Sterols: 4. Hopanoids and Related Lipids
Bacteria and other prokaryotic organisms such as blue-green algae do not in general contain any of the conventional sterols found in plants and animals (there are always exceptions), but rather many species have related molecules, i.e. pentacyclic triterpenoids based on a hopane skeleton with four cyclohexane rings and a cyclopentane E-ring and termed ‘hopanoids’ (from the plant genus Hopea, from which they were first isolated as components of the resin). They were first discovered in living prokaryotes in 1971 in the bacteria Methylococcus capsulatus and Alicyclobacillus acidocaldarius, but it is now known that 10% of bacteria, but especially the Proteobacteria, have the appropriate genes to produce hopanoids. Bacteriohopanepolyols are more complex forms of these molecules from bacteria. In nature, many different structures are now known to occur from simple hopanoids to elongated compounds with polyfunctional side chains, and they extend from living organisms to 'fossil molecules' at least 2.7 billion years back in time in geological sediments.
The rings in hopanoids have chair-chair-chair-chair-chair conformations in comparison to four-ringed sterols, which have chair-boat-chair-boat-open conformations. The resulting ring structures are planar, rigid and hydrophobic with a length corresponding to half the thickness of a membrane bilayer. In general, the molecules are amphiphilic, and in that they modulate the fluidity and the permeability of phospholipid membranes, they have been called ‘sterol surrogates’.
The hopanoids are highly diverse in structure, and they are conveniently grouped into two classes. The first comprises simple hopanoids with a C30 quasi-planar polycyclic hopane skeleton, of which the simplest is diploptene or hop-22(29)-ene, usually found with diplopterol or hopan-22-ol. The second class consists of hopanoids with a polyfunctionalized side chain attached by a carbon-carbon bond, usually derived from ribose, to generate C35 molecules.
Perhaps the most abundant hopanoid in living organisms is tetrahydroxybacteriohopane (bacteriohopane-32,33,34,35-tetrol), i.e. with a distinctive five-carbon terminal side-chain linked by a carbon-carbon bond to the isopropyl group of the hopane framework and with four hydroxyl groups. The polar nature of this moiety changes the physical properties and thence the potential functions in membranes dramatically. Many forms of this are known in which the terminal hydroxyl (on C35) group is substituted, for example with a glycosidic or ether linkage to glucosamine, adenosine or ribose. In some species, the polyhydroxy side-chain can be O- or N-acylated, and Frankia species contain the phenylacetate monoester of tetrahydroxybacteriohopane. Further hydroxylated forms are known, and penta- and hexaols and amino-polyols occur in some genera. Some of these have additional methyl groups (C2β, C2α or C3β), with stereochemical isomers adding to the variability, and for example, 2‑methylbacteriohopanepolyols are common in cyanobacteria and α-proteobacteria, especially those of marine species, but are rarely encountered in other phyla. Hopanoids with double bonds in the ring at C6 and/or C11 have also been characterized. The more complex of these are sometimes referred to as ‘composite structures’, and they are seen as useful taxonomic markers for particular bacterial genera. Some hopanoids are so tightly bound within organisms that they are not easily extracted for structural analysis, so it is likely that many further types will eventually be characterized.
Hopanoids are most abundant in aerobic bacteria (methanotrophs, heterotrophs and cyanobacteria), but they also occur in some anaerobic bacteria, but not in Archaea and rarely in eukaryotes. In particular, they occur in a number of dinitrogen-fixing bacteria but not in all; most species in the genus Azotobacter produce hopanoids, but A. chroococcum does not. In most instances, the hopanoid content of prokaryote cells is comparable to that of sterols in eukaryotic cells.
The lipid A spanning the whole of the outer membrane of a Bradyrhizobium strain (slow-growing rhizobia) has at least one molecule of carboxyl-bacteriohopanediol or its 2‑methyl derivative linked covalently to the (ω-1)-hydroxy group of a very-long-chain fatty acid; its function may be to reinforce the stability and rigidity of the outer membrane of the organism. In addition, the inner leaflet contains free hopanoids that permit a higher ordering and strengthening of the cell envelope.
