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September 23

If raffinose can be hydrolysed to galactose (and fructose and glucose), why doesn't raffinose induce the GAL1 promoter?

So many studies use differential galactose/raffinose media where galactose induces the GAL1 promoter but raffinose does not. My instructor is unable to explain why raffinose does not activate the GAL1 promoter if it can be hydrolyzed to galactose inside the cell. Is GAL1 activation dependent on extracellular galactose binding to a transmembrane receptor, which is insensitive to cytosolic galactose? Yanping Nora Soong (talk) 21:53, 23 September 2019 (UTC)[reply]

I happen to know an expert on galactose metabolism. His answer to your question, Basically SUC2 in yeast, the sucrose invertase hydrolyzes raffinose to melibiose and fructose. Some yeast have the alpha-galactosidase (MEL1), which can break down melibiose but it seems to only be found in a few strains. In fact if raffinose and MEL1 are around mel1 can act on the raff to give D-galactose and sucrose, which can further be broken into 1 glucose and 1 fructose molecule. And now nothing is wasted. This seems more efficient but its not what they do, and since many strains don't even have MEL1 they can't. It's so confusing, everything hates gal. For myself, I'd say I've recently consigned myself to the realization that organisms preferring to ferment under aerobic conditions simply did not evolve to be efficient. They evolved to grow as fast as possible, and this is necessarily wasteful. Someguy1221 (talk) 02:40, 24 September 2019 (UTC)[reply]

Thanks so much!! Yanping Nora Soong (talk) 04:32, 24 September 2019 (UTC)[reply]