Biomedical subjects

Steven L Roderick

Publications and source records attributed to Steven L Roderick.

At least 19 recordsLinked to original sources

PubMed2006

A polymorphism in New Zealand inbred mouse strains that inactivates phosphatidylcholine transfer protein.

New Zealand obese (NZO/HlLt) male mice develop polygenic diabetes and altered phosphatidylcholine metabolism. The gene encoding phosphatidylcholine transfer protein (PC-TP) is sited within the support interval for Nidd3, a recessive NZO-derived locus on Chromosome 11 identified by prior segregation analysis between NZO/HlLt and NON/Lt. Sequence analysis revealed that the NZO-derived PC-TP contained a non-synonymous point mutation that resulted in an Arg120His substitution, which was shared by the related NZB/BlNJ and NZW/LacJ mouse strains. Consistent with the structure-based predictions, functional studies demonstrated that Arg120His PC-TP was inactive, suggesting that this mutation contributes to the deficiencies in phosphatidylcholine metabolism observed in NZO mice.

Amino Acid Substitution↗

PubMed2006

Pentapeptide repeat proteins.

The pentapeptide repeat protein (PRP) family has more than 500 members in the prokaryotic and eukaryotic kingdoms. These proteins are composed of, or contain domains composed of, tandemly repeated amino acid sequences with a consensus sequence of [S,T,A,V][D,N][L,F][S,T,R][G]. The biochemical function of the vast majority of PRP family members is unknown. The three-dimensional structure of the first member of the PRP family was determined for the fluoroquinolone resistance protein (MfpA) from Mycobacterium tuberculosis. The structure revealed that the pentapeptide repeats encode the folding of a novel right-handed quadrilateral beta-helix. MfpA binds to DNA gyrase and inhibits its activity. The rod-shaped, dimeric protein exhibits remarkable similarity in size, shape, and electrostatics to DNA.

Bacterial Proteins↗

PubMed2005

Interaction of serine acetyltransferase with O-acetylserine sulfhydrylase active site: evidence from fluorescence spectroscopy.

Serine acetyltransferase is a key enzyme in the sulfur assimilation pathway of bacteria and plants, and is known to form a bienzyme complex with O-acetylserine sulfhydrylase, the last enzyme in the cysteine biosynthetic pathway. The biological function of the complex and the mechanism of reciprocal regulation of the constituent enzymes are still poorly understood. In this work the effect of complex formation on the O-acetylserine sulfhydrylase active site has been investigated exploiting the fluorescence properties of pyridoxal 5'-phosphate, which are sensitive to the cofactor microenvironment and to conformational changes within the protein matrix. The results indicate that both serine acetyltransferase and its C-terminal decapeptide bind to the alpha-carboxyl subsite of O-acetylserine sulfhydrylase, triggering a transition from an open to a closed conformation. This finding suggests that serine acetyltransferase can inhibit O-acetylserine sulfhydrylase catalytic activity with a double mechanism, the competition with O-acetylserine for binding to the enzyme active site and the stabilization of a closed conformation that is less accessible to the natural substrate.

Acetyltransferases↗

PubMed2005

A fluoroquinolone resistance protein from Mycobacterium tuberculosis that mimics DNA.

Fluoroquinolones are gaining increasing importance in the treatment of tuberculosis. The expression of MfpA, a member of the pentapeptide repeat family of proteins from Mycobacterium tuberculosis, causes resistance to ciprofloxacin and sparfloxacin. This protein binds to DNA gyrase and inhibits its activity. Its three-dimensional structure reveals a fold, which we have named the right-handed quadrilateral beta helix, that exhibits size, shape, and electrostatic similarity to B-form DNA. This represents a form of DNA mimicry and explains both its inhibitory effect on DNA gyrase and fluoroquinolone resistance resulting from the protein's expression in vivo.

Amino Acid Sequence↗

PubMed2005

A novel dimeric structure of the RimL Nalpha-acetyltransferase from Salmonella typhimurium.

