Human beta two microglobulin: Difference between revisions

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=Human Beta two microglobulin (b2m)=
=Human Beta two microglobulin (hb2m)=


==Beta two microglubulin in human class I major histocompatibility complex==
==Beta two microglubulin in human class I major histocompatibility complex (MHCb2m)==
Human β2-Microglobulin (b2m) is the non-covalently bound light chain of the human class I  
Human β2-Microglobulin is the non-covalently bound light chain of the human class I  
major histocompatibility complex (MHC-I).  
major histocompatibility complex (MHC-I).  
its function is to ensure proper folding and cell-surface expression of MHC-1.  
its function is to ensure proper folding and cell-surface expression of MHC-1.  
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==Monomeric human b2m==
==Monomeric human b2m (Mhb2m)==
The first crystal structure of monomeric human b2m (Mhb2m) is solved in 2002. The protein is 99 residue in length and has a
The first crystal structure of monomeric human b2m (Mhb2m) is solved in 2002. The protein is 99 residue in length and has a
seven-stranded β sandwich fold typical of the Immunoglobulin superfamily. β strands A,B,D and E comprise one β sheet, and
seven-stranded β sandwich fold typical of the Immunoglobulin superfamily. It is stabilized by a single disulfide bond between  
whereas β strands C,F,G form the second β sheet. The protein is stabilized by a single disulfide bond between Cys-25 and  
Cys-25 and Cys-80, which links the two β sheets.
Cys-80,which links the two β sheets.
 
{{STRUCTURE_1lds |  PDB=1lds  |  SCENE= }}
{{STRUCTURE_1lds |  PDB=1lds  |  SCENE= }}
==Structural comparison of MHCb2m and Mhb2m==
[[Image:Human b2m bound to MHC-1 .jpg.jpg]]      [[Image:Momeric human b2m.png]] 


Structural comparison of human b2m bound to MHC-1 (left) and monomeric human b2m (right)
Fig.1. crystal structures of MHCb2m and Mhb2m
[[Image:Human b2m bound to MHC-1 .jpg.jpg]]  [[Image:Momeric human b2m.png]]


The
Both of the two strucures adopt seven-stranded β sandwich fold. The most significant difference in the ctrystal structures
of Mhb2m and MHCb2m involves residues in β strand D and the succeeding loop. When complexed with the MHC heavy chain, residues
50-56 of MHCb2m form two short β strands that separated by a two residue β bulge. These strands (depicted as D1 and D2 in Fig.1)
each forms three main-chain-main-chain hydrogen bonds to the adjacent β strand E. The bulge in MHCb2m effectively twists the
edge strand, which facilitate its binding to the surface of the heavy chain. However, β bulge no longer exits in the crytal
strucure of Mhb2m
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