In our last “Papers & Cake” in 2015, we discussed a paper by Wu et al. (2015) which explores the occurrence of 2-methyl hopanoids in the cell membrane of bacterium Rhodopseudomonas palustris TIE-1.
In Bacteria, hopanoids help to rigidify and regulate the cell membrane (Figure 1). These compounds typically consist of a C30 ring system and an extended polyfunctionalised side chain. In 1999, Roger Summons and co-workers identified C2-methylated hopanoids in cyanobacterial cultures and ancient sediments (Fig. 1). They argued that C2-methylhopanes were direct evidence for cyanobacteria and, as a result, oxygenic photosynthesis. This has been used “…as evidence for the antiquity of oxygenic photosynthesis and for the waxing and waning of cyanobacterial primary productivity and nitrogen fixation in the geological past”.
However, in 2010, Paula Welander, a post-doc working with Roger Summons, began to look at 2-methylhopanoids in more detail. Using a genetic and phylogenetic approach, she found that: 1) not all cyanobacteria had the capability to synthesise C2-methylated hopanes and that 2) other organisms (e.g. acidobacteria, α-proteobacteria) had the capability to synthesise C2-methylated hopanes.
Indeed, Jessica Ricci at CalTech has argued that 2-methyhopanes are no longer reliable biomarkers for cyanobacteria (or indeed, for any other taxonomic group). Instead, they may be indicators of a specific environmental niche (low O2 and fixed N2, high osmolarity).
Paper: Wu et al., (2015) Methylation at the C-2 position of hopanoids increases rigidity in native bacterial membranes. eLife.
In this paper, Wu et al. (2015) attempt to find a specific biological function for 2-methylated hopanoids in modern cells and investigate whether particular environmental conditions regulate the production of specific hopanoid variants.
In order to link in vivo hopanoid production with sedimentary biomarker distributions, the authors also evaluate whether the function of these lipids in modern organisms remains stable over millions of years. To achieve this, Wu et al. (2015) use (meta)genomic analysis to explore the biosynthetic pathway of 2Me-hopanoid . They also identify the stress-responsive pathway regulating RNA transcription of the 2-methylase (hpnP) in the model hopanoid producing bacterium (Rhodospeudomonas palustris TIE-1).
What is the role of 2-methylated hopanoids in the cell membrane?
Wu et al. (2015) argue that methylation of hopanoids at the C2 position help the other membrane molecules to pack more tightly together. This makes the membrane more rigid. Another finding is that the extent of membrane rigidity depends on the length of the 2-methylated hopanoid and on the other molecules in the membrane. They also found that the distribution of 2Me-hopanoid in inner (IM) and outer membrane (OM) of R. palustris TIE-1 varies.
To interpret the effect of methylation on the atomic scale, and to understand its biophysical mechanism, a molecular dynamic simulation of 2Me-hopanoid within a relevant lipid context is required; however it is not approached in this paper. Authors of the paper speculated upon a mechanism of rigidification and suggest that when a methyl group is added onto the 2′ position of a hopanoid, the two additional 1,3-diaxial interactions and the stearic effect between the 2-methyl group and the methyl groups at the 4′ and 10′ position of the A-ring are altered. This would affect the preferred molecule shape, and transform the ring conformation from a chair to a twisted state, which may improve the ability of hopanoids to rigidify membranes. Although this speculation seems convincing, it remains as a suggestion and is not proven in this paper.
What’s the implication of their finding in organic geochemistry?
Wu et al. (2015) conclude that a more rigid membrane would protect the bacteria more in times of stress; therefore geological samples containing an increased amount of 2-methylhopanes are likely to indicate time periods when the bacteria experienced elevated levels of stress. However, they do not specify the type of stress (e.g. increased temperature, higher salinity or nutrient deficiencies).
Authors: LB and JH.
Brocks JJ, Logan GA, Buick R, Summons RE. 1999. Archean molecular fossils and the early rise of eukaryotes. Science 285, 1033-1036.
Knoll AH, Summons RE, Waldbauer JR, Zumberge JE. 2007. The geological succession of primary producers in the oceans. In: Falkowski P, Knoll AH, editors. In the evolution of primary producers in the sea. Boston: Academic Press. p. 133–164.
Ricci JN, Coleman ML, Welander PV, Summons RE, Spear JR, Newman DK. 2014 Diverse capacity for 2-methylhopanoid production correlates with a specific ecological niche. The ISME Journal 8, 675-684.
Summons RE, Jahnke LL, Hope JM, Logan GA. 1999. 2-Methylhopanoids as biomarkers for cyanobacterial oxygenic photosynthesis. Nature 400, 554-557.
Welander PV, Coleman ML, Sessions AL, Summons RE, Newman DK. 2010. Identification of a methylase required for 2-methylhopanoid production and implication for the interpretation of sedimentary hopanes. PNAS 107, 8537-8542.
Wu C-H., Bialecka-Fornal M., Newman DK. 2015. Methylation at the C-2 position of hopanoids increases rigidity in native bacterial membranes. eLife 2015;4:e05663.