RimL is responsible for converting the prokaryotic ribosomal protein from L12 to L7 by acetylation of its N-terminal amino group. We demonstrate that purified RimL is capable of posttranslationally acetylating L12, exhibiting a V(max) of 21 min(-1). We have also determined the apostructure of RimL from Salmonella typhimurium and its complex with coenzyme A, revealing a homodimeric oligomer with structural similarity to other Gcn5-related N-acetyltransferase superfamily members. A large central trough located at the dimer interface provides sufficient room to bind both L12 N-terminal helices. Structural and biochemical analysis indicates that RimL proceeds by single-step transfer rather than a covalent-enzyme intermediate. This is the first structure of a Gcn5-related N-acetyltransferase family member with demonstrated activity toward a protein N(alpha)-amino group and is a first step toward understanding the molecular basis for N(alpha)acetylation and its function in cellular regulation.

Acetyltransferases↗

PubMed2005

The serine acetyltransferase reaction: acetyl transfer from an acylpantothenyl donor to an alcohol.

Serine acetyltransferase is a member of the left-handed parallel beta-helix family of enzymes that catalyzes the committed step in the de novo synthesis of l-cysteine in bacteria and plants. The enzyme has an ordered kinetic mechanism with acetyl CoA bound prior to l-serine and O-acetyl-l-serine released prior to CoA. The rate-limiting step along the reaction pathway is the nucleophilic attack of the serine hydroxyl on the thioester of acetyl CoA. Product release contributes to rate-limitation at saturating concentrations of reactants. The reaction is catalyzed by an active site general base with a pK of 7, which accepts a proton from the serine hydroxyl as a tetrahedral intermediate is formed between the reactants, and donates it to the thiol of CoA as the intermediate collapses to give products. This mechanism is likely the same for all O-acyltransferases that catalyze their reaction by direct attack of the alcohol on the acyl donor, using an active-site histidine as the general base. Serine acetyltransferase is regulated by feedback inhibition by the end product l-cysteine, which acts by binding to the serine site in the active site and inducing a conformational change that prevents reactant binding. The enzyme also associates with O-acetylserine sulfhydrylase, the final enzyme in the biosynthetic pathway, which contributes to stabilizing the acetyltransferase.

Acetyl Coenzyme A↗

PubMed2005

Structure and functions of the GNAT superfamily of acetyltransferases.

The Gcn5-related N-acetyltransferases are an enormous superfamily of enzymes that are universally distributed in nature and that use acyl-CoAs to acylate their cognate substrates. In this review, we will examine those members of this superfamily that have been both structurally and mechanistically characterized. These include aminoglycoside N-acetyltransferases, serotonin N-acetyltransferase, glucosamine-6-phosphate N-acetyltransferase, the histone acetyltransferases, mycothiol synthase, protein N-myristoyltransferase, and the Fem family of amino acyl transferases.

Acetyltransferases↗

PubMed2005

The lac operon galactoside acetyltransferase.

Of the proteins encoded by the three structural genes of the lac operon, the galactoside acetyltransferase (thiogalactoside transacetylase, LacA, GAT) encoded by lacA is the only protein whose biological role remains in doubt. Here, we briefly note the classical literature that led to the identification and initial characterization of GAT, and focus on more recent results which have revealed its chemical mechanism of action and its membership in a large superfamily of structurally similar acyltransferases. The structural and sequence similarities of several members of this superfamily confirm the original claim for GAT as a CoA-dependent acetyltransferase specific for the 6-hydroxyl group of certain pyranosides, but do not yet point to the identity of the natural substrate(s) of the enzyme.

Acetyltransferases↗

PubMed2005

The active site of O-acetylserine sulfhydrylase is the anchor point for bienzyme complex formation with serine acetyltransferase.

The biosynthesis of cysteine in bacteria and plants is carried out by a two-step pathway, catalyzed by serine acetyltransferase (SAT) and O-acetylserine sulfhydrylase (OASS; O-acetylserine [thiol] lyase). The aerobic form of OASS forms a tight bienzyme complex with SAT in vivo, termed cysteine synthase. We have determined the crystal structure of OASS in complex with a C-terminal peptide of SAT required for bienzyme complex formation. The binding site of the peptide is at the active site of OASS, and its C-terminal carboxyl group occupies the same anion binding pocket as the alpha-carboxylate of the O-acetylserine substrate of OASS. These results explain the partial inhibition of OASS by SAT on complex formation as well as the competitive dissociation of the complex by O-acetylserine.

