Project Amaze!

In chapter 17, during Ryland’s attempt to focus on teaching science to the involved-and-very-open-about-it pair of DuBois and Shapiro, we get a brief mention of the fact that Astrophage have a Krebs cycle: “It’s identical to what we find in Earth mitochondria, but with one additional step” 17.108. Much later, in 29.220, Ryland states that the Taumoeba also have a Krebs cycle, though we are provided no further details.

The Krebs cycle is also sometimes called the tricarboxylic acid (TCA) cycle or the citric acid cycle. It serves as an important metabolic pathway in cellular respiration, using the pyruvate molecules produced in glycolysis to generate the cellular “energy currency” adenosine triphosphate (ATP) as well as the molecules FADH₂ and NADH which can later be used to generate much more ATP through the electron transport chain (ETC). If all these acronyms have you on the edge of your seat, you’re not alone.

At this time, we will not go into greater detail on the steps of the standard Krebs cycle (in fact, there is a joke that the true Krebs cycle is the endless loop of memorizing and then subsequently forgetting the steps of said cycle). The Krebs cycle of Astrophage is said to have an extra step, or a “variant—” (here, Ryland interrupts himself due to curiosity about the nature of Dubois and Shapiro’s relationship and never completes the statement) 17.108 and in fact, variations on the Krebs cycle do exist in terrestrial life.

There are shunts and bypasses that provide alternatives to the classic Krebs cycle, including the glyoxylate shunt, the GABA shunt, and the α-Ketoglutarate bypass. A team of scientists at the Sloan Kettering Institute discovered what one of them described as “a complete alternative to the canonical TCA cycle” which occurs partially inside and partially outside of the mitochondria.

Some bacteria and archaea even run the Krebs cycle in reverse to generate carbon compounds, a pathway fittingly called the reverse Krebs cycle.

Thanks to Shapiro and DuBois’s shenanigans, we unfortunately do not get to hear more of Ryland’s lecture about the extra step or variant in the Astrophage Krebs cycle. Honestly, who can blame him?