Table 2

Rough approximations showing the energy balance between sugars and fatty acidsa

Process, sugar, or fatty acidEnergy cost of productionaEnergy return upon breakdownaYieldb
ATP/C%
CO2 fixation3 ATP + 2 NADPH = 8 ATP eq.
C3 triose-P9 ATP + 6 NADPH = 24 ATP eq.2 ATP + 5 NADH + 1 FADH2 = 16 ATP eq.5.366.7
C2 acetyl-CoA7 ATP + 6 NADPH − 2 NADH = 17 ATP eq.1 ATP + 3 NADH + 1 FADH2 = 10 ATP eq.558.8
C2 acetate3 NADH + 1 FADH = 9 ATP eq.
C6 sugars (glucose)18 ATP + 12 NADPH = 48 ATP eq.4 ATP + 10 NADH + 2 FADH2 = 32 ATP eq.5.366.7
Starch (C6n)n (18 ATP + 12 NADPH) + n ATP = 49n ATP eq.n (4 ATP + 10 NADH + 2 FADH2) = 32n ATP eq.5.365.3
C2n fatty acidn acetyl-CoA + (n −1) (ATP + 2 NADPH) = 23n − 6 ATP eq.n acetyl-CoA + (n − 1) (NADH + FADH2) = 14n − 4 ATP eq.
C16 fatty acid (palmitic acid)8 acetyl-CoA + 7 ATP + 14 NADPH = 63 ATP + 62 NADPH − 16 NADH = 178 ATP eq.8 acetyl-CoA + 7 NADH + 7 FADH2 = 8 ATP + 31 NADH + 15 FADH2 = 108 ATP eq.6.760.7
C18 fatty acid (oleic acid)9 acetyl-CoA + 8 ATP + 16 NADPH = 71 ATP + 70 NADPH − 18 NADH = 201 ATP eq.9 acetyl-CoA + 8 NADH + 8 FADH2 = 9 ATP + 35 NADH + 17 FADH2 = 122 ATP eq.6.860.7
  • a Regular or roman values show the stoichiometries in ATP and NAD(P)H involved in anabolic or catabolic reactions. These values rather reflect the minimal requirements for synthesis and the maximal return upon breakdown. The overall energies in terms of ATP equivalents (ATP eq.) are shown in italic type. These calculations are based upon chemiosmotic coupling factors of 2.5 ATP produced per NAD(P)H oxidized and 1.5 for succinate (FADH2). Such coupling factors are determined from the stoichiometry of proton to electron transfer in oxidative phosphorylation and from the chemiosmotic coupling of 1 ATP synthesized for 4 H+ transferred, again a rather optimal value. The difference that may exist between NADH and NADPH is ignored in Table 2. The two ATP molecules required for the formation of palmitoyl-CoA from palmitate are ignored.

  • b The ATP/C yield column shows that, per carbon atom, fatty acids return more ATP than sugars but less energy compared to the energy invested in synthesis, as shown in the percent yield column. The estimates in the right column similarly apply to starch, longer-chain fatty acids (C18 or C20) and triacylglycerols (TAGs) because polymerization and synthesis of glycerol-3P and phosphatidic acid involve only a few ATP or NAD(P)H molecules and do not change the ratios much. Simply stated, regular or roman values give reliable data, while italic values show rough estimates.