Purification and properties of plant cytochrome b5

Expression of malic enzyme reveals subcellular carbon partitioning for storage reserve production in soybeans

Central metabolism produces amino and fatty acids for protein and lipids that establish seed value. Biosynthesis of storage reserves occurs in multiple organelles that exchange central intermediates including two essential metabolites, malate, and pyruvate that are linked by malic enzyme. Malic enzyme can be active in multiple subcellular compartments, partitioning carbon and reducing equivalents for anabolic and catabolic requirements. Prior studies based on isotopic labeling and steady-state metabolic flux analyses indicated malic enzyme provides carbon for fatty acid biosynthesis in plants, though genetic evidence confirming this role is lacking. We hypothesized that increasing malic enzyme flux would alter carbon partitioning and result in increased lipid levels in soybeans. Homozygous transgenic soybean plants expressing Arabidopsis malic enzyme alleles, targeting the translational products to plastid or outside the plastid during seed development, were verified by transcript and enzyme activity analyses, organelle proteomics, and transient expression assays. Protein, oil, central metabolites, cofactors, and acyl-acyl carrier protein (ACPs) levels were quantified overdevelopment. Amino and fatty acid levels were altered resulting in an increase in lipids by 0.5-2% of seed biomass (i.e. 2-9% change in oil). Subcellular targeting of a single gene product in central metabolism impacts carbon and reducing equivalent partitioning for seed storage reserves in soybeans.

Tissue-preferential recruitment of electron transfer chains for cytochrome P450-catalyzed phenolic biosynthesis

Cytochrome P450 system consists of P450 monooxygenase and redox pattern(s). While the importance of monooxygenases in plant metabolism is well documented, the metabolic roles of the related redox components have been largely overlooked. Here, we show that distinct electron transfer chains are recruited in phenylpropanoid-monolignol P450 systems to support the synthesis and distribution of different classes of phenolics in different plant tissues. While Arabidopsis cinnamate 4-hydroxylase adopts conventional NADPH-cytochrome P450 oxidoreductase (CPR) electron transfer chain for its para-hydroxylation reaction, ferulate 5-hydroxylase uses both NADPH-CPR-cytochrome b₅ (CB5) and NADH-cytochrome b₅ reductase-CB5 chains to support benzene ring 5-hydroxylation, in which the former route is primarily recruited in the stem for syringyl lignin synthesis, while the latter dominates in the syntheses of 5-hydroxylated phenolics in seeds and seed coat suberin. Our study unveils an additional layer of complexity and versatility of P450 system that the plants evolved for diversifying phenolic repertoires.

Giardia intestinalis incorporates heme into cytosolic cytochrome b₅

The anaerobic intestinal pathogen Giardia intestinalis does not possess enzymes for heme synthesis, and it also lacks the typical set of hemoproteins that are involved in mitochondrial respiration and cellular oxygen stress management. Nevertheless, G. intestinalis may require heme for the function of particular hemoproteins, such as cytochrome b5 (cytb5). We have analyzed the sequences of eukaryotic cytb5 proteins and identified three distinct cytb5 groups: group I, which consists of C-tail membrane-anchored cytb5 proteins; group II, which includes soluble cytb5 proteins; and group III, which comprises the fungal cytb5 proteins. The majority of eukaryotes possess both group I and II cytb5 proteins, whereas three Giardia paralogs belong to group II. We have identified a fourth Giardia cytb5 paralog (gCYTb5-IV) that is rather divergent and possesses an unusual 134-residue N-terminal extension. Recombinant Giardia cytb5 proteins, including gCYTb5-IV, were expressed in Escherichia coli and exhibited characteristic UV-visible spectra that corresponded to heme-loaded cytb5 proteins. The expression of the recombinant gCYTb5-IV in G. intestinalis resulted in the increased import of extracellular heme and its incorporation into the protein, whereas this effect was not observed when gCYTb5-IV containing a mutated heme-binding site was expressed. The electrons for Giardia cytb5 proteins may be provided by the NADPH-dependent Tah18-like oxidoreductase GiOR-1. Therefore, GiOR-1 and cytb5 may constitute a novel redox system in G. intestinalis. To our knowledge, G. intestinalis is the first anaerobic eukaryote in which the presence of heme has been directly demonstrated.

