Comparison of cytochromes b5 from insects and vertebrates

Structure of cytochrome b5 unique to tardigrades

Cytochrome b₅ is an essential electron transfer protein, which is ubiquitously found in living systems and involved in wide variety of biological processes. Tardigrades (also known as water bears), some of which are famous for desiccation resistance, have many proteins unique to them. Here, we report spectroscopic and structural characterization of a cytochrome b₅ like protein from one of the desiccation‐tolerant tardigrades, Ramazzottius varieornatus strain YOKOZUNA‐1 (RvCytb₅). A 1.4 Å resolution crystal structure revealed that RvCytb₅ is a new cytochrome b₅ protein specific to tardigrades.

PDB Code(s): 7BWH;

Role of cytochrome b5 in the modulation of the enzymatic activities of cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1)

Cytochrome b5 (cyt b5) is a small hemoprotein that plays a significant role in the modulation of activities of an important steroidogenic enzyme, cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1, CYP17A1). Located in the zona fasciculata and zona reticularis of the adrenal cortex and in the gonads, P450 17A1 catalyzes two different reactions in the steroidogenic pathway; the 17α-hydroxylation and 17,20-lyase, in the endoplasmic reticulum of these respective tissues. The activities of P450 17A1 are regulated by cyt b5 that enhances the 17,20-lyase reaction by promoting the coupling of P450 17A1 and cytochrome P450 reductase (CPR), allosterically. Cyt b5 can also act as an electron donor to enhance the 16-ene-synthase activity of human P450 17A1. In this review, we discuss the many roles of cyt b5 and focus on the modulation of CYP17A1 activities by cyt b5 and the mechanisms involved.

Functional and structural characterisation of a viral cytochrome b5

Cytochrome b5 is a ubiquitous electron transport protein. The sequenced viral OtV-2 genome, which infects Ostreococcus tauri, was predicted to encode a putative cytochrome b5 enzyme. Using purified OtV-2 cytochrome b5 we confirm this protein has identical spectral properties to purified human cytochrome b5 and additionally that the viral enzyme can substitute for yeast cytochrome b5 in yeast cytochrome P450 51 mediated sterol 14α-demethylation. The crystal structure of the OtV-2 cytochrome b5 enzyme reveals a single domain, comprising four β sheets, four α helices and a haem moiety, which is similar to that found in larger eukaryotic cytochrome proteins. As a product of a horizontal gene transfer event involving a subdomain of the host fumarate reductase-like protein, OtV-2 cytochrome b5 appears to have diverged in function and is likely to have evolved an entirely new role for the virus during infection. Indeed, lacking a hydrophobic C-terminal anchor, OtV-2 encodes the first cytosolic cytochrome b5 characterised. The lack of requirement for membrane attachment (in contrast to all other microsomal cytochrome b5s) may be a reflection of the small size of the host cell, further emphasizes the unique nature of this virus gene product and draws attention to the potential importance of cytochrome b5 metabolic activity at the extremes of cellular scale.

Site-specific covalent attachment of heme proteins on self-assembled monolayers

Naturally occurring hemin cofactor has been functionalized to introduce two terminal alkyne groups. This modified hemin has been successfully covalently attached to mixed self-assembled monolayers of alkanethiols and azide-terminated alkanethiols on gold electrodes using a Cu(I)-catalyzed 1,3-cycloaddition reaction. However these hemin-modified electrodes could not be used to reconstitute apomyoglobin on gold electrodes owing to the hydrophobicity of the alkane thiol self-assembled monolayer. Modification of existing techniques allowed covalent attachment of alkyne-terminated electroactive species onto mixed monolayers of azidothiols and carboxylatoalkanethiols on electrodes using the same Cu(I)-catalyzed 1,3-cycloaddition reaction. Apomyoglobin could be reconstituted using the hemin covalently attached to these hydrophilic electrodes. The electrochemical data, UV-vis absorption data, surface-enhanced resonance Raman spectroscopy data, and atomic force microscopy data indicate the presence of these modified myoglobin proteins on these electrodes. The direct attachment of the heme cofactor of these modified myoglobin proteins to the electrode allows fast electron transfer to the heme center from the electrode and affords efficient O(2)-reducing bioelectrodes under physiological conditions.

