Sandbox Reserved 1051: Difference between revisions

[[Image:Ag85C Hg.jpg |100 xp|left|thumb|'''Figure 4.''' Ag85C-Hg: A modification of Ag85C generated with the addition of p-chloromercuribenzoic acid compared to the native structure. This modified enzyme lacks a hydrogen bond between Glu228 and His260, which relaxes the kink normally found in the native structure and inhibits the active site.]]  

[[Image:E228Q.jpg |100 xp|left|thumb|'''Figure 5.''' Ag85C-E228Q:  The glutamate residue is shifted four angstroms in the mutated form of this enzyme causing a rearrangement of hydrogen bonds within the enzyme. The histidine residue, labeled in pink, takes on two conformations, binding with alternative serine residues, labeled in red.]]  

The mutation introduced in [http://proteopedia.org/wiki/index.php/4qdz Ag85C-E228Q] causes the Glu228 of the catalytic triad to be shifted 4 angstroms from its original position in the native structure of Ag85C. Due to the shift of Glu228 is the loss of hydrogen bonds between Ser124 and His260.  Instead, His260 hydrogen bonds with Ser148, which also results from the shift of Glu228.  A weak electron density difference in the His260 position of the native and mutated structures was also noted, suggesting that the residue may take on two alternative conformations in the Ag85C-E228Q mutant.  Overall, the enzyme functionality is decreased to only 17% activity.<ref name="Favrot"/>

Unlike other Ag85C mutants and modifications aforementioned, the Ag85C-E228Q does not eliminate any hydrogen bonds in the catalytic triad.  Rather, it simply replaces a carboxylate moiety with an amide.  Further, the structural change observed in the low-energy conformation of the E228Q mutant provides additional support for the hypothesis that the natively kinked helix alpha-9 is central to the enzymatic function of Ag85C.<ref name="Favrot"/>

[[Image:H260Q.jpg |100 xp|left|thumb|'''Figure 6.''' Ag85C-H260Q:  Mutated Ag85C enzyme due to conversion of Glu228 to amide-containing side chain.  A shift in helix alpha-9 prevents formation of any stabilizing hydrogen bonds between residues His260 and Glu228, thus decreasing its enzymatic activity.]]

In [http://proteopedia.org/wiki/index.php/4qe3 Ag85C-H260Q], a shift in helix alpha-9 prevents the formation of any stabilizing hydrogen bonds between residues His260 and Glu228, thus decreasing its enzymatic activity.  The conversion of the glutamate, a key player in the catalytic triad, to the corresponding amide-containing side chain as well as a loss of a general base in the charge relay are both key causes for the loss of function.<ref name="Favrot"/>