User:Marvin O'Neal/Antibody OspA and OspB

From Proteopedia

Introduction

Lyme Disease

The causative agent of Lyme disease is Borellia burgdorferi, a spirochaete found in the gut of hard bodied ticks of genus Ixodes. A factor contributing to the severity of Lyme disease is its resistance to complement-dependent lysis. Lysis in the absence of complement is necessary to clear Borrelia burgdorferi.

Complement Independent Antibodies

Monoclonal Antibodies IgG and IgM are complement independent antibodies, and are not dependent on a specific complement in order to successfully bind.

Fragment Antigen Binding Reigons

The key to the bacteriacidal properties of these antibodies are the Fragment Antigen Binding (or fab) reigons of the antibody, which consists of the light chain and N-Terminus of the heavy chain in of the antibody.

Digestion of an Antibody by Paipin clearly shows two Fab reigons present on each antibody

The binding of specific fabs of IgG and IgM monoclonal antibodies to OspA and OspB of the Borrelia leads to a complement-independent lysis of the bacteria. ^[1]

Interaction With OspB

OspB + H6831

IgG antibody fabs CB2 and are structurally similar and both target the C-terminal of . H6831 consists of a and a chain and each chain is composed of a variable and a constant domain. The paratope is located at the N terminal of the variable region of both the heavy and the light chains (Putnam 1979). Fab binding destabilizes the outer membrane (OM) of B. burdorferi with subsequent formation of spheroplasts. It has been observed that the bactericidal action upon binding requires the presence of bivalent cations (Mg2+ and Ca2+). When a similar fab, CB2, is bound to OspB, channels open in the OM allowing rapid infusion of electrolytes,increasing the osmolarity of the periplasm and triggering bivalent cation dependent cascades ^[2]

1. 1. When bound to a similar fab CB2, OspB-CB2 complexes could create physical openings in the OM allowing for rapid infusion of electrolytes, increasing the osmolarity of the periplasm and triggering bivalent cation dependent cascades (Escudero; Halluska et al. 1997)##

Lys-253

Homologous Structures in OspA=

spinqualitylabelsall models OspA-LA2 Complex Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

IgG antibody fabs CB2 and H6831, which are structurally similar, target the C-terminal of OspB. H6831 consists of a heavy and a light chain and each chain is composed of a variable and a constant domain. The paratope (binding site to the antigen) is located in the N terminal of the variable region of both the heavy and the light chains (Putnam 1979). The fab binding destabilizes the outer membrane (OM) of B. burdorferi, with subsequent formation of spheroplasts. It has been observed that the bactericidal action, but not the binding, requires the presence of bivalent cations (Mg2+ and Ca2+). Escudero et al. study demonstrated the inability of fab to kill bacteria in the absence of the bivalent cations. It was speculated that OspB- Cb2 (a fab similar to H6831) complexes could create physical openings in the OM allowing for rapid infusion of electrolytes, increasing the osmolarity of the periplasm and triggering bivalent cation dependent cascades (Escudero; Halluska et al. 1997) Most interactions between H6831 and OspB are between the three adjacent surface-exposed loops at the C-terminal of OspB and some residues on the fab heavy chain that include tyrosine, tryptophan, glutamate, and histidine. The majority of the electrostatic and hydrogen-bonded interactions are between loop 2 (residues 250-254) and the fab heavy chain. Lysine 253 in loop 2 of OspB has a necessary and major role due to its central position in the exposed loops. A mutation at its position abrogates the binding interaction and causes the resistance of the bacteria to the bactericidal effect of the fab. Lys 253 binds to the two aromatic residues on the fab heavy chain, tyrosine and tryptophan. It also makes hydrogen bonds with the glutamate in the heavy chain of the fab. Additional interactions were observed between the carbonyl in loop 1 of the OspB and the histidine in the fab heavy chain and between loop 3 of OspB and fab light chain. (Becker; Bunikis et al. 2004). Whereas Ding et al. found no changes in the C-terminal of OspA upon the binding to the fab, Becker et al. study using crystallography has showed structural changes in OspB upon binding. The most significant difference between the free and the complexed structure of OspB is the loss of the central beta sheet strands 1-4. Both small positional shifts near the Fab binding site and a few larger structural changes away from the binding site were observed. The largest shifts (7– 8 Å) correspond to the repositioning of a loop opposite the Fab-binding site (residues 218 –220). In the free OspB structure, all regions that exhibit shifts are adjacent to the central sheet; in the OspB-H6831 complex they all shift toward, and slightly overlap, the position of the missing sheet. These observations suggest that the larger conformational changes are related to the loss of the central sheet, which could have happened through proteolytic cleavage or fortuitous crystal contacts (Becker; Bunikis et al. 2004)

OspB PDB: 1P4B