Primary structure of flavocytochrome b2 from baker's yeast. Purification by reverse-phase high-pressure liquid chromatography and sequencing of fragment alpha cyanogen bromide peptides

Flavocytochrome b2 of the Methylotrophic Yeast Ogataea polymorpha: Construction of Overproducers, Purification, and Bioanalytical Application

Flavocytochrome b₂ (EC 1.1.2.3; L-lactate cytochrome: c oxidoreductase, FC b₂) from the thermotolerant methylotrophic yeast Ogataea polymorpha is a thermostable enzyme-prospective for a highly selective L-lactate analysis in the medicine, nutrition sector, and quality control of commercial products. Here we describe the construction of FC b₂ producers by overexpression of the CYB2 gene O. polymorpha, encoding FC b₂, under the control of a strong alcohol oxidase promoter in the frame of plasmid for multicopy integration with the next transformation of recipient strain O. polymorpha C-105 (gcr1 catX) impaired in the glucose repression and devoid of catalase activity. The selected recombinant strain O. polymorpha "tr1" (gcr1 catX CYB2), characterized by eightfold increased FC b₂ activity compared to the initial strain, was used as a source of the enzyme. For purification of FC b₂ a new method of affinity chromatography was developed and purified preparations of the enzyme were used for the construction of the highly selective enzymatic kits and amperometric biosensor for L-lactate analysis in human liquids and foods.

Extreme pKa displacements at the active sites of FMN-dependent alpha-hydroxy acid-oxidizing enzymes

Flavocytochrome b2 (or L-lactate dehydrogenase) from baker's yeast is thought to operate by the initial formation of a carbanion, as do the evolutionarily related alpha-hydroxy acid-oxidizing FMN-dependent oxidases. Previous work has shown that, in the active site of the unligated reduced flavocytochrome b2, the group that has captured the substrate alpha-proton has a high pKapp, calculated to lie around 15 through the use of Eigen's equation. A detailed inspection of the now known three-dimensional structure of the enzyme leads to the conclusion that the high pKa belongs to His 373, an active site group that plays the role of general base in the forward reaction and of general acid in the reverse direction. Moreover, consideration of the kinetics of proton transfer during the catalytic cycle suggests that the pKa of the reduced FMN N5 position should be lowered by several pH units compared to its pKa of 20 or more when free. The features of the three-dimensional structure possibly responsible for these pK shifts are analyzed; they are proposed to consist of a network of hydrogen bonds with the solvent and of a mutual electrostatic stabilization of anionic reduced flavin and the imidazolium ion. Finally, it is suggested that similar pK shifts affect the active sites of the alpha-hydroxy acid-oxidizing flavooxidases, which are homologous to flavocytochrome b2. The functional significance of these pK shifts in terms of catalysis and semiquinone stabilization is discussed.

Inactivation of flavocytochrome b2 with fluoropyruvate. Reaction at the active-site histidine

Fluoropyruvate inactivated oxidized flavocytochrome b2 (baker's yeast L-lactate dehydrogenase) in a biphasic process yielding convex semilog plots of residual activity versus time. At each reagent concentration, rate constants k1 and k2 for the two phases could be calculated by simulation studies using one of the schemes proposed by Ray and Koshland [J. Biol. Chem. (1961) 236, 1973-1979]: E----E1 (fully active)----E2 (inactive). When plotted as a function of reagent concentration, the values of k2, but not those of k1, showed a saturation effect. Inactivation was slowed down by D-lactate, a competitive inhibitor, and completely prevented by enzyme reduction. While no enzyme chemical modification could be demonstrated for the first step, the inactivation event of the second step could be ascribed to alkylation of a histidine belonging to proteolytic fragment beta of the enzyme. The only histidine present in the fragment sequence is His-373. In the enzyme three-dimensional structure [Xia et al. (1987) Proc. Natl Acad. Sci. USA 84, 2629-2633] His-373 is well located, close to the cofactor, to play the role of the active-site base required by the chemical mechanism. Alternative chemical interpretations of the kinetic scheme are discussed, so is the difference between flavocytochrome b2 inactivation by fluoropyruvate and bromopyruvate.

Complete amino acid sequence of flavocytochrome b2 from baker's yeast

Each subunit of baker's yeast flavocytochrome b2 can be selectively cleaved by proteases into two fragments, amino-terminal fragment alpha and carboxy-terminal fragment beta. The primary structure of the former has been reported before [Ghrir, B., Becam, A. M. & Lederer, F. (1984) Eur. J. Biochem. 139, 59-74]. The amino acid sequence of the 197-residue fragment beta has now been established. The fragment was cleaved with cyanogen bromide; the three peptides thus obtained were submitted to digestions with Staphylococcus aureus V8 protease, chymotrypsin and trypsin, sometimes after succinylation. The complete fragment was also submitted to tryptic cleavage after citraconylation. Peptides were separated by thin-layer finger-printing or high-pressure liquid chromatography. They were mostly sequenced in a liquid-phase sequenator. The 511-residue amino acid sequence of the mature protein is thus completely established. Secondary structure predictions indicate an alternation of helical and extended structure, with a higher percentage of the former. Comparisons with other flavoproteins do not detect any significant sequence similarity.

Derivation of the globins from type b cytochromes

Similarities in the amino acid sequences of vertebrate and invertebrate globins, b5 and b2 cytochromes and chicken sulfite oxidase point to a common ancestry for all of these proteins. The distal heme ligand (histidine or its equivalent) is common to both sets of proteins, but the proximal histidine ligand of the cytochromes is replaced by another histidine residue in the globins. This explains why the heme is reversed between globins and b5 cytochromes. It seems likely that the genes for primitive globins contained three exons, the first two of which were derived from a cytochromelike DNA sequence. A model is presented to show how globins may have evolved from a pre-existing type b cytochrome; the complexity of the required changes is an indication that all globins are monophyletic.