View source for G04SecL04Tpc1

==Introduction==

Spirochetes are motile, spiral shaped microorganisms. They have an inner membrane, a thin peptidoglycan layer and outer membrane and a periplasmic space. One well known characteristic of spirochetes is that they have lipoproteins that are surface-exposed on their outer membrane<ref name="Connolly,S">PMID:15864264</ref>. These proteins determine the ability of the antigen to trigger an immune response. In [http://en.wikipedia.org/wiki/Lyme_disease_microbiology#Outer_surface_proteins Lyme disease] the lipoproteins are outer-surface proteins. Though it has many outer-surface proteins on its outer membrane, Outer Surfafce Protein B (Osp B) and outer surface protein A (OspA) are very important lipoproteins in Lyme Disease <ref name="Becker M">PMID:15713683</ref>. They have similar structures and they both are bactericidal but they use different antibodies to lyse the bacteria.

Outer surface protein B (OspB) combines with a bactericidal fab H6831 in order to kill the bacteria [http://en.wikipedia.org/wiki/Borrelia_burgdorferi ''Borrelia Burgdorferi''].   This antibody is directed towards the C-terminus and is complement-independent. This means unlike regular antibodies, it does not need to bind to complement protein in order to lyse the bacteria. The mechanism for this amazing anomaly has not yet been discovered. However, much research has been done on the particular aspects that may aid in the process. This includes various amino acid and aromatic residues.

<Structure load='1RJL' size='300' frame='true' align='right' caption='OspB and Fab complex' scene='G04SecL04Tpc1/1rjl/1' />

==Structure==
===Osp-B H6831 complex===

In its original form,Outer Surface Protein B has 12 Beta sheets. However, a  <scene name='G04SecL04Tpc1/Osp_b_fab_complex/1'>truncated form of Osp-B</scene>, purple, is displayed in the complex formed when the C-terminus of Osp B binds to the antibody H6831. The Osp B binds to the Fab <scene name='G04SecL04Tpc1/L_chain/1'> light chain</scene> and <scene name='G04SecL04Tpc1/H_chain/1'>heavy chain</scene>regions of the H6831. 

There are <scene name='G04SecL04Tpc1/Osp_b_fab_complex/8'>three loop regions</scene> on the c-terminal Osp-B that interact with the Fab of H6831. The major interactions of the complex occur between loop region two, seen in pink, and the Fab heavy chain.  More minor hydrogen bonding interactions occur between loop one,green, and the heavy chain as well as loop three,orange, and the light chains
<ref name="Becker M">PMID:15713683</ref>. The essential residue on loop two is the <scene name='G04SecL04Tpc1/Osp_b_fab_complex/6'>Lysine 253</scene>shown in green. This lysine forms an ion pair with a <scene name='G04SecL04Tpc1/Osp_b_fab_complex/4'>Glutamic acid 50</scene>, red, of the Fab heavy chain. This antigen-antibody complex is further stabilized by the presence of a <scene name='G04SecL04Tpc1/Osp_b_fab_complex/5'>Threonine 276</scene>, orange. This bond is locked in by the presence of two 
<scene name='G04SecL04Tpc1/Osp_b_fab_complex/7'>aromatic residues</scene> that reside on the Fab, tryptophan and tyrosine shown in black.  There are some instances in which the H6831 cannot bind to Osp-B. This is a case of a mutant form of the protein in which the essential lysine is replaced by another residue<ref name="Becker M">PMID:15713683</ref>. 

Upon binding, Osp-B experiences structural changes, the largest being the disappearance of the first four beta sheets.  The loop opposite the Fab binding site along with the putative N-terminal region and a non-epitope loop also experience conformational changes all in which shift towards the missing beta sheets<ref name="Becker M">PMID:15713683</ref>.  