Acetyltransferases↗

PubMed2004

Chemical mechanism of the serine acetyltransferase from Haemophilus influenzae.

The pH dependence of kinetic parameters was determined in both reaction directions to obtain information about the acid-base chemical mechanism of serine acetyltransferase from Haemophilus influenzae (HiSAT). The maximum rates in both reaction directions, as well as the V/K(serine) and V/K(OAS), decrease at low pH, exhibiting a pK of approximately 7 for a single enzyme residue that must be unprotonated for optimum activity. The pH-independent values of V(1)/E(t), V(1)/K(serine)E(t), V/K(AcCoA)E(t), V(2)/E(t), V(2)/K(OAS)E(t), and V/K(CoA)E(t) are 3300 +/- 180 s(-1), (9.6 +/- 0.4) x 10(5) M(-1) s(-1), 3.3 x 10(6) M(-1) s(-1), 420 +/- 50 s(-1), (2.1 +/- 0.5) x 10(4) M(-1) s(-1), and (4.2 +/- 0.7) x 10(5) M(-1) s(-1), respectively. The K(i) values for the competitive inhibitors glycine and l-cysteine are pH-independent. The solvent deuterium kinetic isotope effects on V and V/K in the direction of serine acetylation are 1.9 +/- 0.2 and 2.5 +/- 0.4, respectively, and the proton inventories are linear for both parameters. Data are consistent with a single proton in flight in the rate-limiting transition state. A general base catalytic mechanism is proposed for the serine acetyltransferase. Once acetyl-CoA and l-serine are bound, an enzymic general base accepts a proton from the l-serine side chain hydroxyl as it undergoes a nucleophilic attack on the carbonyl of acetyl-CoA. The same enzyme residue then functions as a general acid, donating a proton to the sulfur atom of CoASH as the tetrahedral intermediate collapses, generating the products OAS and CoASH. The rate-limiting step in the reaction at limiting l-serine levels is likely formation of the tetrahedral intermediate between serine and acetyl-CoA.

Acetylation↗

PubMed2004

Kinetic mechanism of the serine acetyltransferase from Haemophilus influenzae.

The kinetic mechanism of serine acetyltransferase from Haemophilus influenzae was studied in both reaction directions. The enzyme catalyzes the conversion of acetyl CoA and L-serine to O-acetyl-L-serine (OAS) and coenzyme A (CoASH). In the direction of L-serine acetylation, an equilibrium ordered mechanism is assigned at pH 6.5. The initial velocity pattern in the absence of added inhibitors is best described by a series of lines converging on the ordinate when L-serine is varied at different fixed levels of acetyl CoA. The initial velocity pattern at pH 7.5 is also intersecting, but the lines are nearly parallel. Product inhibition by OAS is noncompetitive against acetyl CoA, while it is uncompetitive against L-serine. Product inhibition by L-serine in the reverse reaction direction is noncompetitive with respect to both OAS and CoASH. Glycine and S-methyl-L-cysteine (SMC) were used as dead-end analogs of L-serine and OAS, respectively. Glycine is competitive versus L-serine and uncompetitive versus acetyl CoA, while SMC is competitive against OAS and uncompetitive against CoASH. Desulfo-CoA was used as a dead-end analog of both acetyl CoA and CoASH, and is competitive versus both substrates in the direction of L-serine acetylation; while it is competitive against CoASH and noncompetitive against OAS in the direction of CoASH acetylation. All of the above kinetic parameters are consistent with those predicted for an ordered mechanism at pH 6.5 with the exception of the uncompetitive inhibition by OAS vs. serine. The latter inhibition pattern suggests combination of OAS with the central E:acetyl CoA:serine complex. Cysteine is known to regulate its own biosynthesis at the level of SAT. As a dead-end inhibitor, L-cysteine is competitive against both substrates in both reaction directions. These results are discussed in terms of the mechanism of regulation.