Structure of a cDNA for Ciona Cytochrome b(5) and the ubiquitous expression of mRNA in embryonic tissues

A cDNA clone for cytochrome b(5) was isolated from a cDNA library of an ascidian, Ciona savignyi, by a plaque hybridization method using a digoxigenin-labeled cDNA for the soluble form of human cytochrome b(5). The cDNA is composed of 5'- and 3'-noncoding sequences, and a 396-base pair coding sequence. The 3'-noncoding sequence contains polyadenylation signal sequences. The amino acid sequence of 132 residues deduced from the nucleotide sequence of the cDNA showed 61% identity and 82% similarity to the cytochrome b(5) of another ascidian species, Polyandrocarpa misakiensis, which we previously cloned. The amino-terminal hydrophilic domain of 98 residues contains well-conserved structures around two histidine residues for heme binding. A cDNA expression system was constructed to prepare a putative soluble form of Ciona cytochrome b(5). The recombinant soluble cytochrome b(5) showed an asymmetrical absorption spectrum at 560 nm as is shown by mammalian cytochromes b(5) upon reduction with NADH and NADH-cytochrome b(5) reductase. The recombinant Ciona cytochrome b(5) is reduced by NADH-cytochrome b(5) reductase with an apparent K(m) value of 3.3 microM. This value is similar to that of the cytochrome b(5) of Polyandrocarpa misakiensis. The expression of Ciona cytochrome b(5) mRNA during development was examined by an in situ hybridization method and ubiquitous expression in embryonic tissues was observed. The results indicate that cytochrome b(5) plays important roles in various metabolic processes during development.

Heterologous expression of Ascaris suum cytochrome b5 precursor protein: a histidine-tagged full-length presequence is correctly processed to transport the mature protein to the periplasm of Escherichia coli

The cytochrome b(5) of the body wall of adult Ascaris suum, a porcine parasitic nematode, is a novel type of cytochrome b(5). It is a soluble protein that lacks the COOH-terminal membrane-anchoring domain found in erythrocyte cytochrome b(5), but possesses an NH(2)-terminal extension (presequence) of 30 amino acids that are missing from the 82-residue protein purified from the nematode tissues [Yu, Y., Yamasaki, H., Kita, K., and Takamiya, S., 1996, Arch. Biochem. Biophys. 328, 165-172]. The nematode cytochrome b(5) is, therefore, probably synthesized as a precursor protein whose presequence is cleaved to form a mature protein, but the localization of the mature protein is still unknown. To investigate the processing of the putative precursor protein, the wild-type precursor of nematode cytochrome b(5) with a complete presequence (b5wt) and its NH(2) terminus-truncated derivatives, b5Delta18 and b5Delta28, with 18 and 28 residues deleted, respectively, were expressed using pET-28a(+) vector in Escherichia coli. As expected, all transformants, tb5wt, tb5Delta18, and tb5Delta28, produced recombinant proteins with a histidine-tagged NH(2)-terminal extension. However, only the recombinant protein with the full-length presequence, produced in tb5wt, was correctly processed and transported to the periplasm, from which the majority of the induced product was purified as a mature protein chemically and functionally identical to the native protein purified from the nematode body wall. These results clearly show that the nematode histidine-tagged presequence functions as a signal peptide in E. coli.

Biodiversity of the P450 catalytic cycle: yeast cytochrome b5/NADH cytochrome b5 reductase complex efficiently drives the entire sterol 14-demethylation (CYP51) reaction

The widely accepted catalytic cycle of cytochromes P450 (CYP) involves the electron transfer from NADPH cytochrome P450 reductase (CPR), with a potential for second electron donation from the microsomal cytochrome b5/NADH cytochrome b5 reductase system. The latter system only supported CYP reactions inefficiently. Using purified proteins including Candida albicans CYP51 and yeast NADPH cytochrome P450 reductase, cytochrome b5 and NADH cytochrome b5 reductase, we show here that fungal CYP51 mediated sterol 14alpha-demethylation can be wholly and efficiently supported by the cytochrome b5/NADH cytochrome b5 reductase electron transport system. This alternative catalytic cycle, where both the first and second electrons were donated via the NADH cytochrome b5 electron transport system, can account for the continued ergosterol production seen in yeast strains containing a disruption of the gene encoding CPR.

