View source for 2on6

[[Image:2on6.jpg|left|200px]]<br /><applet load="2on6" size="350" color="white" frame="true" align="right" spinBox="true" 
caption="2on6, resolution 2.503&Aring;" />
'''Crystal stucture of human purine nucleoside phosphorylase mutant H257F with Imm-H'''<br />

==Overview==
The X-ray crystal structures of human purine nucleoside phosphorylase, (PNP) with bound inosine or transition-state analogues show His257 within, hydrogen bonding distance of the 5'-hydroxyl. The mutants His257Phe, His257Gly, and His257Asp exhibited greatly decreased affinity for, Immucillin-H (ImmH), binding this mimic of an early transition state as, much as 370-fold (Km/Ki) less tightly than native PNP. In contrast, these, mutants bound DADMe-ImmH, a mimic of a late transition state, nearly as, well as the native enzyme. These results indicate that His257 serves an, important role in the early stages of transition-state formation. Whereas, mutation of His257 resulted in little variation in the PNP.DADMe-ImmH.SO4, structures, His257Phe.ImmH.PO4 showed distortion at the 5'-hydroxyl, indicating the importance of H-bonding in positioning this group during, progression to the transition state. Binding isotope effect (BIE) and, kinetic isotope effect (KIE) studies of the remote 5'-3H for the, arsenolysis of inosine with native PNP revealed a BIE of 1.5% and an, unexpectedly large intrinsic KIE of 4.6%. This result is interpreted as a, moderate electronic distortion toward the transition state in the, Michaelis complex with continued development of a similar distortion at, the transition state. The mutants His257Phe, His257Gly, and His257Asp, altered the 5'-3H intrinsic KIE to -3, -14, and 7%, respectively, while, the BIEs contributed 2, 2, and -2%, respectively. These surprising results, establish that forces in the Michaelis complex, reported by the BIEs, can, be reversed or enhanced at the transition state.

==About this Structure==
2ON6 is a [http://en.wikipedia.org/wiki/Single_protein Single protein] structure of sequence from [http://en.wikipedia.org/wiki/Homo_sapiens Homo sapiens] with <scene name='pdbligand=IMH:'>IMH</scene> as [http://en.wikipedia.org/wiki/ligand ligand]. Active as [http://en.wikipedia.org/wiki/Purine-nucleoside_phosphorylase Purine-nucleoside phosphorylase], with EC number [http://www.brenda-enzymes.info/php/result_flat.php4?ecno=2.4.2.1 2.4.2.1] Full crystallographic information is available from [http://oca.weizmann.ac.il/oca-bin/ocashort?id=2ON6 OCA]. 

==Reference==
Neighboring Group Participation in the Transition State of Human Purine Nucleoside Phosphorylase., Murkin AS, Birck MR, Rinaldo-Matthis A, Shi W, Taylor EA Steven C Almo, Schramm VL, Biochemistry. 2007 May 1;46(17):5038-5049. Epub 2007 Apr 4. PMID:[http://ispc.weizmann.ac.il//pmbin/getpm?pmid=17407325 17407325]
[[Category: Homo sapiens]]
[[Category: Purine-nucleoside phosphorylase]]
[[Category: Single protein]]
[[Category: Almo, S.C.]]
[[Category: Murkin, A.S.]]
[[Category: Rinaldo-Matthis, A.]]
[[Category: Schramm, V.L.]]
[[Category: IMH]]
[[Category: purine nucleoside phosphorylase]]

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