In this case, yes - there’s clearly a metal ion coordinated there. But for the record, it is entirely possible (if somewhat rare) to get acidic side chains directly contacting each other. Look up “carboxyl-carboxylate pair”. They’re typically found at low pH and/or in quite protected environments. Think of it as having one carboxyl group protonated and H-bonding to the other, except that the proton is more-or-less equally shared. Their most common role is as pH-dependent conformational switches - very stably bonded below the pKa, wanting nothing to with each other above.
Tristan Croll Research Fellow Cambridge Institute for Medical Research University of Cambridge CB2 0XY > On 29 Dec 2017, at 14:18, Robbie Joosten <robbie_joos...@hotmail.com> wrote: > > Hi Anurag, > > There is a metal in the middle there. You need to figure out which one. Have > a look at the crystallization conditions. If can also have a look at > anomalous signal. > > Cheers, > Robbie > > From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Anurag > Misra > Sent: Friday, December 29, 2017 14:55 > To: CCP4BB@JISCMAIL.AC.UK > Subject: [ccp4bb] Asp-Asp pair facing each other at <3.0 A distance > > Dear all, > I have solved a protein structure at 2.8 A resolution with Rfree 26%. In this > structure catalytic His interacts with non catalytic Asp. This His-Asp pair > (Chain A) is facing another identical His-Asp pair (in Chain B) which are > related with 2-fold ncs (pics are attached) (The molecule exists as a dimer). > There is another His-Asp pair in chain C (not shown in attached pic) which > faces to the identical pair related to crystallographic 2-fold. (min distance > Asp-Asp 2.58 A, His-His 3.6 A and His1-Asp1/His1-Asp2 ~3.0 A) > How to explain Asp-Asp head on pairs in close proximity? Is there a role of > His in making such pairs. Is there any role of change in pKa to allow such > interactions? Kindly suggest a few references where similar Asp-Asp pairs > have been seen. > > I appreciate your kind attention. > Thank you so much for helping me. > Best regards, > Anurag > > > <image001.png>
