Humans like sugar.
In this, we are not alone: the labyrinthine web of interconnected chemical reactions that support life are centered on the breakdown of varied sugars.
Bacteria like sugar; slime molds like sugar; birds, bees, and most anything that walks, hops, swims or crawls likes sugar. The majority of sugar on Earth comes from that most famous chemical process, photosynthesis.
Scientists at Cornell have recently published a remarkable paper in which they appear to have taken a long-awaited stride toward optimizing Earth’s favorite metabolic pathway. The central enzyme in the sugar-making portion of photosynthesis is called Rubisco. Rubisco is responsible for grabbing CO2 out of the atmosphere and affixing it to carbon structures that will later be processed into usable sugars.
“Once CO2 enters,” UW Molecular Biology professor Mark Gomelsky said, “it’s added to a five-carbon precursor to make a six-carbon intermediate, which is then broken down to a couple of three-carbon molecules. And that’s basically the key reaction that sustains life on Earth.”
Dr. Gomelsky further describes Rubisco as “the most abundant enzyme on this planet.” Given such abundance and importance, one might be excused for thinking that Rubisco must be an awfully good enzyme. The reality, however, is quite different.
“The reason it’s so abundant is because it’s very inefficient,” said Dr. Gomelsky. As it turns out, Rubisco is not an especially selective enzyme. It’s supposed to snag molecules of CO2 and incorporate them into fledgling sugars, but it has an unfortunate predilection for grabbing oxygen molecules instead. When this happens, no sugar is produced, and all of the work that led up to the Rubisco step becomes wasted effort. Plants typically correct for this by producing a lot of Rubisco.
“The reason this is a problem,” said Dr. Gomelsky, “is that [Rubisco] has not evolved – in plants – to be particularly sensitive to CO2. So it’s slow and it’s inefficient.”
Rubisco isn’t exclusive to plants, though: it also exists in bacteria.
“In bacteria, you have many more Rubisco types,” explained Dr. Gomelsky, “and of course they have been evolving on their own. And many of these bacterial types are much more efficient. They have higher affinity for CO2.”
The shortcomings of Rubisco in plants have been known for a long time, and scientists have worked for decades with the aim of addressing the enzyme’s issues. The paper published in Nature on Sept. 17 claims to have reached a new milestone in pursuit of this goal.
Cornell researchers altered a tobacco plant’s genome, knocking out the genes for its native Rubisco. They then took the Rubisco genes from Synechococcus elongates—a photosynthetic cyanobacteria—and inserted them into the chloroplast of the tobacco cells. The tobacco plants were then allowed to grow, and carefully monitored. The researchers found that their altered plants were “photosynthetically competent, supporting autotrophic growth” while using exclusively bacterial Rubisco.
This marks the first experiment in which a plant Rubisco has been successfully replaced, and has the potential to lead to massive advances in agriculture. The ability to fine-tune and swap-out Rubiscos would change the face not just of food production, but any industry reliant on plant products.
That dream is still a long way off: the tobacco plants in the study were actually less efficient overall than those relying on their native Rubiscos. The real victory was simply that the plants survived their transformation. Nonetheless, the success of this experiment represents a true scientific first, and a giant leap in the right direction.
