Table III.

Coenzyme requirements for the biosynthetic outputs from the central metabolic network in heterotrophic Arabidopsis cells

The coenzyme requirements for biosynthesis were determined from the fluxes in Supplemental Table S4 using the reaction stoichiometries in Supplemental Table S6 and are presented as means ± se based on the estimates of label partitioning following metabolism of [U-14C]Glc in Table I. The coenzyme requirements for lipid assume the synthesis of triacylglycerol containing only oleoyl (C18:0) side chains. Costs for amino acid production are based on the assimilation of equal amounts of NO3 and NH4+. The ATP requirement for protein production is calculated from the cost of synthesizing proteins from the component amino acids and is equivalent to 4 ATP per amino acid residue. Accumulating organic acids are withdrawn directly from intermediate pools in the network and incur no additional cost. Generation of coenzymes by the central metabolic network is determined from the net fluxes of the internal interconversions (i.e. excluding output steps) of the model defined in Figure 2 using conventional reaction stoichiometries and ignoring the possible contribution of inorganic pyrophosphate as a phosphoryl donor. NADH and NADPH yields (in italics) assume conversion of isocitrate to 2-oxoglutarate by NADP+-dependent isocitrate dehydrogenase. ATP generation is attributed to substrate-level phosphorylation, and FADH2 production is assumed to be equivalent to 0.6× NADH. Values for coenzyme generation are means ± se of estimates obtained from the best fit flux solutions of 501 Monte Carlo simulations; values in parentheses are determined from the global best fit flux solution of the network in which the oxidative steps of the PPP are located in both the cytosol and plastids, as detailed in Supplemental Table S5.

ProductCoenzyme Requirement (Molar Flux Relative to Glc Uptake)
ATPNADHNADPH
Suc0.076 ± 0.0039
Starch0.042 ± 0.0070
Cell wall0.079 ± 0.0112−0.039 ± 0.0045
Lipid0.066 ± 0.00450.068 ± 0.00450.066 ± 0.0045
Ethanol0.020 ± 0.0023
Amino acids0.726 ± 0.00900.144 ± 0.00281.207 ± 0.0141
Protein0.268 ± 0.0285
Total requirement1.257 ± 0.03320.193 ± 0.00731.273 ± 0.0148
0.999 ± 0.00113.964 ± 0.00240.771 ± 0.0022
3.330 ± 0.00211.407 ± 0.0018
Generated by the central metabolic network(1.001)(4.055)(0.770)
(3.420)(1.405)