A few higher plants and some ferns, mosses and fungi contain hopanoids, although these lack the complex side-chains and often have an oxygen atom at C3. For example, the rare plant metabolite hopanon from dammar resin has the diplopterol structure but with an additional ketone group on C3. Although their function in eukaryotes is not known, it is presumed to be analogous to that of sterols in membranes. Those hopanoids reported from sponges probably originate in symbiotic or ingested microorganisms.
2. Biochemistry and Function
Biosynthesis: As with sterols, squalene is a primary precursor for the biosynthesis of the hopane skeleton. Biosynthesis of isopentenyl pyrophosphate and dimethylallyl pyrophosphate, the intermediates in squalene biosynthesis, is either via the mevalonate pathway or via the ‘non-mevalonate’ or ‘2C-methyl-D-erythritol-4-phosphate (MEP)’ pathway. Indeed, it was anomalies in the results of biosynthetic studies with hopanoids that lead to the elucidation of the MEP pathway (see our web pages dealing with biosynthesis of cholesterol and plant sterols, respectively, for details). Also, the step from presqualene diphosphate to squalene differs in bacteria in that it proceeds via a hydroxy intermediate.
A further important distinction is that hopanoid biosynthesis does not proceed via 2,3‑epoxysqualene, but rather squalene per se undergoes cyclization without migration of the methyl groups to produce C30 diploptene or diplopterol in a one step oxygen-independent process (c.f. the nineteen-enzymatic step conversion of squalene into 24-ethylsterols in higher plants that requires 12 molecules of dioxygen). Therefore, bacterial hopanoids lack the 3β‑hydroxyl group of sterols.
Synthesis occurs in a single concerted reaction by a squalene-hopene cyclase that is quite distinct from the squalene epoxide cyclase. From studies of the crystal structure of the former, it is suggested that the active site is located in a large central cavity of a size and shape to bind squalene in the necessary conformation. The cavity is surrounded by loops containing aromatic residues, which may stabilize the putative ionic intermediates. It is believed that cyclization begins with a reaction in which a carbon-carbon double bond is protonated via an aspartate residue at the top of the cavity. Then, rings A and probably B are formed in a concerted manner before rings C and D are fashioned in ring closure reactions. Finally, the ring E is formed and the carbocation at C-22 is deprotonated to form hopene (or reacts with the elements of water to form diplopterol). Like sterol biosynthesis, it is one of the most complex single-step processes known to biochemistry, with the formation of five ring structures, modification of thirteen covalent bonds and the generation of nine new stereochemical centres, all under precise enzymatic control.
Adenosylhopane (see partial structure above) is the first intermediate produced during the biosynthesis of the side chains of bacteriohopanepolyols by an enzyme designated HpnH, a radical S-adenosylmethionine (SAM) enzyme, usually encoded in the hopanoid biosynthetic gene cluster that converts diploptene into adenosylhopane in the presence of SAM, flavodoxin, flavodoxin reductase, and NADPH. Subsequently, adenosylhopane is utilized to produce the wide range of products formed in microorganisms by down-stream processing. For example, it can lose the adenyl group to produce ribosylhopane, and the D-ribose moiety can in turn be opened to produce the five-carbon side-chain bacteriohopanetetrol either directly or via formylhopane. The enzymology of these reactions is slowly being revealed. In cyanobacteria and Acidobacteria, addition of methyl groups to form 2β- and 3β-methyl hopanoids by hopanoid methylases occurs after synthesis of the hopanoid rings, with SAM as the methyl donor.
The presence of the 3-oxygen atom in some plant hopanoids, e.g. moretenol and mollugogenol, suggests that they are formed via a 2,3‑epoxysqualene intermediate, although in some species there may have been horizontal transfer of the genes for the squalene-hopene cyclase from bacteria.
