Phycocyanobilin, a light-harvesting and photoreceptor pigment in higher plants, algae, and cyanobacteria, is synthesized from biliverdin IXα (BV) by phycocyanobilin:ferredoxin oxidoreductase (PcyA) via two steps of two-proton-coupled two-electron reduction. We determined the neutron structure of PcyA from cyanobacteria complexed with BV, revealing the exact location of the hydrogen atoms involved in catalysis. Notably, approximately half of the BV bound to PcyA was BVH(+), a state in which all four pyrrole nitrogen atoms were protonated. The protonation states of BV complemented the protonation of adjacent Asp105. The "axial" water molecule that interacts with the neutral pyrrole nitrogen of the A-ring was identified. His88 Nδ was protonated to form a hydrogen bond with the lactam O atom of the BV A-ring. His88 and His74 were linked by hydrogen bonds via H3O(+). These results imply that Asp105, His88, and the axial water molecule contribute to proton transfer during PcyA catalysis.

In the presence of calcium ions, human peptidylarginine deiminase (PAD) converts arginine residues in proteins to citrulline. Of the five known human PAD enzymes, the type III isozyme (PAD3) exhibits the highest specificity for synthetic and natural substrates. This study aimed to determine the structure of PAD3 in order to elucidate its selective citrullination mechanism. Crystals of PAD3 obtained using polyethylene glycol 400 as a precipitant diffracted to 2.95 Å resolution using synchrotron radiation. They belonged to space group R3, with unit-cell parameters a = b = 114.97, c = 332.49 Å (hexagonal axes). Assuming two molecules were contained in an asymmetric unit, the calculated Matthews coefficient was 2.83 Å 3 Da À1, corresponding to a solvent content of 56.6%. Initial phases were determined using PAD4 as a molecular-replacement model.

Pb(C102)2 (I), Mr = 340"9, orthorhombic, Ccca, a = 6.004 (1), b = 12.504 (2), c = 6.010 (1) A, V =451.2(1)A 3, z=4, Dx=5"O2Mgm -3, MoKal, h 0108-2701/90/101755-05503.00 = 0-70930 A, /z = 38-76 mm-~, F(000) = 592, T = 300 (1)K, final R = 0.030 for 502 observed unique reflections.Mg(C102)2.6H20 (II), Mr = 267"3,

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