Biomedical subjects

Salam Al-Karadaghi

Publications and source records attributed to Salam Al-Karadaghi.

12 recordsLinked to original sources

PubMed2007

Amino acid residues His183 and Glu264 in Bacillus subtilis ferrochelatase direct and facilitate the insertion of metal ion into protoporphyrin IX.

Ferrochelatase catalyzes the terminal step in the heme biosynthetic pathway, i.e., the incorporation of Fe(II) into protoporphyrin IX. Various biochemical and biophysical methods have been used to probe the enzyme for metal binding residues and the location of the active site. However, the location of the metal binding site and the path of the metal into the porphyrin are still disputed. Using site-directed mutagenesis on Bacillus subtilis ferrochelatase we demonstrate that exchange of the conserved residues His183 and Glu264 affects the metal affinity of the enzyme. We also present the first X-ray crystal structure of ferrochelatase with iron. Only a single iron was found in the active site, coordinated in a square pyramidal fashion by two amino acid residues, His183 and Glu264, and three water molecules. This iron was not present in the structure of a His183Ala modified ferrochelatase. The results strongly suggest that the insertion of a metal ion into protoporphyrin IX by ferrochelatase occurs from a metal binding site represented by His183 and Glu264.

Bacillus subtilis↗

PubMed2006

ATPase activity associated with the magnesium chelatase H-subunit of the chlorophyll biosynthetic pathway is an artefact.

Magnesium chelatase inserts Mg2+ into protoporphyrin IX and is the first unique enzyme of the chlorophyll biosynthetic pathway. It is a heterotrimeric enzyme, composed of I- (40 kDa), D- (70 kDa) and H- (140 kDa) subunits. The I- and D-proteins belong to the family of AAA+ (ATPases associated with various cellular activities), but only I-subunit hydrolyses ATP to ADP. The D-subunits provide a platform for the assembly of the I-subunits, which results in a two-tiered hexameric ring complex. However, the D-subunits are unstable in the chloroplast unless ATPase active I-subunits are present. The H-subunit binds protoporphyrin and is suggested to be the catalytic subunit. Previous studies have indicated that the H-subunit also has ATPase activity, which is in accordance with an earlier suggested two-stage mechanism of the reaction. In the present study, we demonstrate that gel filtration chromatography of affinity-purified Rhodobacter capsulatus H-subunit produced in Escherichia coli generates a high- and a low-molecular-mass fraction. Both fractions were dominated by the H-subunit, but the ATPase activity was only found in the high-molecular-mass fraction and magnesium chelatase activity was only associated with the low-molecular-mass fraction. We demonstrated that light converted monomeric low-molecular-mass H-subunit into high-molecular-mass aggregates. We conclude that ATP utilization by magnesium chelatase is solely connected to the I-subunit and suggest that a contaminating E. coli protein, which binds to aggregates of the H-subunit, caused the previously reported ATPase activity of the H-subunit.

Adenosine Triphosphatases↗

PubMed2006

Recessiveness and dominance in barley mutants deficient in Mg-chelatase subunit D, an AAA protein involved in chlorophyll biosynthesis.

Mg-chelatase catalyzes the insertion of Mg2+ into protoporphyrin IX at the first committed step of the chlorophyll biosynthetic pathway. It consists of three subunits: I, D, and H. The I subunit belongs to the AAA protein superfamily (ATPases associated with various cellular activities) that is known to form hexameric ring structures in an ATP-dependant fashion. Dominant mutations in the I subunit revealed that it functions in a cooperative manner. We demonstrated that the D subunit forms ATP-independent oligomeric structures and should also be classified as an AAA protein. Furthermore, we addressed the question of cooperativity of the D subunit with barley (Hordeum vulgare) mutant analyses. The recessive behavior in vivo was explained by the absence of mutant proteins in the barley cell. Analogous mutations in Rhodobacter capsulatus and the resulting D proteins were studied in vitro. Mixtures of wild-type and mutant R. capsulatus D subunits showed a lower activity compared with wild-type subunits alone. Thus, the mutant D subunits displayed dominant behavior in vitro, revealing cooperativity between the D subunits in the oligomeric state. We propose a model where the D oligomer forms a platform for the stepwise assembly of the I subunits. The cooperative behavior suggests that the D oligomer takes an active part in the conformational dynamics between the subunits of the enzyme.

Amino Acid Sequence↗

PubMed2006

Transmembrane topology of FRO2, a ferric chelate reductase from Arabidopsis thaliana.

Iron uptake in Arabidopsis thaliana is mediated by ferric chelate reductase FRO2, a transmembrane protein belonging to the flavocytochrome b family. There is no high resolution structural information available for any member of this family. We have determined the transmembrane topology of FRO2 experimentally using the alkaline phosphatase fusion method. The resulting topology is different from that obtained by theoretical predictions and contains 8 transmembrane helices, 4 of which build up the highly conserved core of the protein. This core is present in the entire flavocytochrome b family. The large water soluble domain of FRO2, which contains NADPH, FAD and oxidoreductase sequence motifs, was located on the inside of the membrane.

