Long time no write. This post will soon decipher the genes needed to turn normal rice into Golden Rice. Golden Rice produces Beta-Carotene, which is a direct precursor to VitaminA. 20.000 people a day die because of vitamin A deficiency (which is a horrbile death btw, because you become blind first). Why not produce VitaminA? First the human body can produce VitA from Beta-carotene. It would be adding an unneccesary gene to the construct. Secondly, and more importantly, you cannot overdose beta-carotene. If you eat too much vitA you can get intoxication – if ou eat too much beta-carotene your body disposes of it with the urine.
Here is the paper describing the newly engineered pathway.
These are the three genes we need – Psy, Crtl, Lyc.
figure from the open access paper from The American Society for Nutritional Sciences
Plant cells already contain GGPP (Geranylgeranyl pyrophosphate) – it is a pretty important molecule that is the basis of many other pathways. So as you see we need to add a Phytoene Synthase gene which codes for the protein enzyme Phytoene Synthase which turns the plant cells’ GGPP into Phytoene. Usually plants employ two enzymes to go from Phytoene to Lycopene (Lycopene is e.g. one of the healthy compounds in tomatoes) but here we take a shortcut: a characterized bacterial gene Crtl can do this in one step. Last step is an enzyme that converts Lycopene into Beta-Carotene, hence we need a lycopene cyclase.
Each enzyme does one job only – enzymes work like key lock systems. The precursor molecules fit into the cavity or “active center” in the protein enzyme and the enzyme catalyzes their specific reaction. Whatever the media fearmongered you – there are no unforseen side-effects in this regard. Unlike chemical synthesis (where catalysis can e.g. be done at 120°C in sulfuric acid) the enzyme is VERY specific.
A little not at the end of this post: Our far-far ancestors’ bodies were actually able to produce vitamin A themselves.We still have the enzymes in our chromosomal DNA – but there was a mutation that rendered our vitaminA biosynthesis proteins useless. Unlike in horror movies, these mutations don’t made them zombies though. The enzyme’s structure was just destroyed, and the precursor molecule wouldn’t fit in anymore. So speaking, human cells have lots of “corpse-proteins” in them that lost their function but are still expressed. The cell doesn’t know what function proteins have so they keep producing even the mutated forms. Obviosly this wasn’t of harm to us – unless we didn’t eat enough vitamin A.