Tetrameric structures, hydrogen bonds

A biochemical curiosity is the presence in egg white of the glycoprotein avidin.ab Each 68-kDa subunit of this tetrameric protein binds one molecule of biotin tenaciously with Kf 1015 M 1. Nature s purpose in placing this unusual protein in egg white is uncertain. Perhaps it is a storage form of biotin, but it is more likely an antibiotic that depletes the environment of biotin. A closely similar protein streptavidin is secreted into the culture medium by Streptomyces avidinii.c Its sequence is homologous to that of avidin. It has a similar binding constant for biotin and the two proteins have similar three-dimensional structures.3/d i Biotin binds at one end of a P barrel formed from antiparallel strands and is held by multiple hydrogen bonds and a conformational alteration that allows a peptide loop to close over the bound vitamin. 

The crystal structures of both the cis and the trans isomers of 2,8-dihydroxy-2,4,4,6,6,8, 10,10,12,12-decamethylcyclohexasiloxane have very different hydrogen bonded structures. The cis isomer contains an intramolecular hydrogen bond and two further intermolecular hydrogen bonds link the molecules into cyclic dimers. The tram isomer forms intermolecular hydrogen bonds to give tetrameric units, which are further hydrogen bonded to form infinite sheets256. 

There is still a third possible mechanism for the fumarate hydratase reaction. The proton and hydroxyl groups may be added simultaneously in a concerted reaction. However, observed kinetic isotope effects are not consistent with this mechanism. In 1997 the structure of fumarase C of E. coli was reported. Each active site of the tetrameric enzyme is formed using side chains from three different subunits. The H188 imidazole is hydrogen bonded to an active site water molecule and is backed up by the E331 carboxy-late which forms a familiar catalytic pair. However, these results have not clarified the exact mode of substrate binding nor the details of the catalytic mechanism. Structural studies of fumarate hydratase from yeast and the pig are also in progress. 

The compounds [CymB(Pz)3]Tl (136) and [FcB(Pz)3]Tl show polymeric structures, with bridging B(Pz)3 fragments in the solid state. This is a result of unfavorable steric interaction between the substituent on boron and the hydrogen atoms on the pyrazolyl ring 5-position. The structure of [Cym B(Pz)3]Tl is somewhat related, but it adopts a macrocyclic tetrameric structure rather than a linear polymeric structure. Ligands with secondary donors on the backbone may form additional bonds to the thallium atom. For example, in [HB(3-(2-pyridyl)Pz)3]Tl, weak TF"N interactions between pyridyl nitrogens and the T1 atom have been observed. HB(3-(2-MeOC6Fl4)Pz)3]Tl features close intramolecular TF 0 interactions. ... 

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