=== Comparison between OspB-H6831 and OspA-LA2===

<Structure load='1FJ1' size='300' frame='true' align='right' caption='OspA and LA2 bound complex' scene='G04SecL04Tpc1/Ospa/1' />


Another surface protein of note in the B. Burgdorferi is outer surface protein A <scene name='G04SecL04Tpc1/Ospa_complex/1'>OspA</scene>. It is composed of 21 anti-parallel beta-sheets and one alpha-helix.  It contains a B-cell epitope on its C-terminal that binds with monoclonal antibody LA-2.  LA-2 is a regular-structured antibody that is composed of two heavy chains and two light chains. <ref name="Ding,W">PMID: 11183781</ref>.  
Complex proteins OspB and OspA are shown to be 53% similar due to structural similarities<ref name="Li,H">PMID: 9108020</ref>.  The H6831 and LA-2 antibodies are also structurally similar, and each binds near the C-terminus on its respective surface protein.  The H6831 and LA-2 epitopes on OspB and OspA are positioned at the opposite end of the molecule as the N-terminus of each antigen.  On the C-terminal tips on each protein, the epitopes contain three exposed loops of residues that are involved with the binding interaction between antigen and antibody.  For OspB, the middle loop (loop 2) has been shown to have the strongest interaction with H6831. The residue Lys253 in this loop is especially important in the interaction.  For OspA, one of the outer loops (loop 1) has the strongest interaction with LA-2. The <scene name='G04SecL04Tpc1/Ospa_complex/6'>Alanine 208</scene> residue in the loop plays the important role here.  The importance of these residues is confirmed by observing antibody reactivity using different species of Borrelia<ref name="Ding,W">PMID: 11183781</ref><ref name="Li,H">PMID: 9108020</ref>. Also, researchers found that buried surface area of OspA in LA-2 Fab is larger than OspB in the H6831Fab<ref name="Becker M">PMID:15713683</ref>). 








==Possible Mechanism of Bactericidal Properties of Antibody Bound Complex==
Although the exact mechanism of the bacterial effect of OspB and Fab H6831 complex is not know there are several hypotheses on how bacteria lysis occurs. One such hypothesis is that bacteria lysis is the direct result of oxidative reaction of singlet oxygen and water to yield hydrogen peroxide, ozone, and hydroxide radicals <ref name="Becker M">PMID:15713683</ref>. Experiments done by Jorge Nieva and Paul Wentworth Jr of The Scripps Research Institute describe this reaction further. They suggest that this antibody-catalyzed water oxidation pathway is a property of nearly all antibodies and is similar to phagocytosis and that this may prove to be an important defense of the immune system.  However, in order for phagocytosis to occur the immune system must use a complement indirect cascade initiating a membrane attack complex <ref name="Connolly,S">PMID:15864264</ref>. In the experiments by Nieva and Wentworth they provide evidence that this oxidative reaction is independent of this compliment system. They believe that singlet oxygen acts as the substrate and binds to binding sites within the outer surface protein folds <ref name="Nieva,J.">PMID:15130564</ref>. A [http://en.wikipedia.org/wiki/Singlet_oxygen singlet oxygen] acts a nucleophile and attacks one water molecule to form the dihydrogentrioxide (H<sub>2</sub>O<sub>3</sub>) intermediate which then reacts to form hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) which when given an oxygen source proves to be bactericidal. Catalase prevents the reaction of hydrogen peroxide and ozone that produces hydroxide radicals which is the potential antibacterial agent <ref name="Nieva,J.">PMID:15130564</ref>. This oxidation reaction leads to damage in the cell wall and plasma membrance which leads to lysis of the cell which was observed through observation under an electron microscope <ref name="Nieva,J.">PMID:15130564</ref>. Because B. burgdorferi survives best in environments with limited oxygen and its genome does not encode for a [http://en.wikipedia.org/wiki/Catalase catalase] it may be vulnerable to this oxidative reaction <ref name=BeckerM>PMID:15713683</ref>.


==3D Structures==
[http://proteopedia.org/wiki/index.php/Antibody Antibody]

[http://www.proteopedia.org/wiki/index.php/1rjl OspA]

[http://www.proteopedia.org/wiki/index.php/1fj1 OspB]





==Notes==
===References===
<references/>