Function: Hopanoids are believed to have very similar functions to those of sterols in the membranes of animals and plants in that they can intercalate into phospholipid bilayers and modulate the fluidity of membranes by interacting with their complex lipid components. Physical chemical studies with diplopterol and model membranes have shown that it induces phase segregation and increases lipid compaction, while retaining membrane fluidity and compressibility in a similar manner to sterols such as cholesterol, stigmasterol and ergosterol. Also, hopanoids are important in decreasing membrane permeability, including the diffusion of oxygen, which may facilitate aerobic nitrogen fixation in the ocean by protecting the extremely O2-sensitive nitrogenase enzyme in cyanobacteria.
They also enable adaptation to stress caused by extreme environmental conditions, including high temperatures, low pH, detergents and antibiotics, and while hopanoids usually comprise around 1 to 5% of the total lipids in cells, the proportion can rise appreciably in response to stress. For example, the thermophile Bacillus acidocaldarius synthesises roughly seven times more hopanoids at 65°C than at 60°C, presumably to counteract the increased fluidity of the lipid portion of the membrane and thereby reinforce it. Resting cells (akinetes) of cyanobacteria produce ten times as much hopanoids as vegetative cells, i.e. when they are stressed by desiccation or exposure to prolonged cold. However, hopanoids may differ from sterols in their ability to direct vesicle formation.
Molecular dynamics simulations suggest that diploptene is located between the two leaflets of a membrane bilayer, partitioning to the midplane, where it may decrease membrane permeability by providing an added layer of protection to the cell. In contrast, the extended hopanoid bacteriohopanetetrol in membranes is believed to adopt an upright orientation inside the lipid bilayer, where it can interact with other membrane lipids to condense the membrane. By increasing the Van der Waals forces between lipids, they limit the penetration of small molecules. For example, in Frankia species, most of the lipids in the membrane barrier that prevents diffusion of oxygen into the nitrogen-fixing cells are hopanoids; bacteriohopanetetrol phenylacetate monoester is specific to the vesicle envelope. Hopanoids have been located in the plasma membrane and outer membranes of Gram-negative bacteria, and in the outer membrane and thylakoid membrane of cyanobacteria, where they interact with glycolipids in bacterial outer membranes to form a highly ordered bilayer in a manner analogous to the interaction of sterols with sphingolipids in forming raft-like microdomains in eukaryotic plasma membranes. As hopanoids have a key role in the structure and function of pentameric ligand-gated ion channels in membranes in prokaryotes, it has been suggested that these ion channels and sterols may have co-evolved in eukaryotes.
With cultures of single species, bacteriohopanepolyols have only been detected in cyanobacteria capable of nitrogen fixation, implicating them as markers for diazotrophy in the oceans. Similarly in plants, hopanoids of Bradyrhizobium spp. appear to be involved in legume–rhizobia root nodule symbioses, although there is no known mechanistic link between hopanoid production and nitrogen fixation.
It is apparent that hopanoids are essential for growth in many if not all hopanoid-producing organisms, as inhibition of hopanoid biosynthesis can limit their growth markedly and selectively in comparison with other bacteria. While some species appear to grow when hopanoid synthesis is inhibited, they are much less resistant to environmental challenges. Hopanoid biosynthesis is therefore seen as a potential target to fight pathogenic multidrug resistance.
As the pentacyclic ring structure of hopanoids is very stable and not readily degraded, geochemists tend to look upon them as molecular fossils ('geohopanoids' or 'homohopanoids') that serve as biological markers for particular organisms in geological formations from recent sediments to petroleum deposits and rocks (and potentially even for extra-terrestrial life). Different bacterial genera possess characteristic hopanoid distributions, which may point to the origins of the organisms producing the deposits. However, 2α‑methylhopanes that were thought to be unique constituents of photosynthetic cyanobacteria and found in 2.7 billion year old shales in Australia, i.e. from a period long before the atmosphere was oxidizing, are now known to be produced by proteobacteria and acidobacteria. Rather, current thinking appears to be that they may be indicators of an origin in a specific environmental niche, i.e. closely packed microbial communities or microbial mats such as those found in hot springs. Similarly, 3-methyl hopanoids can no longer be used as biomarkers for aerobic methanotrophs.
