermal stability was investigated by studying the inactivation kinetics at different temperatures and by differential scanning calorimetry (DSC). As a result, single-point (Ala267Met) and double mutants (Ala267Met/Ile272Val) were found to be more stable than the wild-type enzyme at high temperatures. The stabilization effect depends on temperature, and at 52°C it was 3.6- and 11-fold, respectively. These mutants also showed higher melting temperatures in DSC experiments - the differences in maxima of the melting curves (T(m)) for the single and double mutants were 2.7 and 4.6°C, respectively. For mutations Leu24Asp and Val127Arg, the thermal stability at 52°C decreased 5- and 2.5-fold, respectively, and the T(m) decreased by 3.5 and 1.7°C, respectively. There were no differences in thermal stability of six mutant forms of SoyFDH - Gly18Ala, Lys23Thr, Lys109Pro, Asn247Glu, Val281Ile, and Ser354Pro. Analysis of kinetic data showed that for the enzymes with mutations Val127Arg and Ala267Met the catalytic efficiency increased 1.7- and 2.3-fold, respectively.

Previous experiments on substitution of the residue Phe290 to Asp, Asn and Ser in NAD(+)-dependent formate dehydrogenase from soya Glycine max (SoyFDH) showed important role of the residue in enzyme thermal stability and catalytic properties (Alekseeva et al. Prot. Eng. Des. Sel., 2012a; 25: :781-88). In this work, we continued site-directed mutagenesis experiments of the Phe290 and the residue was changed to Ala, Thr, Tyr, Glu and Gln. All amino acid changes resulted in increase of catalytic constant from 2.9 to 3.5-4.7 s(-1). The substitution Phe290Ala led to KM (NAD+) decrease from 13.3 to 8.6 μM, and substitutions Phe290Tyr and Phe290Glu resulted in decrease and increase of KM (HCOO-) from 1.5 to 0.9 and -2.9 mM, respectively. The highest improvement of catalytic properties was observed for SoyFDH Phe290Ala which showed 2-fold higher catalytic efficiency with both substrates. Stability of mutants was examined by study of thermal inactivation kinetics and differential scanning calorimetry (DSC). All five amino acids provided increase of thermal stability of mutant SoyFDH in comparison with wild-type enzyme. Mutant SoyFDH Phe290Glu showed the highest improvement-the stabilization effect was 43 at 56°C. The DSC data agree with results of thermal inactivation kinetics. Substitutions Phe290Tyr, Phe290Thr, Phe290Gln and Phe290Glu provided Tm value increase 0.6°-6.6°. SoyFDH Phe290Glu and previously prepared SoyFDH Phe290Asp show similar thermal stability as enzymes from Candida boidinii and Mycobacterium vaccae N10 and have higher catalytic efficiency with NAD(+) compared with all described FDHs. Therefore, these mutants are very perspective enzymes for coenzyme regeneration in processes of chiral synthesis with dehydrogenases.

Резюме. В настоящее время в литературе активно обсуждается неоднозначная роль распознаю-щих рецепторов врожденного иммунитета, в частности TLRs, в иммунопатогенезе бронхиальной астмы (БА).Цель нашей работы -исследование экспрессии ТLR2 и TLR4 на уровне клеток слизистой полости носа и лейкоцитов периферической крови (ЛПК) больных БА разной степени тяжести.В исследование были включены 40 детей с БА (3-12 лет) и 10 здоровых детей того же возраста. Методом ПЦР-РВ оценивали экспрессию генов TLR2 и TLR4 в соскобах со слизистой полости носа и в ЛПК; методом проточной цитометрии определяли процент моноцитов, лимфоцитов и гранулоци-тов, экспрессирующих TLR2 TLR4, и интенсивность их экспрессии; методом мультиплексного им-мунофлуоресцентного анализа оценивали уровень про-и противовоспалительных цитокинов (IL-1β, IL-1ra, IL-6, IL-8, IL-10, TNFα) в назальных смывах.В результате проведенного исследования выявлена гиперактивация факторов врожденного имму-нитета на уровне слизистой оболочки полости носа у больных с БА, проявляющаяся повышением экспрессии генов TLR2, TLR4 и выработки как провоспалительных, так и противовоспалительных цитокинов. Выявлена связь между уровнем цитокинов и степенью тяжести бронхиальной астмы. В периферической крови определено достоверное увеличение экспрессии TLR2 и TLR4 на циркули-рующих CD14 + моноцитах у детей с БА. Таким образом, показано увеличение экспрессии генов TLRs слизистой полости носа и повыше-ние поверхностной экспрессии TLR2 и TLR4 на циркулирующих моноцитах больных бронхиальной астмой по сравнению со здоровыми детьми. Выявленные изменения свидетельствуют о вовлечении системы TLRs в иммунопатогенез бронхиальной астмы. В дальнейшем TLRs могут быть использова-ны в качестве маркеров прогноза течения БА и возможных терапевтических мишеней.

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