Can I take this opportunity to introduce the concept of substrate-level phosphorylation?
Let’s see if cells can really live without ATP synthase. Hmm, how about we read this paper?
Hunt KA, Flynn JM, Naranjo B, Shikhare ID, Gralnick JA. Substrate-level phosphorylation is the primary source of energy conservation during anaerobic respiration of Shewanella oneidensis strain MR-1. J Bacteriol. 2010 Jul;192(13):3345-51. doi: 10.1128/JB.00090-10.
Abstract
It is well established that respiratory organisms use proton motive force to produce ATP via F-type ATP synthase aerobically and that this process may reverse during anaerobiosis to produce proton motive force. Here, we show that Shewanella oneidensis strain MR-1, a nonfermentative, facultative anaerobe known to respire exogenous electron acceptors, generates ATP primarily from substrate-level phosphorylation under anaerobic conditions. Mutant strains lacking ackA (SO2915) and pta (SO2916), genes required for acetate production and a significant portion of substrate-level ATP produced anaerobically, were tested for growth. These mutant strains were unable to grow anaerobically with lactate and fumarate as the electron acceptor, consistent with substrate-level phosphorylation yielding a significant amount of ATP. Mutant strains lacking ackA and pta were also shown to grow slowly using N-acetylglucosamine as the carbon source and fumarate as the electron acceptor, consistent with some ATP generation deriving from the Entner-Doudoroff pathway with this substrate. A deletion strain lacking the sole F-type ATP synthase (SO4746 to SO4754) demonstrated enhanced growth on N-acetylglucosamine and a minor defect with lactate under anaerobic conditions. ATP synthase mutants grown anaerobically on lactate while expressing proteorhodopsin, a light-dependent proton pump, exhibited restored growth when exposed to light, consistent with a proton-pumping role for ATP synthase under anaerobic conditions. Although S. oneidensis requires external electron acceptors to balance redox reactions and is not fermentative, we find that substrate-level phosphorylation is its primary anaerobic energy conservation strategy. Phenotypic characterization of an ackA deletion in Shewanella sp. strain MR-4 and genomic analysis of other sequenced strains suggest that this strategy is a common feature of Shewanella.
The goal of this work was to elucidate the primary source of ATP generation under anaerobic conditions in MR-1. Data presented here support a model of anaerobic metabolism where substrate-level phosphorylation is the primary mechanism for ATP generation and where some amount of the ATP pool is used to generate PMF.
If substrate-level phosphorylation is the primary means of conserving energy anaerobically, a strain lacking the ATP synthase operon (Δ atp ) should remain viable and eliminate the possibility of ATP generation via PMF. The entire ATP synthase operon (SO4746 to SO4754) was deleted to avoid futile cycling of protons or ATP through a partial complex.
And yet it lived, and produced ATP by substrate-level phosphorylation instead.
Well I guess this isn’t the chicken-and-egg paradox we’ve been led to believe. Weird.