Although they are highly resistant, hopanoids are not immune to change in the geological environment over millions of years, and stereomutation, reduction and defunctionalization occur to produce hopanols and hopanoic acids, for example, commonly with 28 to 35 carbon atoms; these processes are termed 'diagenesis'. For example, the concentration of 25,28-bisnorgammacerane relative to steranes in key geological deposits is treated as a biomarker to trace the change from predominantly eukaryotic life forms to bacterial and back again during the epoch known as the "snowball earth". The pattern of the various products can have indicative value in oil exploration, so their analysis is of great practical importance. Hopanoids are sometimes stated to be the most abundant lipids on earth, and in one estimate, their mass in sedimentary rocks and oil reservoirs is said to be ~1012 tons, an amount comparable to the total mass of carbon in all living organisms!
4. Tetrahymanol and Related Lipids
A number of pentacyclic compounds related to the hopanoids are known that are derived from the gammacerane skeleton in which the E-ring is six-membered (5-six-membered rings in total). Of these, the most important is tetrahymanol (gammaceran-21α-ol), which was first isolated from the ciliate protozoan Tetrahymena pyriformis. It was subsequently detected in several other eukaryotic organisms that inhabit oxygen-poor environments, including fungi, protozoa and even ferns, before it was found in other ciliates and increasingly in anaerobic bacteria, such as the purple non-sulfur bacterium Rhodopseudomonas palustris.
Various structural variants have been found including 20α-methyltetrahymanol, 2β-methyltetrahymanol and 2β,20α‑dimethyltetrahymanol, which occur with tetrahymanol per se and various hopanoids in the nitrogen-fixing, symbiotic root-nodule forming bacterium Bradyrhizobium japonicum.
Like the hopanoids, tetrahymanol is formed by a squalene-tetrahymanol cyclase, with the nature of the E-ring depending on the orientation of the terminal methyl groups during the final cyclization step. It is believed that genes encoding the squalene-tetrahymanol cyclase have been transferred laterally between eukaryotes. When sterols are added to cultures of Tetrahymena pyriformis, synthesis of tetrahymanol is inhibited completely, suggesting that sterols and tetrahymanol have similar functions in this organism, i.e. like the hopanoids, tetrahymanol is a sterol surrogate. Bacteria produce tetrahymanol by a mechanism very different from the eukaryotic system. Instead, they couple the cyclization of squalene to a hopene molecule by squalene-hopene cyclase followed by a ring expansion involving tetrahymanol synthase.
Gammacerane structures have been found in sediments and other geological formations, together with the homohopanoids, where they were initially believed to be useful geochemical markers for ciliate protozoa, although it is now recognized that they could also originate in bacteria.
Sporulenes are C35-terpenoid hydrocarbons with an unusual pentacyclic structure produced by Bacillus subtilis during sporulation that increase their resistance to reactive oxygen species (akin to bacteriohopanetetrol phenylacetate monoester in Frankia species). Rather than squalene, they are believed to originate from a linear heptaprenyl unit with all head-to-tail isoprene linkages via an enzyme related to the squalene synthase.
Hopanoids and those derived from the bacteriohopanetetrols especially require special extraction methods because of their high polarity. Indeed some are so tightly bound that their presence in many organisms may have gone undetected. At one time, the molecular structures were simplified by removal of part of the side-chain by chemical means to facilitate analysis, so much information was lost. This no longer appears to be necessary, and HPLC allied to modern mass spectrometric methods involving atmospheric-pressure chemical ionization or electrospray ionization appears to be the way forward.
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|Credits/disclaimer||Updated: September 20th, 2021||Author: William W. Christie|