A-cyclase

The first G-protein a subunit to be identified was Gs. The a subunit of Gs (as) is responsible for stimulating adenylate cyclase (hence, the subscript s ) and is ADP-ribosylated and activated by CTx. Gs has at least four molecular variants. Some evidence exists that as can also enhance the activity of cardiac L-type Ca2+ channels, independently of their phosphorylation by cAMP-stimu-lated protein kinase A. Golf is a cyclase-stimulating homolog in the olfactory epithelium, activated by the large family of olfactory receptors. 

Figure 29 A common pathway of trichothecene biosynthesis. FPP is cyclized to TDN (125) by a cyclase encoded by Tri5. In the biosynthesis of both Fusarium trichothecenes and non-Fusarium trichothecenes, three oxygenations catalyzed by a CYP encoded by Tri4 follow to yield ITdiol (128).

Methyl shifts are now concerted Hydrides move along the chain To lanosterol converted Suddenly four rings we gain Ring arounding quite astounding Catalysed by a cyclase (to amaze) Within the microsomal phase. 

However, despite many attempts,it is only within the last few years that progress has been made toward the successful cyclization of hexose derivatives to inositol derivatives. Ferndndez, Izquierdo and Martinez " have reported the presence of a cyclase in the leaves of Lactuca virosa L. which converts D-glucose to a material giving the Scherer test for inositol. Since this color test is general for the inositols a definite statement regarding the nature of the product will have to await its isolation and identification. 

Porfirova, S., Bergmuller, E., Tropf, S., Lemke, R., and Dormann, R, Isolation of an Arabidopsis mutant lacking vitamin E and identification of a cyclase essential for all tocopherol biosynthesis, Proc. Natl. Acad. Sci. U.S.A. 99 (19), 12495-12500, 2002. 

Several specific enzyme activities associated with precursor conversions in the aflatoxin pathway " " have been partially purified (Fig.l 1.1) whereas others such as methyltransferases " have been purified to homogeneity. Several other enzymes involved in aflatoxin biosynthesis, such as a reductase and a cyclase, " have also been purified from A. parasiticus. A desaturase that converts VERA to VERB has been found in cell-free fungal extracts. Matsushima et al. have purified and characterized two versiconal hemiacetal acetate reductases involved in toxin synthesis, whereas Kusumoto and Hsieh purified to homogeneity an esterase that converts VHA to versiconal. ... 

A cyclase from Pogostemon cablin (syn. P. patchouli and P. heyneanus) leaves is capable of converting famesyl pyrophosphate to (- )-patchoulol (14, Fig. 21.7). Cell-free... 

The only other 3, 5 -cyclic nucleotide found in nature to date is cyclic GMP. Like cyclic AMP, the guanine derivative is excreted in the urine in relatively large amounts. The amount of cyclic GMP excreted in rat urine docs vary with the endocrine status of the animal, but specific hormones affect it differently than cyclic AMP. Cyclic GMP is widely distributed in nature, but is present in tissues in lower concentrations than cyclic AMP. Its synthesis is catalyzed by a cyclase distinct from adenyl cyclase. 

Guanylate cyclases exist in both the soluble and membrane fractions of most tissue homogenates, probably across all animals (Foster et al. 1999). The conservations of amino acids within signature is pronounced, and thus a cyclase homology domain, known to process either adenylyl or gua-nylyl cyclase activity, and an amphipathic region found in both soluble and membrane forms of guanylate cyclase are easily recognised at the primary amino acid level. 

Ueberbacher, B.T., Oberdorfer, G., Gruber, K. and Faber, K. (2009) Epoxide-hydrolase-initiated hydrolysis/rearrangement cascade of a methylene-interrupted bis-epoxide yields chiral THF moieties without involvement of a "cyclase". ChemBioChem, 10,1697-1704. 

Formation of sterols from squalene involves cyclization. First a microsomal mixed-function oxidase (squalene epoxidase) forms squalene-2,3-oxide in the presence of NADPH, FAD and O2 (there is no requirement for cytochrome P450 in this reaction). The cyclization of the oxide to lanosterol then takes place by a concerted reaction without the formation of any stable intermediates. This conversion, which has been described as the most complex known enzyme-catalysed reaction, depends on a cyclase with a molecular mass of only 90kDa. In plants and algae squalene-2,3-epoxide is cyclized to cycloartenol which is the precursor of stigmasterol whereas lanosterol is the precursor of cholesterol and ergosterol (Figure 7.19). 

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