What were to happen if RuBisCO didn't exist?

[Irving4Prez]

RuBisCO, is an enzyme involved in the Calvin Cycle.The enzyme RuBisCO catalyses the carboxylation of ribulose-1,5-bisphosphate, a 5-carbon compound, by carbon dioxide. If however, RuBisCO didn't exist, would this step still be able proceed but at a much slower rate or would it just not occur?

Secondly, through the basis of my first question, how will this affect humans? Cyanobacteria converted the early reducing atmosphere into an oxidising one, but even they require RuBisCO to carry out photosynthesis. If RuBisCO didn't exist on our planet, would we still be able to sustain life?

Thanks in advance

[alondouek]

Here's a fairly in-depth look at the biochemistry involved in the Light-Independent stage of photosynthesis, involving RuBisCO from a first-year uni perspective.

We define the function of enzymes as lowering the required activation energy for a reaction to occur. Hence, it is reasonable to state that if RuBisCO didn't exist, the same reaction could take place given ideal conditions (i.e. in solution of correct pH and temperature, with the adequate amounts of substrate). However, as the activation energy required for this reaction to occur is quite high, the reaction would only occur extremely, extremely slowly - or even not at all without an adequate input of environmental energy.

This allows for the answer to your second question. If you look at evolutionary history, you'll see that complex organisms such as animals are fairly recent. Millions of years ago, the most complex life-forms that existed were either unicellular or basic multicellular organisms who relied on photosynthesis to obtain their chemical energy. Assuming they used a similar or identical mode of photosynthesis as seen in modern plants (and some photosynthetic bacteria), without RuBisCO these animals would have become extinct in a very short time - they could not obtain the necessary chemical energy to survive. Given that these organisms are likely to have evolved over time to the more complex organisms we see today, we might not be alive if RuBisCO had never existed.

However, given what we are fairly sure about regarding evolution, there would have likely been some variation in the ancestor species that would have allowed for the adaptation and evolution of other means of photosynthesis/other forms of chemical energy harvesting. Evolution is nature's contingency!

[Irving4Prez]

On that basis, would it be remiss to say that if somehow RuBisCO didn't exist as from now, all organisms (even marine and terrestrial) would eventually die? How long will the amount of oxygen present in the atmosphere last us before there'd be insufficient amounts to be utilised by cellular respiration?

[alondouek]

On that basis, would it be remiss to say that if somehow RuBisCO didn't exist as from now, all organisms (even marine and terrestrial) would eventually die? How long will the amount of oxygen present in the atmosphere last us before there'd be insufficient amounts to be utilised by cellular respiration?

I guess the answer to your first question depends on the adaptability of modern photosynthetic organisms. Let's look at a couple of simple flowchart, examining two situations:

Situation 1: Some photosynthetic organisms can adapt to an alternative form of photosynthesis, without the need of the RuBisCO enzyme

No RuBisCO No complete photosynthesis in RuBisCo-requiring organisms Over a short period of time, these organisms die out, and only those organisms that have the variation that allows them to photosynthesise by alternate means can survive and reproduce (in evolutionary terms, we call this Survival of the Fittest) In the time it takes for many generations of these alternative-photosynthesising organisms to become populous, other oxygen-requiring organisms would probably be selected against due to lack of conversion of significant amounts of CO2 to O2 Due to potential variation within species, only those organisms that could (hypothetically) perform aerobic respiration in an extremely efficient manner with little O2 input can survive and reproduce.

Also, this assumes that all existing photosynthesis relies on the same chemical pathway. This may be correct (and it probably is), but I don't know that. We'll assume that's true for theory's sake.

Situation 2: The non-existence of RuBisCO is sudden, and organisms lack adaptability

No RuBisCO No required adaptability in photosynthetic organisms These organisms die out We also die out, as we can't perform aerobic respiration.

As for the second question; this is a little tricky, and I definitely can't give you a direct answer.
The effective volume of the Earth's atmosphere is about 4.2 billion cubic kilometres. 21% of that is oxygen, so .

I have absolutely NO idea what the collective rate of oxygen use for aerobic respiration by all organisms in the entire world is, but you would divide the above value by the rate of oxygen use to get an (extremely) approximate value of how long we'd have before the oxygen would run out with no photosynthesis. This also doesn't take into account any loss of O2 by any process other than aerobic respiration, so - again - it would be really, really approximate.

But, this all goes to show just how freaking important one single enzyme is in the context of every other organism. Really makes you think about how fragile everything around us really is. I hope I answered your questions

[Yacoubb]

(1) In the Calvin Cycle, carbon dioxide reacts with the 5-Carbon compound ribulose bi-phosphate (RuBP) to form the unstable 6-carbon compound later split into two PGA 3-C compounds. The formation of this unstable 6-carbon compound is catalysed by RuBisCO, and enzyme that increases the rate of this reaction by lowering activation energy. Thus, it can be concluded that the Calvin Benson Cycle would be occuring at a rate too slow for sufficient glucose production per unit time.

(2) Heterotrophic organisms ultimately depend on photosynthetic organisms or parts of them to obtain glucose, the organic carbohydrate that is involved in ATP production. If an autotroph cannot synthesise glucose at a rate sufficient to meet the needs of the plant and consumer organisms, the cycle will be disrupted and heterotrophs will be forced to obtain dietary requirements in amounts insufficient for the effective functioning of the organism.

[Irving4Prez]

Thanks so much alondouek, especially for the freakish calculations of the amount of oxygen within our atmosphere