rent bisubstrate analogue inhibitors PAPU or PALO. Each subunit has a characteristic shape consisting of a hemispheric PTC-PAPU Data collection ESRF LY-411575 chemical information Beamline Wavelength Space group Unit cell a, b, c a, b, c Resolution range a BM16 0.980 P6322 117.2, 117.2, 225.3 90, 90, 120 302.50 Reflections, total/unique Completeness a I/sa Rsymb a Refinement Statistics Resolution range R-factor/Rfree c PTC-PALO Truncated PTC-PALO ID23-2 0.873 P1 81.5, 81.7, 82.3 105, 103, 101 302.00 194,037/114,553 87.5 10.9 5.5 ID14-4 0.979 P6322 90.0, 90.0, 184.3 90, 90, 120 40-1.59 611,850/60,518 100 5.7 8.2 692,675/32,509 100 35.1 11.5 232.50 18.7/21.6 302.00 19.6/23.7 30-1.59 16.2/18.0 Molecules and atoms refined Polypeptide chains Protein atoms PAPU or PALO RMSDd bonds /angles Average B-factor Protein PAPU or PALO Ramachandran Plot Favored Allowed Generously allowed Disallowed a e 2 5,422 2 0.011/1.16 6 15,480 6 0.008/1.01 1 2,446 1 0.007/1.13 19.8 11.1 33.8 30.2 12.7 8.5 93.3 6.1 0 0.7 92.2 7.1 0 0.7 92.6 6.4 0 0.7 Values in parenthesis are data for the highest resolution shell. Rsym = SI2,I./SI, where I is the observed intensity and,I. the average intensity. c R-factor = Sh IFobs|2|FcalcI/Sh |Fobs|, where |Fobs| and |Fcalc| are observed and calculated structure factors amplitudes for all reflections. Rfree, R based on 5% of the data, withheld for the cross-validation test. d RMSD: root mean square deviation. e Using PROCHECK. doi:10.1371/journal.pone.0031528.t001 b 4 February 2012 | Volume 7 | Issue 2 | e31528 Putrescine Transcarbamylase Structure subunit body and a projecting tail formed by a C-terminal helix that, as shown below, is a characteristic and exclusive PTC feature. The subunit body presents the classical transcarbamylase fold, consisting of two domains of similar size and gross fold, the N-domain, and the C-domain, corresponding respectively 5 February 2012 | Volume 7 | Issue 2 | e31528 Putrescine Transcarbamylase Structure to the polar and equatorial domains of ATC and to the CP and ornithine domains of OTC. These domains are folded as open aba sandwiches nucleated by parallel b sheets . The two domains are interconnected by helices 5 and 12, with additional interdomain gluing being provided by helix 1, which runs transversely over helices 5 and 12, at the interdomain divide on the convex face of the subunit. The flat face of the hemispheric subunit buries the bisubstrate analog inhibitor at the interdomain divide. Characteristically for transcarbamylases, the b sheets C-edges of both domains look towards this flat face, and the loops emerging from the b strands C-ends participate in the active center, including the b5-a5 loop, the 230-loop, the b10-a11 loop, and the 80-loop of an adjacent subunit. The participation of two subunits in the active center supports the observation made with aspartate transcarbamylase that the trimeric architecture is essential for activity. Although the composition, topology, and even the length of the secondary structure elements of the “ 23977191 PTC subunit are highly similar to those of the corresponding domains of “ 24786787 pfOTC and hOTC, there are some differences with these enzymes. These differences affect particularly the specificitydetermining 230-loop which encompasses helix 9, and the 
Cterminus of the subunit, where helix 12 is 1.5-turn longer than in OTCs and is connected via a 4-residue linker with the already mentioned extra C-terminal helix. The latter 5-turn helix is exclusive of PTC, it is highly prominent, and it is
