Reactions involving the fixation of carbon monoxide (CO) into activated acetyl groups on surfaces containing the sulfides of nickel and iron have been implicated in models of the chemoautotrophic origin of life (1, 2). These models adopt sequences of the acetyl-CoA pathway (Wood͞Ljungdahl pathway), which is operative in CO 2 fixation by autotrophic acetogens, sulfidogens, and methanogens, as well as in acetate utilization by methanogens (3-7). The synthesis of acetyl-CoA in the pathway involves the functions of a NiFeS acetyl-CoA synthase (ACS), a NiFeS CO dehydrogenase (CODH), and a cobalt-containing corrinoid iron-sulfur protein (CoFeSP). ACS and CODH form a tight complex in all microorganisms that have been previously examined in that respect (4-6). ACS catalyzes the synthesis of acetyl-CoA from a methyl group donated by CoFeSP, CO, and CoA (Eq. 1) (5, 6). CODH catalyzes the reduction of CO 2 to CO, which reappears in the carboxyl group of the acetyl residue formed (Eq. 2) (5, 6).Carboxydothermus hydrogenoformans is a hydrogenogenic bacterium that utilizes CO as a sole source of carbon and energy under anaerobic chemolithoautotrophic conditions (8). The bacterium presumably employs the acetyl-CoA pathway for the assimilation of carbon (V.S., personal communication). The genomic sequence of C. hydrogenoformans contains an Ϸ10-kb region that assembles the predicted functions of the genes cooSIII (CODHIII Ch , 73.3 kDa), acs (ACS, 82.2 kDa), cfsA and cfsB (the 48.8-kDa large and the 33.9-kDa small subunits of the heterodimeric CoFeSP), and mtr (a 29.3-kDa methyltransferase) (Fig. 1A). The deduced amino acid sequence of acs from C. hydrogenoformans shows 74% identity (86% similarity) to the complexed ACS Mt from Moorella thermoacetica (9).According to a recent model (9), the generation of energy and reducing equivalents in C. hydrogenoformans involves the activities of the monofunctional CODHI Ch and CODHII Ch . A first crystal structure of a NiFeS-CODH, the CODHII Ch , at 1.6 Å resolution in the dithionite-reduced state showed five metal clusters, of which clusters B, BЈ, and a subunit-bridging, surface-exposed cluster D are cubane-type [4Fe-4S] clusters (10). The active-site clusters C and CЈ are asymmetric [Ni-4Fe-5S] clusters. Their integral Ni ion, which is the likely site of CO oxidation, is coordinated by four sulfur ligands with square planar geometry. The CODH Rr crystal structure from Rhodospirillum rubrum determined at 2.8-Å resolution shows a similar overall structure, except that cluster C is interpreted as a [Ni-3Fe-4S] cubane, to which a mononuclear Fe site is coordinated (11). Cluster C in the CODH component of the ACS Mt ͞CODH Mt complex from M. thermoacetica has been described as a Ni-4Fe-4S cage that can be viewed as a [Ni-Fe] Doukov et al. (13) and Darnault et al. (12) are generally similar and have an ␣ 2 ␤ 2 (␣, ACS; ␤, CODH) quaternary structure. The structures of the CODH Mt subunits are similar in the two crystal forms and closely resemble the structures of CODHII Ch (10) and CODH R...

Polyamines are involved in many fundamental cellular processes. Common polyamines are putrescine, spermidine and spermine. Spermine is synthesized by transfer of an aminopropyl residue derived from decarboxylated S-adenosylmethionine to spermidine. Thermospermine is an isomer of spermine and assumed to be synthesized by an analogous mechanism. However, none of the recently described spermine synthases was investigated for their possible activity as thermospermine synthases. In this work, putative spermine synthases from the diatom Thalassiosira pseudonana and from Arabidopsis thaliana could be identified as thermospermine synthases. These findings may explain the previous result that two putative spermine synthase genes in Arabidopsis produce completely different phenotypes in knock-out experiments. Likely, part of putative spermine synthases identifiable by sequence comparisons represents in fact thermospermine synthases.

Bone is a connective tissue and guarantees protection and support of organ function. Contrary to the common view, bone is a dynamic tissue that constantly undergoes turnover in order to maintain stability and integrity. In this process called bone turnover or bone remodelling, two effector cell types are involved. Osteoclasts, specialised for bone resorption, and osteoblasts, responsible for bone formation, are key players in bone turnover. In the past decade, a lot of information about signal pathways, osteoblast-osteoclast communication and osteoclast activation concerning bone remodelling has arisen. In this publication, we aim to review molecular and biochemical insights with respect to the bone remodelling process. The bone remodelling process is of fundamental importance for craniofacial growth, orthodontic tooth movement and regenerative dentistry.