G04SecL04Tpc1

From Proteopedia

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.

Structure

Osp-B H6831 complex

In its original form,Outer Surface Protein B has 12 Beta sheets. However, a , 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 and regions of the H6831.

There are 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 ^[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

Another surface protein of note in the B. Burgdorferi is outer surface protein A . 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. ^[3]. Complex proteins OspB and OspA are shown to be 53% similar due to structural similarities^[4]. 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 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^[3][4]. Also, researchers found that buried surface area of OspA in LA-2 Fab is larger than OspB in the H6831Fab^[2]).

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 ^[5]. 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 ^[5]. 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 ^[5]. 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 ^[6].

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} Ding W, Huang X, Yang X, Dunn JJ, Luft BJ, Koide S, Lawson CL. Structural identification of a key protective B-cell epitope in Lyme disease antigen OspA. J Mol Biol. 2000 Oct 6;302(5):1153-64. PMID:11183781 doi:10.1006/jmbi.2000.4119
4. ↑ ^{4.0 4.1} Li H, Dunn JJ, Luft BJ, Lawson CL. Crystal structure of Lyme disease antigen outer surface protein A complexed with an Fab. Proc Natl Acad Sci U S A. 1997 Apr 15;94(8):3584-9. PMID:9108020
5. ↑ ^{5.0 5.1 5.2} 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
6. ↑ 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