Alkaline Phosphatase↗

PubMed2006

Chelatases: distort to select?

Chelatases catalyze the insertion of a specific metal ion into porphyrins, a key step in the synthesis of metalated tetrapyrroles that are essential for many cellular processes. Despite apparent common structural features among chelatases, no general reaction mechanism accounting for metal ion specificity has been established. We propose that chelatase-induced distortion of the porphyrin substrate not only enhances the reaction rate by decreasing the activation energy of the reaction but also modulates which divalent metal ion is incorporated into the porphyrin ring. We evaluate the recently recognized interaction between ferrochelatase and frataxin as a way to regulate iron delivery to ferrochelatase, and thus iron and heme metabolism. We postulate that the ferrochelatase-frataxin interaction controls the type of metal ion that is delivered to ferrochelatase.

Animals↗

PubMed2006

The structures of frataxin oligomers reveal the mechanism for the delivery and detoxification of iron.

Defects in the mitochondrial protein frataxin are responsible for Friedreich ataxia, a neurodegenerative and cardiac disease that affects 1:40,000 children. Here, we present the crystal structures of the iron-free and iron-loaded frataxin trimers, and a single-particle electron microscopy reconstruction of a 24 subunit oligomer. The structures reveal fundamental aspects of the frataxin mechanism. The trimer has a central channel in which one atom of iron binds. Two conformations of the channel with different metal-binding affinities suggest that a gating mechanism controls whether the bound iron is delivered to other proteins or transferred to detoxification sites. The trimer constitutes the basic structural unit of the 24 subunit oligomer. The architecture of this oligomer and several features of the trimer structure demonstrate striking similarities to the iron-storage protein ferritin. The data reveal how stepwise assembly provides frataxin with the structural flexibility to perform two functions: metal delivery and detoxification.

Amino Acid Sequence↗

PubMed2005

Metallation of the transition-state inhibitor N-methyl mesoporphyrin by ferrochelatase: implications for the catalytic reaction mechanism.

Insertion of metals into various tetrapyrroles is catalysed by a group of enzymes called chelatases, e.g. nickel, cobalt, magnesium and ferro-chelatase. It has been proposed that catalytic metallation includes distorting the porphyrin substrate by the enzyme towards a transition state-like geometry in which at least one of the pyrrole rings will be available for metal chelation. Here, we present a study of metal insertion into the transition-state inhibitor of protoporphyrin IX ferrochelatase, N-methyl mesoporphyrin (N-MeMP), by time-resolved crystallography and mass spectrometry with and without the presence of ferrochelatase. The results show that metallation of N-MeMP has a very limited effect on the conformation of the residues that participate in porphyrin and metal binding. These findings support theoretical data, which indicate that product release is controlled largely by the strain created by metal insertion into the distorted porphyrin. The results suggest that, similar to non-catalytic metallation of N-MeMP, the ferrochelatase-assisted metallation depends on the ligand exchange rate for the respective metal. Moreover, ferrochelatase catalyses insertion of Cu(II) and Zn(II) into N-MeMP with a rate that is about 20 times faster than non-enzymatic metallation in solution, suggesting that the catalytic strategy of ferrochelatase includes a stage of acceleration of the rate of ligand exchange for the metal substrate. The greater efficiency of N-MeMP metallation by Cu(II), as compared to Zn(II), contrasts with the K(m) values for Zn(II) (17 microM) and Cu(II) (170 microM) obtained for metallation of protoporphyrin IX. We suggest that this difference in metal specificity depends on the type of distortion imposed by the enzyme on protoporphyrin IX, which is different from the intrinsic non-planar distortion of N-MeMP. A mechanism of control of metal specificity by porphyrin distortion may be general for different chelatases, and may have common features with the mechanism of metal specificity in crown ethers.

Bacillus subtilis↗

PubMed2005

Cloning, expression, characterisation and three-dimensional structure determination of Caenorhabditis elegans spermidine synthase.

The polyamine synthesis enzyme spermidine synthase (SPDS) has been cloned from the model nematode Caenorhabditis elegans. Biochemical characterisation of the recombinantly expressed protein revealed a high degree of similarity to other eukaryotic SPDS with the exception of a low affinity towards the substrate decarboxylated S-adenosylmethionine (Km = 110 microM) and a less pronounced feedback inhibition by the second reaction product 5'-methylthioadenosine (IC50 = 430 microM). The C. elegans protein that carries a nematode-specific insertion of 27 amino acids close to its N-terminus was crystallized, leading to the first X-ray structure of a dimeric eukaryotic SPDS.

