Reconstitution of the two terminal enzymes of the heme biosynthetic pathway into phospholipid vesicles

Phosphatidylglycerol-derived phospholipids have a universal, domain-crossing role in stress responses

Phosphatidylglycerol and phospholipids derived from it are widely distributed throughout the three domains of life. Cardiolipin is the best characterized of these phospholipids, and plays a key role in the response to environmental variations. Phosphatidylglycerol-derived phospholipids confer cell membranes with a wide range of responses, including changes in surface charge, fluidity, flexibility, morphology, biosynthesis and remodeling, that adapt the cell to these situations. Furthermore, the synthesis and remodeling of these phospholipids is finely regulated, highlighting the importance of these lipids in cell homeostasis and responses during stressful situations. In this article, we review the most important roles of these anionic phospholipids across domains, focusing on the biophysical basis by which these phospholipids are used in stress responses.

Control of intracellular heme levels: heme transporters and heme oxygenases

Heme serves as a co-factor in proteins involved in fundamental biological processes including oxidative metabolism, oxygen storage and transport, signal transduction and drug metabolism. In addition, heme is important for systemic iron homeostasis in mammals. Heme has important regulatory roles in cell biology, yet excessive levels of intracellular heme are toxic; thus, mechanisms have evolved to control the acquisition, synthesis, catabolism and expulsion of cellular heme. Recently, a number of transporters of heme and heme synthesis intermediates have been described. Here we review aspects of heme metabolism and discuss our current understanding of heme transporters, with emphasis on the function of the cell-surface heme exporter, FLVCR. Knockdown of Flvcr in mice leads to both defective erythropoiesis and disturbed systemic iron homeostasis, underscoring the critical role of heme transporters in mammalian physiology. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Crystal structure of protoporphyrinogen oxidase from Myxococcus xanthus and its complex with the inhibitor acifluorfen

Protoporphyrinogen IX oxidase, a monotopic membrane protein, which catalyzes the oxidation of protoporphyrinogen IX to protoporphyrin IX in the heme/chlorophyll biosynthetic pathway, is distributed widely throughout nature. Here we present the structure of protoporphyrinogen IX oxidase from Myxococcus xanthus, an enzyme with similar catalytic properties to human protoporphyrinogen IX oxidase that also binds the common plant herbicide, acifluorfen. In the native structure, the planar porphyrinogen substrate is mimicked by a Tween 20 molecule, tracing three sides of the macrocycle. In contrast, acifluorfen does not mimic the planarity of the substrate but is accommodated by the shape of the binding pocket and held in place by electrostatic and aromatic interactions. A hydrophobic patch surrounded by positively charged residues suggests the position of the membrane anchor, differing from the one proposed for the tobacco mitochondrial protoporphyrinogen oxidase. Interestingly, there is a discrepancy between the dimerization state of the protein in solution and in the crystal. Conserved structural features are discussed in relation to a number of South African variegate porphyria-causing mutations in the human enzyme.

Characterization of the iron-binding site in mammalian ferrochelatase by kinetic and Mössbauer methods

All organisms utilize ferrochelatase (protoheme ferrolyase, EC 4.99.1.1) to catalyze the terminal step of the heme biosynthetic pathway, which involves the insertion of ferrous ion into protoporphyrin IX. Kinetic methods and Mössbauer spectroscopy have been used in an effort to characterize the ferrous ion-binding active site of recombinant murine ferrochelatase. The kinetic studies indicate that dithiothreitol, a reducing agent commonly used in ferrochelatase activity assays, interferes with the enzymatic production of heme. Ferrochelatase specific activity values determined under strictly anaerobic conditions are much greater than those obtained for the same enzyme under aerobic conditions and in the presence of dithiothreitol. Mössbauer spectroscopy conclusively demonstrates that, under the commonly used assay conditions, dithiothreitol chelates ferrous ion and hence competes with the enzyme for binding the ferrous substrate. Mössbauer spectroscopy of ferrous ion incubated with ferrochelatase in the absence of dithiothreitol shows a somewhat broad quadrupole doublet. Spectral analysis indicates that when 0.1 mM Fe(II) is added to 1.75 mM ferrochelatase, the overwhelming majority of the added ferrous ion is bound to the protein. The spectroscopic parameters for this bound species are delta = 1.36 +/- 0.03 mm/s and delta EQ = 3.04 +/- 0.06 mm/s, distinct from the larger delta EQ of a control sample of Fe(II) in buffer only. The parameters for the bound species are consistent with an active site composed of nitrogenous/oxygenous ligands and inconsistent with the presence of sulfur ligands. This finding is in accord with the absence of conserved cysteines among the known ferrochelatase sequences. The implications these results have with regard to the mechanism of ferrochelatase activity are discussed.

