Rubisco and Evolution

By: Matt

Ribulose-1,5-bisphosphate carboxylase oxygenase; more commonly known as “Rubisco” sounds like something you might find in the cookie aisle at the grocery store. In fact you find Rubisco almost everywhere you look. It is the most common protein in green plants, and is by and large responsible for life as we know it. Many of us have seen Chuck Missler’s misguided video “Peanut Butter-the Atheist’s Nightmare”. Missler argues that if evolutionary theory is correct, then occasionally life should spontaneously occur in a jar of peanut butter. He is missing the boat on more than one issue. First evolutionary theory doesn’t address the origin of life; it states only that over time the frequency of genes in a population will change. Secondly it’s extremely unlikely that a jar of Peter Pan® is going to be exposed to an atmosphere similar to earth’s 3 billion years ago. He has set up a straw man, hoping that an uneducated public wouldn’t notice, and then tried to knock him down. In this essay I would like to argue that Rubisco is, in effect, “the creationist’s nightmare”. Rather than set up an imaginary straw argument I will ask a legitimate question and hope for an honest discussion.

Like all proteins Rubicso has a function, its job is to grab carbon from CO2 early on in photosynthesis and make place it onto a 5 carbon sugar (Ribulose biphosphate) in order to create a pair of 3 carbon sugars. There are several more steps involved before the plant has converted Ribulose Biphosphate into the much more familiar glucose, but all that is for another day. If you really want to understand how it all works Google “Calvin Cycle” Figure 1 shows where Rubisco fits in the Calvin cycle.

But here’s the rub. Rubisco is painfully ineffective. Virtually all life as we know it depends on green plants ability to fix carbon from the atmosphere, but the enzyme that does it is barely functional. Unlike most enzymes, which do their task and turn over in hundreds or thousands of times every second, Rubisco can fix only three carbon atoms/second. Worse, it isn’t very selective about what molecules it grabs. 27% of the time (more than a quarter of all reactions) our beknighted enzyme grabs Oxygen instead of Carbon. When this happens the oxygenated Ribulose Biphosphate molecule travels along the Calvin cycle to its completion, thus wasting not only Rubisco’s efforts, but those of every other enzyme along the way, and then a further set of enzymes that are required to break down this faulty molecule. The term for this miscue is “photorespiration” and if you were a plant you’d really hate it. The reason Rubisco is so common in plants is that it is so ineffective. If it was as speedy and precise as the other enzymes in the photosynthetic pathway its numbers could be reduced by a factor of at least 100.

So how did this come about? Why are plants saddled with this weak enzyme? Of course we can’t know for sure, as gene sequences from the pre-Cambrian are not preserved, but it’s a reasonable guess that the Rubisco was one of the very first enzymes created by the earliest organisms on our planet. There are over 400 varieties of Rubisco, indicating that it has had plenty of time to evolve, and identifying the variety of Rubisco in a leaf is one way (albeit kind of an unnecessary one) to identify the species. On the way to becoming photosynthetic, a necessary early step would have been simply capturing carbon atoms and using them as building blocks. It is easy to imagine at the time this happened our atmosphere didn’t have any Oxygen, so photorespiration wouldn’t have been an issue. Unfortunately for plants evolution isn’t a magic bullet, and even though we can imagine better ways to fix carbon, none have evolved. It’s not for lack of trying. Several plants have modified the system somewhat through either the C4 pathway or the Crassulacean acid metabolism (CAM) pathway. Neither eliminated the need for Rubisco, but both found ways of having the enzyme do its work in a region of the leaf with either less Oxygen or more CO2.

I believe that if a thoughtful creator was involved the single most important enzyme for the pinnacle of His creation (presumably that’s us humans) would not have an error rate of 27% and a speed orders of magnitude slower than others. Without green plants we don’t exist, and without photosynthesis there are no green plants, and without Rubisco there is no photosynthesis. Clearly this doesn’t prove there is no creator. An idea that can’t be proved is equally hard to disprove, but to my way of thinking this is even more telling than the litany of other questions out there. Ones like: Why do whales have finger bones? How come the Panda’s thumb is made from a different bone than my thumb? Where did the T-Rex’s go? What is does prove is that if there is a creator, the creator was forced to work from a toolbox of available pieces and parts to put together life on our planet. And THAT flies in the face of a good many religious doctrines.

My feeling is that there was no creator involved, but it can’t be proved either way and isn’t arguable. However, “young earth creationism” is wrong on every level, and can be countered by looking at the ages of rocks, the ages of stars, the geology of the Hawaiian Islands and a host of other big ticket items. We can look at the inside of each and every leaf on the planet and see the truth that stares us in the face.