Eubacteria, enzymes found

A second example of the differences between important biomolecules in archaebacteria and eubacteria is their DNA-dependent RNA polymerase. The enzyme found in archaea resembles that in eukaryotes more than it does those in bacteria ... 

Mn superoxide dismutases are found in both eubacteria and archaebacteria as well as in eukaryotes, where they are frequently found in mitochondria. They (Figure 16.1) have considerable structural homology to Fe SODs both are monomers of 200 amino acid and occur as dimers or tetramers, and their catalytic sites are also very similar. They both catalyse the two-step dismutation of superoxide anion and, like the Cu-Zn SODs, avoid the difficulty of overcoming electrostatic repulsion between two negatively charged superoxide anions by reacting with only one molecule at a time. As in the case of Cu-Zn SOD, a first molecule of superoxide reduces the oxidized (Mn3+) form of the enzyme, releasing... 

Enzymes that are structurally related to the eukaryotic V-ATPase are also found in certain eubacteria (Speelmans etal., 1994 Takase etal., 1994 Yokoyama etal., 1990). Based on nucleotide sequence analysis, it is believed that these bacterial V-like ATPases have been introduced into the eubacteria via horizontal gene transfer from Archaea (Hilario and Gogarten, 1993, 1998). The subunit composition of the bacterial V-like ATPase is indeed more similar to the archaeal A-ATPase than to the eukaryotic V-ATPase, and we will therefore treat the bacterial V-ATPase—like enzyme together with the archaeal A-ATPase (see below). In the following, we will use the name V-ATPase only for the eukaryotic enzyme, and we will call the bacterial enzyme the A/V-type ATPase as suggested by Hilario and Gogarten (1998). 

There were 37 distinct enzymes that contain molybdenum or tungsten known by the end of 1997. The enzymes are diverse in function, broadly distributed, and include oxidases, reductases, dehydrogenases, a transhydroxylase, and a hydratase. The Mo enzymes are found in eubacteria, archae, protista, fungi, plants, and animals (including humans) and are essential for respiration and carbon and nitrogen assimilation. Several of the enzymatic substrates and products are key components in the nitrogen, sulfur, selenium, carbon, and arsenic cycles and have major biological and environmental impact. 

In aerobic environments, eukaryotes and many eubacteria oxidatively decarboxylate the 2-oxoacids via pyruvate and 2-oxoglutarate dehydrogenase complexes [36]. For comparison with the archaebacterial oxidoreductases, the catalytic mechanism of these complexes is also shown in Fig. 4. Three enzymic activities are involved, catalysed by three distinct enzymes a 2-oxoacid decarboxylase (El), a dihydrolipoyl acyltransferase (E2) and dihydrolipoamide dehydrogenase (E3). Multiple copies of these three enzymes are found in each complex molecule, resulting in relative molecular masses in excess of 2x10 ... 

It is the purpose of this review to describe and discuss the pathways of central metabolism in the archaebacteria however, for the reasons given above, this will not be done in isolation but as a comparative survey with those found in eubacteria and eukaryotes. Indeed, not only will the pathways be compared, but the comparison will be extended to the enzymes catalysing the reactions of these pathways. For previous reviews the reader is referred to Danson[l,2]. 

TGT is the key enzyme involved in the biosynthesis of queuosine and archaeosine (see next section for a discussion of queuosine biosynthesis). TGT occurs in all three kingdoms of life with very few exceptions (yeast and mycoplasma). In eukarya, queuine (the base of queuosine) is a dietary factor (found in many sources ) and is incorporated directly into tRNA by TGT. In eubacteria, TGT incorporates a queuine precursor (preQj,... 

In eubacteria, the bifunctional riboflavin kinase/FAD synthetase type appears to be widely distributed. ° ° A monofunctional kinase that is specific for dihydroriboflavin has been found in B. subtilis, which also has the bifunctional enzyme type that operates with oxidized flavin substrates. 

Molybdenum (see also Part III, Chapter 18) The molybdenum cofactor (MoCo) is an essential component of a large family of enzymes involved in important transformations in carbon, nitrogen, and sulfur metabolism. The MoCo biosynthetic pathway is evolutionarily conserved and found in arch-aea, eubacteria, and eucaryotes. In humans. 

In conclusion, these results indicate that Sulfolobus solfataricus ceU-free system contains a significant ADP-ribosyl transferase activity. It seems of relevant interest that this archaebacterium is the most primitive organism in which ADP-ribosylation has been demonstrated. Furthermore the enzymatic activity appears to be thermophilic, a unique property never observed for the same kind of enzyme isolated from other sources (17). Further studies are in progress on the purification and characterization of the enzyme to investigate its biological role. As mentioned above, several biochemical properties demonstrated that sulfur-dependent archaebacteria are closer related to eukaryotes than to eubacteria (3). These findings support the hypothesis that the ADP-ribosyl transferase activity which we found associated primarily with the nucleoprotein fraction of Sulfolobus solfataricus, could play a role in any cellular event in which the enzyme is known to be involved in eukaryotic cells (17). 

Some eubacteria and archaea do not possess carotenogenesis gene clusters. The genes for early steps can be found from homologous searches. Some genes and enzymes are found by molecular biological techniques (Table 106.3), but some genes for the final steps of complex carotenoids remain unknown. 

Enzymes containing the molybdenum (or tungsten) molybdopterin cofactor are found in eubacteria and archaebacteria and in lower and higher eukaryotes,... 

Next to the mevalonate biosynthesis is the reduction of HMG-CoA to mevalonic acid by HMG-CoA reductase [EC 1.1.1.34] (Scheme 5.1). The enzyme is found in eukaryotes, archaebacteria, and some eubacteria. The conversion of the... 

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