I've been looking at crystal structures of murE from here:
There are 2 structures from M. tuberculosis. The first shows murE bound to UAG and magnesium. You can see that the ADP binding pocket between the brown and blue subdomains is open.
The second M. tuberculosis murE structure includes ADP:
In E. coli, it's clear that the structure is very similar:
But where do we find the murE point mutations? Looking at the M. tuberculosis structure, we find them here (based on a BLAST alignment):
murE751 is a L -> S substitution found on an alpha helix (the backside where the arrow is pointing).
It looks pretty close to the active site, but it points away from all the action. The other murE point mutations (murE749, G -> R; murE750, G -> W) appear on the same brown domain but far away on the surface of the protein:
The first mutation mentioned may hinder the activity since it is relatively close to the ADP binding area but for the most part, I'm skeptical that any of these point mutations have a big impact on activity. I looked at the location of the point mutations for all three species (M. tuberculosis, E. coli and S. aureus) and it seems very consistent.