Acetyltransferases↗

PubMed2004

Structure of serine acetyltransferase in complexes with CoA and its cysteine feedback inhibitor.

Serine acetyltransferase (SAT, EC 2.3.1.30) catalyzes the CoA-dependent acetylation of the side chain hydroxyl group of l-serine to form O-acetylserine, as the first step of a two-step biosynthetic pathway in bacteria and plants leading to the formation of l-cysteine. This reaction represents a key metabolic point of regulation for the cysteine biosynthetic pathway due to its feedback inhibition by cysteine. We have determined the X-ray crystal structure of Haemophilus influenzae SAT in complexes with CoA and its cysteine feedback inhibitor. The enzyme is a 175 kDa hexamer displaying the characteristic left-handed parallel beta-helix (LbetaH) structural domain of the hexapeptide acyltransferase superfamily of enzymes. Cysteine is bound in a crevice between adjacent LbetaH domains and underneath a loop excluded from the coiled LbetaH. The proximity of its thiol group to the thiol group of CoA derived from superimposed models of the cysteine and CoA complexes confirms that cysteine is bound at the active site. Analysis of the contacts of SAT with cysteine and CoA and the conformational differences that distinguish these complexes provides a structural basis for cysteine feedback inhibition, which invokes competition between cysteine and serine binding and a cysteine-induced conformational change of the C-terminal segment of the enzyme that excludes binding of the cofactor.

Acetyltransferases↗

PubMed2004

A bacterial acetyltransferase capable of regioselective N-acetylation of antibiotics and histones.

The Salmonella enterica chromosomally encoded AAC(6')-Iy has been shown to confer broad aminoglycoside resistance in strains in which the structural gene is expressed. The three-dimensional structures reported place the enzyme in the large Gcn5-related N-acetyltransferase (GNAT) superfamily. The structure of the CoA-ribostamycin ternary complex allows us to propose a chemical mechanism for the reaction, and comparison with the Mycobacterium tuberculosis AAC(2')-CoA-ribostamycin complex allows us to define how regioselectivity of acetylation is achieved. The AAC(6')-Iy dimer is most structurally similar to the Saccharomyces cerevisiae Hpa2-encoded histone acetyltransferase. We demonstrate that AAC(6')-Iy catalyzes both acetyl-CoA-dependent self-alpha-N-acetylation and acetylation of eukaryotic histone proteins and the human histone H3 N-terminal peptide. These structural and catalytic similarities lead us to propose that chromosomally encoded bacterial acetyltransferases, including those functionally identified as aminoglycoside acetyltransferases, are the evolutionary progenitors of the eukaryotic histone acetyltransferases.

Acetylation↗

PubMed2003

Crystal structure of mycothiol synthase (Rv0819) from Mycobacterium tuberculosis shows structural homology to the GNAT family of N-acetyltransferases.

Mycothiol is the predominant low-molecular weight thiol produced by actinomycetes, including Mycobacterium tuberculosis. The last reaction in the biosynthetic pathway for mycothiol is catalyzed by mycothiol synthase (MshD), which acetylates the cysteinyl amine of cysteine-glucosamine-inositol (Cys-GlcN-Ins). The crystal structure of MshD was determined in the presence of coenzyme A and acetyl-CoA. MshD consists of two tandem-repeated domains, each exhibiting the Gcn5-related N-acetyltransferase (GNAT) fold. These two domains superimpose with a root-mean-square deviation of 1.7 A over 88 residues, and each was found to bind one molecule of coenzyme, although the binding sites are quite different. The C-terminal domain has a similar active site to many GNAT members in which the acetyl group of the coenzyme is presented to an open active site slot. However, acetyl-CoA bound to the N-terminal domain is buried, and is apparently not positioned to promote acetyl transfer. A modeled substrate complex indicates that Cys-GlcN-Ins would only fill a portion of a negatively charged channel located between the two domains. This is the first structure determined for an enzyme involved in the biosynthesis of mycothiol.

