Tuesday, November 29, 2022

More utter metabolic nonsense

When will researchers stop writing utter nonsense about metabolism? I was reading this review:

Molecular Metabolism
12 November 2022, 101635
Nitin Patil Orla Howe, Paul Cahill and Hugh J.Byrne

Monitoring and modelling the dynamics of the cellular glycolysis pathway: A review and future perspectives and spotted this sentence in the text:

"Alternately, the activity of rate-limiting glycolytic enzymes can be determined by quantifying their catalytic products in vivo [115]. "

I thought, that can't be true, it's not possible to ascertain rate limitingness just from the products. So I checked out the citation they used:

Methods in Enzymology
Volume 542, 2014, Pages 91-114
Chapter Five - Techniques to Monitor Glycolysis
Tara TeSlaa and Michael A.Teitell

Well I was in for a treat, so many illogical statements in one paper. Here is one such sentence:

"While the activity of any single glycolytic enzyme is not a proxy for carbon flux through the entire pathway, specific enzymes limit the rate of glycolysis, and therefore, their activities control the maximum possible flux. Hexokinase, phosphofructokinase, and PK are the main rate-limiting enzymes in glycolysis."
v So authors are mixing up the Vmax for rate-limitation as if enzymes are even running at their Vmax. These enzymes, using the more classical definition of rate-limitingness, are not rate-limiting other than HK, but that depends on the organism and conditions. The reason they aren't rate-limiting is that they are regulated, which means any changes to their activity results in no change to the pathway flux. Not only that, it turns out that for the mentioned enzymes, particularly HK and PK, don't even have low Vmax's so even if they were running at Vmax they wouldn't be limiting the flux anyway. PFK is also not that low, since PDC and ENO have lower Vmaxs (this is in yeast). The point is there is no necessary correlation between rate-limitingness (however they define it) and an enzyme's Vmax. There are other reasons at play for determining a Vmax.

Here is another one:

"while determining the activity of rate-limiting glycolytic enzymes can provide insights into points of metabolic regulation."

There is no reason why rate-limiting steps are regulated, quite the opposite in fact. I'm not sure how much experimental data, theory, or computational research need to be done to convince them this isn't true. I never did find any reference to my orginal quesiton in the 2014 paper. Me thinks the orginal authors didn't read the 2014 review. There is simply no way to tell if a step is rate-limiting or not just from its products. The authors also have a modeling section where they state:

"Ordinary and partial differential equation can be used for both deterministic and stochastic modelling."

what were they thinking?

And another:

Mass action kinetics, rate law (Michaelis Menten models, Hill kinetics),

MM and Hill models don't follow clasical mass-action kinetics, they show fractional kinetic orders.

The authors never mention Metabolic Control Analysis at all, which is a primary language for talking objectively about metabolic control and regualation.

We've obviously gone very wrong somewhere in our education and training of systems biology researchers.

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