Methylococcus, sterols

Until recently it seemed that prokaryotic organisms (blue-green algae and bacteria) were unable to synthesize steroids or triterpenoids. Several reports, however, now indicate that this is not the case. Hop-22(29)-ene (128), squalene, and several 4,4-dimethyl-sterols have been identified in the bacterium Methylococcus capsulatus. Hop-22(29)-ene has also been isolated from an... 

Although the major pathway of demethylation is 4,4,14a-trimethyl - 4a,14a-dimethyl - 4a-methyl, occasional deviations from the pattern can exist as for example 4,4-dimethyl sterols are major components in Methylococcus capsulatus and macdougallin (I), a 14 -methyl sterol, occurs in Peniocerus macdougallii. 

Jackson, C.J., D.C. Lamb, T.H. Marczylo, A.G.S. Warrilow, N.J. Manning, D.J. Lowe et al. (2002). A novel sterol 14a-Demethylase/Ferredoxin fusion protein (MCCYP51FX) from Methylococcus capsu-latus represents a new class of the cytochrome P450 superfamily. J. Biol. Chem. 211, 46959 6965. 

The bacterial genus Mycoplasma Is well known for the occurrence of and dependence on sterols [3], but these bacteria without cell walls do not synthesize their sterols by themselves [4]. Bacterial species which contain and synthesize sterols by themselves are Methylococcus capsulatus [5,6], Hannocystls exedens [7] and an L-form of Staphylococcus aureus [8]. It Is Interesting to note that Methylococcus capsulatus simultaneously contains hopanolds [6]. 

Methylococcus capsulatus (Bird et ai, 1971 Bouvier ef ai, 1976), the only well-authenticated case of a bacterium which synthesized sterols. Similar compounds have been detected in algae grown in the presence of certain inhibitors (e.g., Patterson et al., 1974). Such experiments also result in the accumulation of 14a-methyl sterols, which indicates that, if demethylation at C-14 is blocked, the second 4a-methyl group can still be removed. However, there is no indication that this is the normal pathway in Chlorella, but it is clear that in M. capsulatus the 14a-methyl group is removed first because a major portion of the sterols present are 4,4-dimethyl derivatives. 

Jackson CJ, Lamb DC, Marczylo TH, Warrilow AG, Manning NJ, Lowe DJ, Kelly DE, Kelly SL (2002) A novel sterol 14alpha-demethylase/ferredoxin fusion protein (MCCYP51FX) from Methylococcus capsulatus represents a new class of the eytoehrome P450 superfamily. J Biol Chem 277 46959 6965... 

Whether some procaryotes contain sterols and others do not has, of course, an important bearing on evolutionary relationships. One may ask, for example, whether Methylococcus and related methanotrophs were ancestral to a line of aerobic bacteria which then abandoned the sterol pathway or whether in another independently evolving branch of aerobes sterol biosynthesis was never invented. These issues are of interest not only by themselves but also because they bear on the matter of how the wide evolutionary gap between procaryotes and eucaryotic cells was bridged. As pointed out earlier, the possession of the sterol molecule is a universal feature of eucaryotic forms of life. For eucaryotes, phylogenetic relationships are much more certain than they are for procaryotic cells so that one can speak with some confidence of eucaryotes that are primitive and those that are more advanced. In the present context it is therefore significant that the red algae, supposedly the most primitive extant eucaryotes, not only contain sterols in substantial amounts but also that in these cells the biosynthetic pathway is modern, i.e. fully developed. Cholesterol or related cholestane derivatives are the dominant sterols in all members of the order Rhodophytae so far examined. ... 

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