The polyketo ester (Figure 3.25), formed from four acetate units (one acetate starter group and three malonate chain extension units) is capable of being folded in at least two ways, A and B (Figure 3.25). For A, ionization of the a-methylene allows aldol addition on to the carbonyl six carbons distant along the chain, giving the tertiary alcohol. Dehydration occurs as in most chemical aldol reactions, giving the alkene, and enolization follows to attain the stability conferred by the aromatic ring. The thioester bond (to coenzyme A or ACP) is then hydrolysed to produce orsellinic acid. Alternatively, folding of the polyketo ester as in B allows a Claisen reaction to occur, which, although mechanistically analogous to the aldol reaction, is
folding aldol addition
SEnz on to carbonyl o o aldol reaction
SEnz dehydration favoured II H by formation of O O
N^SEnz oq re-formation of carbonyl possible by expulsion of leaving group
enolization favoured by formation of aromatic ring co2h
OH orsellinic acid enolization hydrolysis co2h
OH orsellinic acid
HO ^ OH phloracetophenone enolization favoured by formation of aromatic ring
HO ^ OH phloracetophenone
enolization terminated by expulsion of the thiol leaving group, and direct release from the enzyme. Enolization of the cyclohexatrione produces phloracetophenone. As with fatty acid synthases, the whole sequence of reactions is carried out by an enzyme complex which converts acetyl-CoA and malonyl-CoA into the final product without giving any detectable free intermediates. These enzyme complexes combine polyketide synthase and polyketide cyclase activities and share many structural similarities with fatty acid synthases, including an acyl carrier protein with a phosphopantatheine group, a reactive cysteine residue, and an analogous P-ketoacyl syn-thase activity.
A distinctive feature of an aromatic ring system derived through the acetate pathway is that several of the carbonyl oxygens of the poly-^-keto system are retained in the final product. These end up on alternate carbons around the ring system. Of course, one or more might be used in forming a carbon - carbon bond, as in orsellinic acid. Nevertheless, this oxygenation on alternate carbon atoms, a meta oxygenation pattern, is usually easily recognizable, and points to the biosynthetic origin of the molecule. This meta oxygenation pattern contrasts to that seen on aromatic rings formed via the shikimate pathway (see Chapter 4).
6-methylsalicylic acid (Figure 3.26) is a metabolite of Penicillium patulum, and differs from orsellinic acid by the absence of a phenol group at position 4. It is also derived from acetyl-CoA
and three molecules of malonyl-CoA, and the 'missing' oxygen function is removed during the biosynthesis. Orsellinic acid is not itself deoxy-genated to 6-methylsalicylic acid. The enzyme 6-methylsalicylic acid synthase requires NADPH as cofactor, and removes the oxygen function by reduction of a ketone to an alcohol, followed by a dehydration step (Figure 3.26). Whilst on paper this could be carried out on an eight-carbon intermediate involved in orsellinic acid biosynthesis (Figure 3.25), there is evidence that the reduction/dehydration actually occurs on a six-carbon intermediate as the chain is growing (compare fatty acid biosynthesis, page 36), prior to the final chain extension (Figure 3.26). Aldol condensation, eno-lization, and release from the enzyme then generate 6-methylsalicylic acid. Important evidence for reduction occurring at the C6 stage as shown in Figure 3.26 comes from the formation of tri-acetic acid lactone if NADPH is omitted from the enzymic incubation.
The folding of a polyketide chain can be established by labelling studies, feeding carbon-labelled sodium acetate to the appropriate organism and establishing the position of labelling in the final product by chemical degradation and counting (for the radioactive isotope 14C), or by NMR spectrometry (for the stable isotope 13C). 13C NMR spectrometry is also valuable in establishing the location of intact C2 units derived from feeding 13C2-labelled acetate. This is exemplified in
SEnz enolization then ester formation
NADPH reduction of carbonyl to alcohol dehydration favoured by formation of conjugated system
O SEnz malonyl-CoA
triacetic acid lactone aldol reaction - H2O
SEnz chain extension follows after O O
dehydration / removal of oxygen function
SEnz enolization hydrolysis
refers to intact acetate C2 unit
"OO "SEnz C14 poly-P-keto chain
"OO "SEnz C14 poly-P-keto chain
~OH lecanoric acid
~OH lecanoric acid
Figure 3.27, where alternariol, a metabolite from the mould Alternaria tenuis, can be established to be derived from a single C14 polyketide chain, folded as shown, and then cyclized. Whilst the precise sequence of reactions involved is not known, paper chemistry allows us to formulate the essential features. Two aldol condensations followed by enolization in both rings would give a biphenyl, and lactonization would then lead to alternariol. The oxygenation pattern in alternariol shows alternate oxygens on both aromatic rings, and an acetate origin is readily surmised, even though some oxygens have been used in ring formation processes. The lone methyl 'start-of-chain' is also usually very obvious in acetate-derived compounds, though the carboxyl 'end-of-chain' can often react with convenient hydroxyl functions, which may have arisen through enolization, and lactone or ester functions are thus reasonably common. For example, lecanoric acid is a depside (an ester formed from two phenolic acids) found in lichens and produced from two orsellinic acid molecules (Figure 3.28).
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