Amino Acid Sequence↗

PubMed2003

Metal binding to Bacillus subtilis ferrochelatase and interaction between metal sites.

Ferrochelatase, the terminal enzyme in heme biosynthesis, catalyses metal insertion into protoporphyrin IX. The location of the metal binding site with respect to the bound porphyrin substrate and the mode of metal binding are of central importance for understanding the mechanism of porphyrin metallation. In this work we demonstrate that Zn(2+), which is commonly used as substrate in assays of the ferrochelatase reaction, and Cd(2+), an inhibitor of the enzyme, bind to the invariant amino acids His183 and Glu264 and water molecules, all located within the porphyrin binding cleft. On the other hand, Mg(2+), which has been shown to bind close to the surface at 7 A from His183, was largely absent from its site. Activity measurements demonstrate that Mg(2+) has a stimulatory effect on the enzyme, lowering K(M) for Zn(2+) from 55 to 24 micro M. Changing one of the Mg(2+) binding residues, Glu272, to serine abolishes the effect of Mg(2+). It is proposed that prior to metal insertion the metal may form a sitting-atop (SAT) complex with the invariant His-Glu couple and the porphyrin. Metal binding to the Mg(2+) site may stimulate metal release from the protein ligands and its insertion into the porphyrin.

Bacillus subtilis↗

PubMed2002

Metal binding to Saccharomyces cerevisiae ferrochelatase.

Ferrochelatase is the terminal enzyme in the heme biosynthetic pathway. It catalyzes the insertion of ferrous iron into protoporphyrin IX to produce protoheme IX. The crystal structures of ferrochelatase from Saccharomyces cerevisiae in free form, in complex with Co(II), a substrate metal ion, and in complex with two inhibitors, Cd(II) and Hg(I), are presented in this work. The enzyme is a homodimer, with clear asymmetry between the monomers with regard to the porphyrin binding cleft and the mode of metal binding. The Co(II) and Cd(II) complexes reveal the metal binding site which consists of the invariant amino acids H235, E314, and S275 and solvent molecules. The shortest distance to the metal reveals that amino acid H235 is the primary metal binding residue. A second site with bound Cd(II) was found close to the surface of the molecule, approximately 14 A from H235, with E97, H317, and E326 participating in metal coordination. It is suggested that this site corresponds to the magnesium binding site in Bacillus subtilis ferrochelatase. The latter site is also located at the surface of the molecule and thought to be involved in initial metal binding and regulation.

Amino Acid Sequence↗

PubMed2002

Occurrence, conformational features and amino acid propensities for the pi-helix.

The most abundant helix type in proteins is the alpha-helix, accounting for about 31% of amino acid secondary structure states, while the 3(10)-helix accounts for about 4%. The pi-helix appears to be extremely rare and is considered to be unstable. Existing secondary structure definition methods find very few within the Protein Data Bank. Using an improved pi-helix definition algorithm to search a non-redundant subset of high-resolution and well-refined protein structures, we found that almost every tenth protein contained a pi-helix. This enabled us to show for the first time that the pi-helix has structural parameters that are different from the hypothesized model values. It also has distinctive amino acid preferences and it is conserved within functionally related proteins. Features that may contribute to the stability of the pi-helical structure have also been identified. In addition to hydrogen bonds, several other factors contribute to the stability of pi-helices. The pi-helix may have some functional advantages over other helical structures. Thus, we describe cases where the side chains of functionally important residues at every fourth position within a pi-helix could be aligned and brought close together in a way that would not be allowed by any other helix type.

Algorithms↗

PubMed2002

In vivo and in vitro studies of Bacillus subtilis ferrochelatase mutants suggest substrate channeling in the heme biosynthesis pathway.

Ferrochelatase (EC 4.99.1.1) catalyzes the last reaction in the heme biosynthetic pathway. The enzyme was studied in the bacterium Bacillus subtilis, for which the ferrochelatase three-dimensional structure is known. Two conserved amino acid residues, S54 and Q63, were changed to alanine by site-directed mutagenesis in order to detect any function they might have. The effects of these changes were studied in vivo and in vitro. S54 and Q63 are both located at helix alpha3. The functional group of S54 points out from the enzyme, while Q63 is located in the interior of the structure. None of these residues interact with any other amino acid residues in the ferrochelatase and their function is not understood from the three-dimensional structure. The exchange S54A, but not Q63A, reduced the growth rate of B. subtilis and resulted in the accumulation of coproporphyrin III in the growth medium. This was in contrast to the in vitro activity measurements with the purified enzymes. The ferrochelatase with the exchange S54A was as active as wild-type ferrochelatase, whereas the exchange Q63A caused a 16-fold reduction in V(max). The function of Q63 remains unclear, but it is suggested that S54 is involved in substrate reception or delivery of the enzymatic product.

Bacillus subtilis↗