Recombinant glycerol dehydratase of Klebsiella pneumoniae was purified to homogeneity. The subunit composition of the enzyme was most probably a 2 b 2 c 2 . When (R)-and (S)-propane-1,2-diols were used independently as substrates, the rate with the (R)-enantiomer was 2.5 times faster than that with the (S)-isomer. In contrast to diol dehydratase, an isofunctional enzyme, the affinity of the enzyme for the (S)-isomer was essentially the same or only slightly higher than that for the (R)-isomer (K m(R) /K m(S) ¼ 1.5). The crystal structure of glycerol dehydratase in complex with cyanocobalamin and propane-1,2-diol was determined at 2.1 Å resolution. The enzyme exists as a dimer of the abc heterotrimer. Cobalamin is bound at the interface between the a and b subunits in the so-called Ôbase-onÕ mode with 5,6-dimethylbenzimidazole of the nucleotide moiety coordinating to the cobalt atom. The electron density of the cyano group was almost unobservable, suggesting that the cyanocobalamin was reduced to cob(II)alamin by X-ray irradiation. The active site is in a (b/a) 8 barrel that was formed by a central region of the a subunit. The substrate propane-1,2-diol and essential cofactor K + are bound inside the (b/a) 8 barrel above the corrin ring of cobalamin. K + is heptacoordinated by the two hydroxyls of the substrate and five oxygen atoms from the active-site residues. These structural features are quite similar to those of diol dehydratase. A closer contact between the a and b subunits in glycerol dehydratase may be reminiscent of the higher affinity of the enzyme for adenosylcobalamin than that of diol dehydratase. Although racemic propane-1,2-diol was used for crystallization, the substrate bound to glycerol dehydratase was assigned to the (R)-isomer. This is in clear contrast to diol dehydratase and accounts for the difference between the two enzymes in the susceptibility of suicide inactivation by glycerol.Keywords: coenzyme B 12 ; adenosylcobalamin; glycerol dehydratase; crystal structure; radical enzyme catalysis.Adenosylcobalamin is one of the most unique compounds in nature. It is a water-soluble organometallic compound possessing a Co-C r bond and serves as a cofactor for enzymatic radical reactions including carbon skeleton rearrangements, heteroatom eliminations and intramolecular amino group migrations [1]. Diol dehydratase (EC 4.2.1.28) of Klebsiella oxytoca is an adenosylcobalamin (AdoCbl 1 ) dependent enzyme that catalyzes the conversions of 1,2-diols, such as propane-1,2-diol, glycerol, and 1,2-ethanediol, to the corresponding aldehydes [2,3] (Fig. 1). This enzyme has been studied intensively to establish the mechanism of action of AdoCbl [4][5][6][7]. The structurefunction relationship of the coenzyme has also been investigated extensively with this enzyme [5][6][7][8]. Recently, we have reported the three-dimensional structures of its complexes with cyanocobalamin [9] and adeninylpentylcobalamin [10] and theoretical calculations of the entire energy profile along the reaction pathway with a simplifie...

The terminator regions of eukaryotes encode functional elements in the 3' untranslated region (3'-UTR) that influence the 3'-end processing of mRNA, mRNA stability, and translational efficiency, which can modulate protein production. However, the contribution of these terminator regions to gene expression remains unclear, and therefore their utilization in metabolic engineering or synthetic genetic circuits has been limited. Here, we comprehensively evaluated the activity of 5302 terminator regions from a total of 5880 genes in the budding yeast Saccharomyces cerevisiae by inserting each terminator region downstream of the P TDH3 - green fluorescent protein (GFP) reporter gene and measuring the fluorescent intensity of GFP. Terminator region activities relative to that of the PGK1 standard terminator ranged from 0.036 to 2.52, with a mean of 0.87. We thus could isolate the most and least active terminator regions. The activities of the terminator regions showed a positive correlation with mRNA abundance, indicating that the terminator region is a determinant of mRNA abundance. The least active terminator regions tended to encode longer 3'-UTRs, suggesting the existence of active degradation mechanisms for those mRNAs. The terminator regions of ribosomal protein genes tended to be the most active, suggesting the existence of a common regulator of those genes. The ″terminatome″ (the genome-wide set of terminator regions) thus not only provides valuable information to understand the modulatory roles of terminator regions on gene expression but also serves as a useful toolbox for the development of metabolically and genetically engineered yeast.

The human adenosyltransferase hATR converts exogenous cobalamin into coenzyme B12 by transferring the adenosyl group from cosubstrate ATP to a transiently formed Co1+cobalamin (Co1+Cbl) species. A particularly puzzling aspect of hATR function is that the midpoint potential for Co2+Cbl --> Co1+Cbl reduction is below that of readily available biological reductants. Our magnetic circular dichroism and electron paramagnetic resonance spectroscopic studies reported here reveal that, in the absence of ATP, the interaction between Co2+Cbl and hATR promotes partial conversion of the cofactor to its "base-off" form in which a water molecule occupies the lower axial position. This interaction becomes much stronger in the presence of ATP, leading to the formation of an unprecedented Co2+Cbl species with spectroscopic signatures consistent with an essentially four-coordinate, square-planar Co2+ center. This unusual Co2+Cbl coordination is expected to raise the Co2+/1+ reduction potential well into the physiological range.

Coenzyme B12 dependent diol dehydratase undergoes mechanism‐based inactivation by glycerol, accompanying the irreversible cleavage of the coenzyme Co–C bond. Bachovchin et al. [Biochemistry16, 1082–1092 (1977)] reported that glycerol bound in the GS conformation, in which the pro‐S‐CH2OH group is oriented to the hydrogen‐abstracting site, primarily contributes to the inactivation reaction. To understand the mechanism of inactivation by glycerol, we analyzed the X‐ray structure of diol dehydratase complexed with cyanocobalamin and glycerol. Glycerol is bound to the active site preferentially in the same conformation as that of (S)‐1,2‐propanediol, i.e. in the GS conformation, with its 3‐OH group hydrogen bonded to Serα301, but not to nearby Glnα336. kinact of the Sα301A, Qα336A and Sα301A/Qα336A mutants with glycerol was much smaller than that of the wild‐type enzyme. kcat/kinact showed that the Sα301A and Qα336A mutants are substantially more resistant to glycerol inactivation than the wild‐type enzyme, suggesting that Serα301 and Glnα336 are directly or indirectly involved in the inactivation. The degree of preference for (S)‐1,2‐propanediol decreased on these mutations. The substrate activities towards longer chain 1,2‐diols significantly increased on the Sα301A/Qα336A double mutation, probably because these amino acid substitutions yield more space for accommodating a longer alkyl group on C3 of 1,2‐diols. Database Structural data are available in the Protein Data Bank under the accession number http://www.rcsb.org/pdb/search/structidSearch.do?structureId=3AUJ. Structured digital abstract  http://www.uniprot.org/uniprot/Q59472, http://www.uniprot.org/uniprot/Q59471 and http://www.uniprot.org/uniprot/Q59470 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0915 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0114 (http://mint.bio.uniroma2.it/mint/search/interaction.do?interactionAc=MINT-8301985)