G04SecL04Tpc1

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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[1]. These proteins determine the ability of the antigen to trigger an immune response. In 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 [2]. 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 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.

OspB and Fab complex

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Structure

Osp-B H6831 complex

The complex formed by the C-terminal end of Outer Surface Protein B and H6831 displays a , purple,Binding with the Fab region of the H6831, light blue.

There are three loop regions on the c-terminal Osp-B that interact with the Fab of H6831. The major interactions of the complex occur between loop region two and the Fab heavy chain. More minor hydrogen bonding interactions occur between loop one and the heavy chain as well as loop three and the light chains [2]. The essential residue on loop two is the shown in green. This lysine forms an ion pair with a , red, of the Fab heavy chain. This antigen-antibody complex is further stabilized by the presence of a , orange. This bond is locked in by the presence of two 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[2].

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[2].


Comparison between OspB-H6831 and OspA-LA2

OspA and LA2 bound complex

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Another surface protein of note in the B. Burgdorferi is , which is 53% similar to OspB. This is due to some structural similarities in the proteins. Also, the change in conformation of OspA after binding to the LA2 antibody is very similar to that of OspB when binding with H6831. The H6831 and LA2 antibodies are also structurally similar and both are bind at C-terminal of the particular surface protein[2]. OspA has a structure composed of a single alpha helix and 21 anti-parallel beta strands. OspA also contains a B-cell epitope that binds with LA-2 Fab, an antigen combining fragment of the LA-2 monoclonal antibody, which is a major component in effective OspA vaccinations (Ding et al. 2000). Early vaccination for OspA is critical as OspA is only up-regulated during late stages of the disease; anti-OspA antibodies can only kill spirochetes in the tick gut, but is ineffective once it has migrated into host (Rupprecht et al. 2008). Through NMR chemical shift-perturbation and crystallographic identification, the LA-2 antibody is shown to make direct contact with the three loops in the C-terminal tip, which consists of approximately 39 amino acid residues (Ding et al. 2000). Specifically, the LA-2 Fab makes a concave groove over the three loops using all six of its light and heavy variable chain domains seen by NMR-perturbation analysis (Ding et al. 2000). The sequence variation of Loop 1 (~17 residues) has even shown to dominate LA-2 recognition of the OspA antigen; furthermore, is a major dynamic for current studies of antibody cross-reactivity between different strains of the Borrelia that transmit Lyme disease (Ding et al 2000). Similar to the LA-2 epitope, the H6831 epitope is positioned opposite to the N-terminus near the end of the antigen. The buried surface area of OspB in the H6831 Fab complex is smaller than that of the OspA-LA2 complex. Loop 1 in the OspA-LA2 complex has the most interactions with the Fab, whereas Loop 1 in the OspB-H6831 complex has the fewest interactions with the Fab (Golde 1997)


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 [2]. 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 [1]. 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 [3]. A singlet oxygen acts a nucleophile and attacks one water molecule to form the dihydrogentrioxide (H2O3) intermediate which then reacts to form hydrogen peroxide (H2O2) 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 [3]. 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 [3]. Because B. burgdorferi survives best in environments with limited oxygen and its genome does not encode for a catalase it may be vulnerable to this oxidative reaction [4].


3D Structures

Antibody

OspA

OspB



Notes

References

  1. ↑ 1.0 1.1 Connolly SE, Benach JL. The versatile roles of antibodies in Borrelia infections. Nat Rev Microbiol. 2005 May;3(5):411-20. PMID:15864264 doi:10.1038/nrmicro1149
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Becker M, Bunikis J, Lade BD, Dunn JJ, Barbour AG, Lawson CL. Structural investigation of Borrelia burgdorferi OspB, a bactericidal Fab target. J Biol Chem. 2005 Apr 29;280(17):17363-70. Epub 2005 Feb 15. PMID:15713683 doi:10.1074/jbc.M412842200
  3. ↑ 3.0 3.1 3.2 Nieva J, Wentworth P Jr. The antibody-catalyzed water oxidation pathway--a new chemical arm to immune defense? Trends Biochem Sci. 2004 May;29(5):274-8. PMID:15130564 doi:10.1016/j.tibs.2004.03.009
  4. ↑ Becker M, Bunikis J, Lade BD, Dunn JJ, Barbour AG, Lawson CL. Structural investigation of Borrelia burgdorferi OspB, a bactericidal Fab target. J Biol Chem. 2005 Apr 29;280(17):17363-70. Epub 2005 Feb 15. PMID:15713683 doi:10.1074/jbc.M412842200

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Rebecca Fisher, Young Park, Anusha Rankoth, Jalissa Hall

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