View source for ModG

 {{STRUCTURE_1atg|  PDB=1atg  | SIZE=400| SCENE= |right|   CAPTION=Molybdate-binding protein complex with tungstate, ethylene glycol, acetate and sulfate, [[1atg]] }

<StructureSection load='1atg' size='350' side='right' caption='Molybdate-binding protein complex with tungstate, ethylene glycol, acetate and sulfate (PDB entry [[1atg]])' scene=''>
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=Introduction to ModG=
'''ModG''' is a cytoplasmic molybdate-binding protein exclusive to the aerobic nitrogen-fixer ''Azobacter vinelandii''.<ref name="MODGR1">PMID:11352591</ref> Molybdate is a molybdenum oxyanion (MoO42-). The group 6 element molybdenum is required by many enzymes that catalyze reactions associated with carbon, nitrogen, or sulfur metabolism.<ref name="MODGR2">PMID:21454640</ref> It is also part of the cofactor of the molybdoenzyme ModG. Not surprisingly, studies have linked ModG to molybdenum homeostasis within the cell.<ref name="MODGR1"/>

=Structure=

The molybdoenzyme is a homotrimer.<ref name="MODGR1"/> It can bind up to 8 molybdate molecules between 4 different types of <scene name='Sandbox_Reserved_306/1h9j/4'>active sites</scene> on subunit interfaces (BS1, BS1’, BS2, and BS2’). Binding site 1 and binding site 2 are found at opposite ends of the protein; binding site 1’ and binding site 2’ are found off-axis near like ends.<ref name="MODGR3">PMID:9862806</ref> The sites are connected by hydrogen bonds and thus a cooperative binding mechanism has been proposed for ModG whereby ligand engagement with type 2 sites induces conformational changes to asparagine residues at type 1 sites, reading the site for ligand interactions.<ref name="MODGR4">PMID:7665518</ref> The structure of ModG was solved by Delarbe et al. using multi-wavelength anomalous dispersion (MAD).<ref name="MODGR5"/> Crystallization required salt-free conditions established with polyethylene glycol (PEG), at which point the authors solved the PEG crystal form using molecular replacement.<ref name="MODGR5"/>

The protein is made up of 3 identical subunits that have 67 amino acid pairs and are 14.3 kDa in size.<ref name="MODGR1"/> In trimer form the subunits are perpendicular to each other and intersect at the 3-fold axis (as seen in the crystallized form).<ref name="MODGR3"/> Each subunit is composed of two β-barrel domains (Domain I and Domain II) that each feature a short 310-helix.<ref name="MODGR3"/> Each β-barrel domain includes five antiparallel β-strands arranged in a <scene name='Sandbox_Reserved_306/1h9j/7'>Greek Key</scene> motif that is capped by two-turn α-helices.<ref name="MODGR3"/> The folding arrangement of the ModG domains is that of an oligomer-binding (OB) fold, characteristic of toxins and other intracellular oxyanion-binding proteins.<ref name="MODGR3"/> In trimerization, N and C termini (both found in Domain I) of a subunit interact with the β5-β6 loop (Domain II) of the adjoining subunit by sharing antiparallel β-sheets. Approximately 40% of monomer surface area is buried when the protein is in trimer form.<ref name="MODGR1"/>








=Properties and binding affinity=
About 60% of each subunit interface is <scene name='Sandbox_Reserved_306/1h9j/8'>non-polar</scene> (40% is polar); furthermore, there is an unequal charge distribution that is attributed to the presence of 4 side chains.<ref name="MODGR2"/><ref name="MODGR4"/> These include: Lys60, Lys132, Arg6, and Arg78.<ref name="MODGR1"/> The <scene name='Sandbox_Reserved_306/1h9j/10'>lysine residues</scene> are both found on the type 2 binding sites and are entirely buried in the protein; the <scene name='Sandbox_Reserved_306/1h9j/5'>arginine residues</scene> are partly exposed.<ref name="MODGR1"/> This leaves the interface particularly electropositive; electrostatic repulsion is balanced by several favorable interactions that include the formation of 24 hydrogen bonds, 2 salt bridges and approximately 24% of total hydrophobic surface buried (per subunit).<ref name="MODGR4"/> The trimer is further stabilized by the binding of an oxyanion which would have a neutralizing effect.<ref name="MODGR3"/>

Type 1 binding sites utilize hydrogen bonds and interactions with uncharged polar residues to bind molybdate.<ref name="MODGR4"/> The tight pocket volume and rigidity of the site suggest that there is high selectivity for molybdate (or tungstate because ModG cannot differentiate between the two oxyanions).<ref name="MODGR4"/> Type 2 sites are comparatively larger in pocket volume, supple, and feature electropositive lysine residues that would contribute to high affinity for negatively charged oxyanions like molybdate.<ref name="MODGR4"/> Because the type 2 sites are larger and more flexible, there would be less specificity for molybdate and thus other oxyanions such as phosphate would likely compete for binding.<ref name="MODGR3"/><ref name="MODGR4"/>


=Function in Bacteria=
The ability of ''A. vinelandii'' to differentiate between molybdate and other oxyanions is of particular interest to researchers.<ref name="MODGR1"/><ref name="MODGR2"/><ref name="MODGR5"/> Phosphate is distinguished from molybdate by the divergence in protonation states: phosphate is protonated at physiological pH whereas molybdate is not.<ref name="MODGR2"/> The distinction between sulfate and molybdate is reliant upon differences in ligand size.<ref name="MODGR2"/> Interestingly, molybdate permeases are not able to distinguish between tungstate and molybdate.<ref name="MODGR1"/><ref name="MODGR2"/> Regulation of these intracellular metabolites is attributed to other molybdate-binding proteins collectively known as molbindins.<ref name="MODGR1"/><ref name="MODGR5">PMID:10393312</ref>


=Fate=
ModG is eventually degraded and incorporated into molybdopterin, a cofactor of molybdenum enzymes, or the iron-molybdenum cofactor of a nitrogenase enzyme.<ref name="MODGR1"/><ref name="MODGR2"/> Currently there is little known about cofactor biosynthesis involving the ModG protein.<ref name="MODGR1"/>
</StructureSection>
=3D structures of ModG=

[[1h9j]] – AvModG + phosphate + molybdate – ''Azotobacter vinelandii''<br />
[[1atg]] – AvModG + tungstate + sulfate + acetate<br />
[[1h9k]] - AvModG + tungstate + phosphate<br />
[[1h9m]] - AvModG + molybdate

=References=
<references/>


=See also=
[[Category:Molybdenum-containing enzyme]]
[[Category:Topic Page]]
This page originally authored by Corbin Black