NMR structural studies reveal a novel protein fold for MerB, the organomercurial lyase involved in the bacterial mercury resistance system.
Mercury resistant bacteria have developed a system of two enzymes (MerA and MerB), which allows them to efficiently detoxify both ionic and organomercurial compounds. The organomercurial lyase (MerB) catalyzes the protonolysis of the carbon-mercury bond resulting in the formation of ionic mercury and a reduced hydrocarbon. The ionic mercury [Hg(II)] is subsequently reduced to the less reactive elemental mercury [Hg(0)] by a specific mercuric reductase (MerA). To better understand MerB's unique enzymatic activity, we used nuclear magnetic resonance (NMR) spectroscopy to determine the structure of the free enzyme. MerB is characterized by a novel protein fold consisting of three noninteracting antiparallel beta-sheets surrounded by six alpha-helices. By comparing the NMR data of free MerB and the MerB/Hg/DTT complex, we identified a set of residues that likely define a Hg/DTT binding site. These residues cluster around two cysteines (C(96) and C(159)) that are crucial to MerB's catalytic activity. A detailed analysis of the structure revealed the presence of an extensive hydrophobic groove adjacent to this Hg/DTT binding site. This extensive hydrophobic groove has the potential to interact with the hydrocarbon moiety of a wide variety of substrates and may explain the broad substrate specificity of MerB.
Di Lello
Paola
P
Department of Biochemistry & Molecular Biology, University of Georgia, Athens, Georgia 30602, USA.
Benison
Gregory C
GC
Valafar
Homayoun
H
Pitts
Keith E
KE
Summers
Anne O
AO
Legault
Pascale
P
Omichinski
James G
JG
D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry*; D001665:Binding Sites; D002244:Carbon / Q000737:chemistry; D002384:Catalysis; D003545:Cysteine / Q000737:chemistry; D004351:Drug Resistance; D006838:Hydrocarbons; D006863:Hydrogen-Ion Concentration; D007477:Ions; D008190:Lyases / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008628:Mercury / Q000737:chemistry / Q000494:pharmacology*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009941:Organomercury Compounds / Q000737:chemistry; D010957:Plasmids / Q000378:metabolism; D011487:Protein Conformation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D013379:Substrate Specificity; D013696:Temperature
D001426:Bacterial Proteins; D006838:Hydrocarbons; D007477:Ions; D009941:Organomercury Compounds; D002244:Carbon; D008190:Lyases; C097247:MerB protein, Bacteria; D008628:Mercury; D003545:Cysteine