Acetyltransferases↗

PubMed2002

Human phosphatidylcholine transfer protein: purification, crystallization and preliminary X-ray diffraction data.

We have expressed, purified and crystallized recombinant human phosphatidylcholine transfer protein (PC-TP) and selenomethionyl PC-TP bound to dilinoleoyl phosphatidylcholine. The biochemical properties of native and selenomethionyl PC-TP were indistinguishable, and the two proteins crystallized under similar conditions. Both native and selenomethionyl PC-TP crystallized in two distinct space groups and diffracted X-rays to 2.4 A resolution.

Androgen-Binding Protein↗

PubMed2002

Crystal structure of Vat(D): an acetyltransferase that inactivates streptogramin group A antibiotics.

The streptogramin class of antibiotics act to inhibit bacterial protein synthesis, and their semisynthetic derivatives, such as dalfopristin-quinupristin (Synercid), are used to treat serious or life-threatening infections due to multiply antibiotic resistant bacteria. Acquired resistance of the nosocomial pathogen Enterococcus faecium to the group A component of natural and semisynthetic streptogramin mixtures is a prerequisite for the streptogramin resistance phenotype and is mediated by a streptogramin acetyltransferase. The crystal structure of Vat(D), a streptogramin acetyltransferase from a human urinary isolate of E. faecium, has been determined as an apoenzyme and in complex with either acetyl-CoA or virginiamycin M1 and CoA. These structures illustrate the location and arrangement of residues at the active site, and point to His 82 as a residue that may function as a general base. The structural similarity of Vat(D) to the xenobiotic acetyltransferase from Pseudomonas aeruginosa indicates similarities in the catalytic mechanism for these enzymes as well as several shared and distinctive antibiotic binding interactions between these enzymes and their respective substrates. These results reveal the molecular basis for a reaction by which Gram-positive cocci acquire resistance to a last resort antibiotic.

Acetyltransferases↗

PubMed2002

Structure of the lac operon galactoside acetyltransferase.

The galactoside acetyltransferase (thiogalactoside transacetylase) of Escherichia coli (GAT, LacA, EC 2.3.1.18) is a gene product of the classical lac operon. GAT may assist cellular detoxification by acetylating nonmetabolizable pyranosides, thereby preventing their reentry into the cell. The structure of GAT has been solved in binary complexes with acetyl-CoA or CoA and in ternary complexes with CoA and the nonphysiological acceptor substrates isopropyl beta-D-thiogalactoside (IPTG) or p-nitrophenyl beta-D-galactopyranoside (PNPbetaGal). A hydrophobic cleft that binds the thioisopropyl and p-nitrophenyl aglycones of IPTG and PNPbetaGal may discriminate against substrates with hydrophilic substituents at this position, such as lactose, or inducers of the lac operon. An extended loop projecting from the left-handed parallel beta helix domain contributes His115, which is in position to facilitate attack of the C6-hydroxyl group of the substrate on the thioester.

Acetyltransferases↗

PubMed2002

Structure of human phosphatidylcholine transfer protein in complex with its ligand.

Phosphatidylcholines (PtdChos) comprise the most common phospholipid class in eukaryotic cells. In mammalian cells, these insoluble molecules are transferred between membranes by a highly specific phosphatidylcholine transfer protein (PC-TP) belonging to the steroidogenic acute regulatory protein related transfer (START) domain superfamily of hydrophobic ligand-binding proteins. The crystal structures of human PC-TP in complex with dilinoleoyl-PtdCho or palmitoyl-linoleoyl-PtdCho reveal that a single well-ordered PtdCho molecule occupies a centrally located tunnel. The positively charged choline headgroup of the lipid engages in cation-pi interactions within a cage formed by the faces of three aromatic residues. These binding determinants and those for the phosphoryl group may be exposed to the lipid headgroup at the membrane-water interface by a conformational change involving the amphipathic C-terminal helix and an Omega-loop. The structures presented here provide a basis for rationalizing the specificity of PC-TP for PtdCho and may identify common features used by START proteins to bind their hydrophobic ligands.

Androgen-Binding Protein↗