The secondary metabolism of pineberry strawberries

Thursday 16 May 2019

The secondary metabolism of pineberry strawberries

For an upcoming open-day we will extract DNA from strawberries. For this I made a slide that explains how DNA mutations lead to protein variants, than in turn lead to different phenotypes (redness in the strawberry's case). In doing this, I got fascinated by a strawberry cultivar called "Pineberry". But not because it is unpigmented, but because the reviews online say it is bland, which means that a rather early enzyme is missing resulting in a unpigmented phenotype and a bland phenotype.


Nobody has studies pineberry metabolics/genetics, but one can guess what is going on.
Strawberry pigments are derived from two main chromophores (with a variety of glycosyl handles), both anthocyanins. Cyanidin is a dark red pigment, which is oxidised by a cytochrome P450 enzyme to pelargonidin, which is has a brighter red. (I found a nice graph of the spectra in a paper: here).
Simple model of possible differences in three cultivars. As discussed below, naringenin is likely not produced in pineberries, but the conversion of an earlier metabolite is the bottleneck.
Pineberry strawberries, lack pigmentatione entirely in the flesh —the seeds seem fine so, it's a gene regulation loss-of-function somewhere and not a structural loss-of-function. To be unpigmented these strawberries don't produce cyanidin and must stop at a precursor. Cyanidin is made by reducing (in a biochemically horrendous way) naringenin, which is a flavonone, so is colourless —as the planar middle ring does not have resonance structures. If the enzyme that converts naringenin to cyandin, flavonone 3-hydroxylase, were not expressed, you would indeed have unpigmented fruit, i.e. white with maybe a touch of yellow from the cinnamic acids (no hydroxyls), coumaric acids (one) and caffeic acid (two), namely your usual plant phenolic acids (note that they have nothing to do with the plants they are named after). But this is not what is going on!


However, pigmentation should not affect taste. If anything the lack of anthocyanins should make it less bitter —not that strawberries are due to their sugar content. Yet, all the reviews online say it tastes bland. The name pineberry is because it tastes a bit like pineapple. This means the metabolic disruption affects both the pigmentation and some aroma compounds, so it is very upstream. So naringenin is most likely not made.

Strawberry flavour is caused by a panel of aromatic compounds. Several of which have strong strawberry smells by themselves, albeit with an artificial quality as is often found with aromatic compounds in isolation, e.g. isoamyl acetate is a banana aroma compound, but in isolation is causes the quintessential Nesquik fake banana flavour. Ethyl methylphenylglycidate is the compound used to imitate strawberries, while other distintive aroma compounds include benzyl acetate (strawberry/jasmin), p-Anisaldehyde (floral/aniseed), furaneol (strawberry/pineapple) along with the usual fruity short fatty acid acetate esters. Furaneol is not produced from cinnamic/cumaric acid, while the other are, and is a compound described as smelling like pineapple. As pineberries taste like pineapple, I would bet they can still make furaneol fine, while the levels of the other compounds are affected. The pigments, as mentioned, are anothocyanins, which are derived from naringenin, which is formed by condensing a polyketide that has a cinnamic acid component.

Benzyl acetate is also produced from cinnamic acid. Actually, I mentioned cinnamoyl-CoA in the whatif/Breaking Bad/synbio post about methamphetamine biosynthesis—back when Breaking Bad still aired...
The ethyl methylphenylglycidate looks complicated, but if you look at it, you can see that the backbone is an ethyl cinnamate that has been given a methyl group. It is rather unusual and I did not find any biosynthetic pathways in a quick glance, but to get an epoxyl you need an allylic alcohol so my guess is that it was not methylated but was hydroxylated first then an allyl group added somehow (decarboxylation?) or via radical SAM dependent methylation. Nevertheless, the cinnamic acid is the key part that is possibly missing.

Cinnamic acid and coumaric acid are generally produced by a dedicated phenylalanine amonia-lyase and tyrosine ammonia-lyase, but often coumaric acid is produced by 4-cinnamic acid hydroxylase from cinnamic acid. So the best candidate enzyme that could be under expressed is phenylalanine amonia-lyase. This would also explain why they are so white.

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