The Fate of Glutamine in Human Metabolism. Comparison with Glucose

Genome-scale models of metabolism (GEM) are now used to study how metabolism varies in different physiological conditions or environments. However, the great number of reactions involved in GEM makes it difficult to understand the results obtained in these studies. In order to have a more understandable tool, we develop a reduced metabolic model of central carbon metabolism, C2M2 with 63 reactions, 46 internal metabolites and 3 compartments, taking into account the actual stoichiometry of the reactions, including the stoichiometric role of the cofactors and the irreversibility of some reactions. In order to model OXPHOS functioning, the proton gradient through the inner mitochondrial membrane is represented by two pseudo-metabolites DPH (ΔpH) and DPSI (Δψ). To illustrate the interest of such a reduced model of metabolism in mammalian cell, we used Flux Balance Analysis (FBA), to systematically study all the possible fates of glutamine in central carbon metabolism. Our analysis shows that glutamine can supply carbon sources for cell energy production and can be used as a carbon source to synthesize essential metabolites thus sustaining cell proliferation. We show how C2M2 can also be used to explore the results of more complex metabolic models in comparing our results with those of a medium size model MitoCore.


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In C2M2, the model of OXPHOS functioning is actually based on a proton gradient through the inner 41 mitochondrial membrane, represented here by two pseudo-metabolites DPH (∆pH) and DPSI (∆ψ).

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This allows us to specifically take into account the 'vectorial' protons [12] across the mitochondrial  To illustrate the interest of such a reduced model of metabolism in mammalian cell, we studied 46 the metabolism of glutamine and compared it with the metabolism of glucose. Glutamine is the most 47 abundant amino acid in plasma and has long been recognized to be essential in proliferating cell.

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The synthesis of nucleotide bases (NUC) is represented by a simplification of purine and 83 pyrimidine biosynthesis obtained by averaging the stoichiometry of the different metabolites and 84 cofactors in the metabolic pathway of each nucleotide and taking into account their different amounts 85 in human (30% of A and T or U and 20% of G and C). It should be stressed that nucleotide synthesis 86 necessitates glutamine which is converted to glutamate.

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The synthesis of serine from 3-phosphoglycerate involves 3 steps: a dehydrogenase, a 88 transaminase involving the glutamate/ α-ketoglutarate pair and a phosphatase. These three steps are 89 assembled in one reaction: SERSYNT.

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The

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The synthesis of fatty acids is a major pathway in proliferating cells. It starts with citrate lyase 98 (CL) and is represented in the case of palmitate by the reaction PL1 with the corresponding 99 stoichiometries.

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The respiratory chain is represented by three reactions, RC1 which is the respiratory complex I, 101 RC2 (succinate dehydrogenase or complex II which also belongs to the TCA cycle + fumarase) and 102 RC34 which represents complex III + IV.

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The production of pyruvate (precursor of alanine) from glutamine can follow the production of 167 glutamate and α-ketoglutarate (AKG) entering the "left", -oxidative-part of TCA cycle to produce 168 OAA. From OAA, mitochondrial PEPCK2 produces PEP, which comes out of the mitochondria 169 through citrate and malate cycling (grey arrows on Figure S3a) and generates pyruvate (with PK).

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Aspartate is an amino acid which participates in many reactions, particularly in nucleotides 209 synthesis. Due to its low concentration in blood, aspartate synthesis is crucial for cell survival [24-

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In recent papers [24][25][26] some authors evidenced "an essential role of the mitochondrial electron 232 transport chain…. in aspartate synthesis". This is not unexpected if we note that mitochondrial

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The synthesis of nucleotides from glucose requires glutamine, aspartate and R5P synthesis.

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With glucose as carbon substrate (Figure 6b) Figure S6 in the supplementary materials.

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The advantage of C2M2 is that, due to the low number of reactions and metabolites, the

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With C2M2, we demonstrated that glutamine is a precursor as good as glucose for the syntheses 365 of the main metabolites necessary for cell proliferation and energy production and we are able to give 366 the quantitative yield in these productions (

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We would like to stress, however, that a big advantage of C2M2 is that it can be approached by