Photosynthetic cytochrome c550

Cytochrome c550 (cyt c550) is a membrane component of the PSII complex in cyanobacteria and some eukaryotic algae, such as red and brown algae. Cyt c550 presents a bis-histidine heme coordination which is very unusual for monoheme c-type cytochromes. In PSII, the cyt c550 with the other extrinsic proteins stabilizes the binding of Cl(-) and Ca(2+) ions to the oxygen evolving complex and protects the Mn(4)Ca cluster from attack by bulk reductants. The role (if there is one) of the heme of the cyt c550 is unknown. The low midpoint redox potential (E(m)) of the purified soluble form (from -250 to -314mV) is incompatible with a redox function in PSII. However, more positive values for the Em have been obtained for the cyt c550 bound to the PSII. A very recent work has shown an E(m) value of +200mV. These data open the possibility of a redox function for this protein in electron transfer in PSII. Despite the long distance (22Å) between cyt c550 and the nearest redox cofactor (Mn(4)Ca cluster), an electron transfer reaction between these components is possible. Some kind of protective cycle involving a soluble redox component in the lumen has also been proposed. The aim of this article is to review previous studies done on cyt c550 and to consider its function in the light of the new results obtained in recent years. The emphasis is on the physical properties of the heme and its redox properties. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Accommodating a nonconservative internal mutation by water-mediated hydrogen bonding between β-sheet strands: a comparison of human and rat type B (mitochondrial) cytochrome b5

Mammalian type B (mitochondrial) b(5) cytochromes exhibit greater amino acid sequence diversity than their type A (microsomal) counterparts, as exemplified by the type B proteins from human (hCYB5B) and rat (rCYB5B). The comparison of X-ray crystal structures of hCYB5B and rCYB5B reported herein reveals a striking difference in packing involving the five-strand β-sheet, which can be attributed to fully buried residue 21 in strand β4. The greater bulk of Leu21 in hCYB5B in comparison to that of Thr21 in rCYB5B results in a substantial displacement of the first two residues in β5, and consequent loss of two of the three hydrogen bonds between β5 and β4. Hydrogen bonding between the residues is instead mediated by two well-ordered, fully buried water molecules. In a 10 ns molecular dynamics simulation, one of the buried water molecules in the hCYB5B structure exchanged readily with solvent via intermediates having three water molecules sandwiched between β4 and β5. When the buried water molecules were removed prior to a second 10 ns simulation, β4 and β5 formed persistent hydrogen bonds identical to those in rCYB5B, but the Leu21 side chain was forced to adopt a rarely observed conformation. Despite the apparently greater ease of access of water to the interior of hCYB5B than of rCYB5B suggested by these observations, the two proteins exhibit virtually identical stability, dynamic, and redox properties. The results provide new insight into the factors stabilizing the cytochrome b(5) fold.

Regulation of hemocytes in Drosophila requires dappled cytochrome b5

A major category of mutant hematopoietic phenotypes in Drosophila is melanotic tumors or nodules, which consist of abnormal and overproliferated blood cells, similar to granulomas. Our analyses of the melanotic mutant dappled have revealed a novel type of gene involved in blood cell regulation. The dappled gene is an essential gene that encodes cytochrome b5, a conserved hemoprotein that participates in electron transfer in multiple biochemical reactions and pathways. Viable mutations of dappled cause melanotic nodules and hemocyte misregulation during both hematopoietic waves of development. The sexes are similarly affected, but hemocyte number is different in females and males of both mutants and wild type. Additionally, initial tests show that curcumin enhances the dappled melanotic phenotype and establish screening of endogenous and xenobiotic compounds as a route for analysis of cytochrome b5 function. Overall, dappled provides a tractable genetic model for cytochrome b5, which has been difficult to study in higher organisms.

Enhancing the thermal stability of mitochondrial cytochrome b5 by introducing a structural motif characteristic of the less stable microsomal isoform

Outer mitochondrial membrane cytochrome b5 (OM b5) is the most thermostable cytochrome b5 isoform presently known. Herein, we show that OM b5 thermal stability is substantially enhanced by swapping an apparently invariant motif in its heme-independent folding core with the corresponding motif characteristic of its less stable evolutionary relative, microsomal cytochrome b5 (Mc b5). The motif swap involved replacing two residues, Arg15 with His and Glu20 with Ser, thereby introducing a Glu11-His15-Ser20 H-bonding triad on the protein surface along with a His15/Trp22 pi-stacking interaction. The ferric and ferrous forms of the OM b5 R15H/E20S double mutant have thermal denaturation midpoints (Tm values) of approximately 93 degrees C and approximately 104 degrees C, respectively. A 15 degrees C increase in apoprotein Tm plays a key role in the holoprotein thermal stability enhancement, and is achieved by one of the most common natural mechanisms for stabilization of thermophilic versus mesophilic proteins: raising the unfolding free energy along the entire stability curve.