Characteristics of murine protoporphyrinogen oxidase

Protoporphyrinogen oxidase (EC 1.3.3.4) (PPO) is the penultimate enzyme of the heme biosynthetic pathway. Mouse PPO has been purified in low yield and kinetically characterized by this laboratory previously. A new more rapid purification procedure is described herein, and with this protein we detect a noncovalently bound flavin moiety. This flavin is present at approximately stoichiometric amounts in the purified enzyme and has been identified by its fluorescence spectrum and high performance liquid chromatography as flavin mononucleotide (FMN). Fluorescence quenching studies on the flavin yielded a Stern-Volmer quenching constant of 12.08 M-1 for iodide and 1.1 M-1 for acrylamide. Quenching of enzyme tryptophan fluorescence resulted in quenching constants of 6 M-1 and 10 M-1 for iodide and acrylamide, respectively. Plasma scans performed on purified enzyme preparations did not reveal the presence of stoichiometric amounts of protein-bound metal ions, and we were unable to detect any protein-associated pyrroloquinoline quinone (PQQ). Data from circular dichroism studies predict a secondary structure of the native protein consisting of 30.5% alpha helix, 40.5% beta sheet, 13.7% turn, and 15.3% random coil. Denaturation of PPO with urea resulted in a biphasic curve when ellipticity is plotted against urea concentration, typical of amphipathic proteins.

Interaction of phospholipids with proteins, peptides and amino acids. New advances 1987-1989

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins, peptides and amino acids. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. The effect of the interaction on the structure and biological activity of the interacting biomolecules is discussed.

Lipid regulation of bovine cytochrome P450(11) beta activity

Purified bovine adrenocortical cytochrome P450(11) beta has been reconstituted into phospholipid vesicles using a detergent dialysis procedure. Using this reconstituted system, we have examined the effect of changes in the fatty acyl substituents of the lipids on the catalytic activity of the enzyme. The studies reported here show that cytochrome P450(11) beta exhibits a completely different response to changes in the fatty acyl groups from that shown by cytochrome P450scc. Cytochrome P450(11) beta displays maximal activity in lipid vesicles composed of saturated lipids, such as dipalmitoyl and dimyristoyl phosphatidylcholines, with turnover numbers ranging from 35 to 60 min-1. Incremental increases of phospholipids such as diphytanoyl and dioleoyl phosphatidylcholines result in a progressive inhibition of 11 beta hydroxylase activity; most of this kinetic effect is attributable to a significant decrease in Vmax accompanied by modest changes in Km for the steroid substrate deoxycorticosterone. Diphosphatidyl glycerol (cardiolipin), which has been previously shown to activate cytochrome P450scc, is a potent inhibitor of the 11 beta hydroxylase activity of cytochrome P450(11) beta, with half maximum inhibition observed in vesicles containing 4-5 mol% diphosphatidyl glycerol. Kinetic analysis demonstrates that this inhibition by diphosphatidyl glycerol is reflected in both a decrease in Vmax and relatively large increases (up to sevenfold) in Km for the steroid substrate. These effects on the 11 beta hydroxylase activity may have important implications for the in vivo regulation of not only the 11 beta hydroxylase activity, but also the other catalytic activities of this enzyme, particularly 18- and 19-hydroxylase and oxidase activities.

Interaction of free porphyrins and metalloporphyrins with mouse ferrochelatase. A model for the active site of ferrochelatase

The ability of purified mouse ferrochelatase (protoheme ferro-lyase, EC 4.99.1.1) to bind and catalytically utilize a variety of porphyrins has been examined. In all, the kd, Km or Ki values for eleven different porphyrins, the Ki values for four metalloporphyrins and the kd values for two metalloporphyrins were determined. The data obtained demonstrate that mouse ferrochelatase binds a wide variety of porphyrins and metalloporphyrins with kd values ranging from 6 nM for N-methylprotoporphyrin to 1.08 microM for coproporphyrin III. However, the enzyme shows a degree of catalytic specificity for the substituents at the 2.4 positions and utilizes only proto-, hemato-, meso-, deutero-, 2,4-monohydroxy-ethylmonovinyl- and 2,4-monohydroxymethylmonovinyl deuteroporphyrin as substrates. The data show that the magnitude of the kd is not an accurate indicator of the ability of the porphyrin to serve as a substrate or inhibitor and, with the exception of N-methylprotoporphyrin, the size of the kd is several orders of magnitude less than that of the Km or Ki. Of the metalloporphyrins examined (Fe, Co, Zn and Sn) all inhibited ferrochelatase at micromolar concentrations, although tin protoporphyrin was the least effective. These data are discussed in terms of an active site model for mammalian ferrochelatase.