Temporal and spatial gene expression of cytochrome B5 during flower and fruit development in olives

We report the characterisation of two cytochrome b5 genes and their spatial and temporal patterns of expression during development in olive, Olea europaea. A PCR-generated probe, based on a tobacco cytochrome b5 sequence, was used to isolate two full-length cDNA clones (cytochrome b5-15 and cytochrome b5-38) from a library derived from 13 WAF olive fruits. The cDNAs encoded proteins of 17.0 and 17.7 kDa, which contained all the characteristic motifs of cytochromes b5 from other organisms and exhibited 63% identity and 85% similarity with each other. The olive cytochrome b5-15 cDNA was then used as a probe for more detailed analysis. Southern blotting revealed a gene family of at least 4-6 members while northern blotting and in situ hybridisation showed a highly specific pattern of gene expression. Very low levels of cytochrome b5 mRNA were detected in tissues characterised by high rates of lipid accumulation, such as young expanding leaves, maturing seeds and ripening mesocarp. The cytochrome b5 genes were not induced at 6 degrees C and their response to ABA was relatively slow compared with fatty acid desaturase genes. In contrast, high levels of cytochrome b5 gene expression were found in young fruits at the pattern formation (globular/heart) stage of embryogenesis and in vascular and transmitting tissues of male and female reproductive organs. The data are consistent with a major role for cytochrome b5 in developmental processes related to plant reproduction in addition to being an electron donor to microsomal desaturases.

Microsomal electron transfer in higher plants: cloning and heterologous expression of NADH-cytochrome b5 reductase from Arabidopsis

AtCBR, a cDNA encoding NADH-cytochrome (Cyt) b5 reductase, and AtB5-A and AtB5-B, two cDNAs encoding Cyt b5, were isolated from Arabidopsis. The primary structure deduced from the AtCBR cDNA was 40% identical to those of the NADH-Cyt b5 reductases of yeast and mammals. A recombinant AtCBR protein prepared using a baculovirus system exhibited typical spectral properties of NADH-Cyt b5 reductase and was used to study its electron-transfer activity. The recombinant NADH-Cyt b5 reductase was functionally active and displayed strict specificity to NADH for the reduction of a recombinant Cyt b5 (AtB5-A), whereas no Cyt b5 reduction was observed when NADPH was used as the electron donor. Conversely, a recombinant NADPH-Cyt P450 reductase of Arabidopsis was able to reduce Cyt b5 with NADPH but not with NADH. To our knowledge, this is the first evidence in higher plants that both NADH-Cyt b5 reductase and NADPH-Cyt P450 reductase can reduce Cyt b5 and have clear specificities in terms of the electron donor, NADH or NADPH, respectively. This substrate specificity of the two reductases is discussed in relation to the NADH- and NADPH-dependent activities of microsomal fatty acid desaturases.

Primary Structure of Cytochrome b(5) from Cauliflower (Brassica oleracea L.) Deduced from Peptide and cDNA Sequences

Cytochrome b(5) is a microsomal protein that functions as an intermediate electron donor in fatty acid desaturation and other oxidation/reduction reactions. cDNA clones were isolated from cauliflower (Brassica oleracea L.) by using oligonucleotides based on the partial amino acid sequence of the protein. The deduced amino acid sequence of the polypeptide exhibited approximately 30% sequence identity with the homologous protein from vertebrates.

The role of cytochrome b5 in delta 12 desaturation of oleic acid by microsomes of safflower (Carthamus tinctorius L.)

The electron donors for the membrane-bound fatty acid desaturases of higher plants have not previously been identified. In order to assess the participation of cytochrome b5 in microsomal fatty acid desaturation, the cytoplasmic domain of microsomal cytochrome b5 was purified from Brassica oleracea, and murine polyclonal antibodies were prepared. The IgG fraction from ascites fluid inhibited 62% of NADH-dependent cytochrome c reduction in safflower (Carthamus tinctorius L.) microsomes. These antibodies also blocked desaturation of oleic acid to linoleic acid in lipids of C. tinctorius microsomes by 93%, suggesting that cytochrome b5 is the electron donor for the delta 12 desaturase.

Electron-transport components of the 1-acyl-2-oleoyl-sn-glycero-3-phosphocholine delta 12-desaturase (delta 12-desaturase) in microsomal preparations from developing safflower (Carthamus tinctorius L.) cotyledons

The major cytochrome in microsomal membrane preparations from developing seeds of safflower (Carthamus tinctorius, var High Linoleate), has a reduced-minus-oxidized difference spectrum characteristic of a b-type cytochrome, and was identified from its midpoint-potential (E'7.2) value as cytochrome b5. Cytochromes P-450 and P-420 were also present. The cytochrome b5 content of microsomal preparations from a number of oilseed species was found to be in the order of 200-300 pmol/mg of protein. The cytochrome b5 was reduced in the membrane preparations by NADH, demonstrating the presence of an NADH: cytochrome b5 reductase; NADPH was a less effective donor. Microsomal membranes catalysed the NAD(P)H-dependent conversion of radioactive oleate into linoleate, indicating acyl-CoA: lysophosphatidylcholine acyltransferase and 1-acyl-2-oleoyl-sn-glycero-3-phosphocholine delta 12-desaturase (delta 12-desaturase) activity. Desaturation of oleate to linoleate was unaffected by CO, but inhibited by CN-. The addition of oleoyl-CoA to the NADH-reduced membranes resulted in the CN(-)-sensitive partial re-oxidation of cytochrome b5, indicating that electrons from NADH were transferred to the site of desaturation via this cytochrome. The delta 12-desaturase in safflower, therefore, is CN(-)-sensitive and appears to require cytochrome b5 and NADH: cytochrome b5 reductase for activity.

Interference of electron transport inhibitors with desaturation of monogalactosyl diacylglycerol in intact chloroplasts

Isolated intact chloroplasts are able to desaturate fatty acids in newly synthesized monogalactosyl diacylglycerol. By analogy with other systems, this desaturation might be expected to involve electron carriers. The effects of electron transport inhibitors on chloroplast lipid-linked desaturation were therefore investigated. Because desaturation occurs in the dark and is not inhibited by compounds specifically blocking photosystem II, it appeared that the photosystems themselves did not participate. Several compounds that prevent enzymatic reoxidation of plastoquinol in thylakoid membranes at the Qz site or withdraw electrons from this lipophilic electron carrier inhibited desaturation in the dark. This inhibition could not be reversed by adding chemicals that donate electrons to photosystem I, indicating that carriers past the cytochrome b/f complex were not involved. Inhibitors of cyclic electron transport interfered with desaturation only at rather high concentrations or not at all. Additional compounds that block the reduction of quinones were slightly inhibitory. Dithioerythritol and KCN also inhibited desaturation, although their exact mode of action is unknown. Dinitrophenyl-iodonitrothymol (DNP-INT), stigmatellin, and myxothiazol did not block desaturation at concentrations that inhibited photosynthetic electron flow through the Qz site very efficiently. Therefore, these results argue against an involvement of the Qz site in desaturation. Accordingly, the inhibition by the other compounds seemingly interfering at the same site as well as that by electron acceptors could be due to interference at a different redox step in desaturation. In vitro these compounds function also as electron acceptors in diaphorase reactions catalyzed by ferredoxin:NADP oxidoreductase.

Purification and Characterization of Microsomal Cytochrome b(5) and NADH Cytochrome b(5) Reductase from Pisum sativum

In this communication we document the reproducible protocols for the purification of milligram quantities of cytochrome b(5) and NADH-cytochrome b(5) reductase from the microsomal fraction of Pisum sativum. The cytochrome b(5) component of this NADH linked electron transport chain was found to have a molecular mass of 16,400 daltons and the reductase a molecular mass of 34,500 daltons. These components could be reconstituted into a functional NADH oxidase activity active in the reduction of exogenous cytochrome c or ferricyanide. In the latter assay the purified reductase exhibited a turnover number of 22,000 per minute. The amino-terminal amino acid sequence of the cytochrome b(5) component was determined by sequential Edmund degredation, thus providing crucial information for the efficient cloning of this central protein of plant microsomal electron transfer.