Figure 1. The glyoxylate cycle, a combination of the TCA cycle with the isocitrate lyase and malate synthase reactions. 1, citrate synthase; 2, cis-aconitase; 3, isocitrate dehydrogenase; 4, α-oxoglutarate dehydrogenase; 5, succinate thiokinase; 6, succinate dehydrogenase; 7, fumarase; 8, malate dehydrogenase; 9, isocitrate lyase. Reactions 1–8 represent the TCA cycle.

Figure 2. Aerobic oxidation of methane by methanotrophic bacteria. 1, methane mono-oxygenase, membrane-integrated; 2, methanol dehydrogenase. Electrons are transferred to pyrroloquinoline quinone, also called methoxatin. From there, they are transferred to the respiratory chain; 3 to 6, “the archaeal box,” see Figure 3; 7, formate dehydrogenase; RC, respiratory chain. Formaldehyde is the precursor for cell mass production in these organisms.

Figure 3. Degradation of long-chain hydrocarbons. The hydrocarbon “dissolves” in the cytoplasmic membrane with the help of detergents produced by the cells. 1, hydrocarbon (n-alkane) mono-oxygenase; 2, n-alkane alcohol dehydrogenase; 3, aldehyde dehydrogenase; 4, acyl-CoA synthetase.

Figure 4. Oxidation of D-glucose to L-ascorbate. Note that sorbitol is designated as having a D configuration because the OH-group located at C-atom 5 stands is on the right. The sorbose molecule has to be turned 180 °, then the OH group at C-atom 5 stands is on the left. Therefore, sorbose as well as ascorbate have the L-configuration.

Figure 5. CO ₂ fixation via the CBB cycle. Ru-P ₂, ribulose 1,5-bisphosphate; PGA, 3-phosphoglycerate; GAP, glyceraldehyde 3-phosphate; DAP, dihydroxyacetone phosphate; F-P ₂, fructose 1,6-bisphosphate; F-6-P, fructose 6-phosphate; E-4-P, erythrose 4-phosphate; Su-P ₂, sedoheptulose 1,7-bisphosphate; Su-7-P, sedoheptulose 7-phosphate; Xu-5-P, xylulose 5-phosphate; Ri-5-P, ribose 5-phosphate; Ru5-P, ribulose 5-phosphate.

Figure 6. CO ₂ fixation via the Wood-Ljungdahl pathway. 1, formate dehydrogenase; 2, formyl-THF synthetase; 3, methenyl-THF cyclohydrolase; 4, methylene-THF dehydrogenase; 5, methylene-THF reductase; 6, methyltransferase; 7, CO dehydrogenase/acetyl-CoA synthase, an enzyme system combining CO, the methyl group from reaction 6 and CoA to acetyl-CoA; 8, phosphotransacetylase + acetate kinase. THF, tetrahydrofolate.

Figure 7. Alcohol fermentation via the EMP and the ED pathways. (a) Fermentation of glucose to ethanol and CO ₂ by yeast. 1, initial enzymes of the Embden-Meyerhof-Parnas pathway 2, glyceraldehyde-3-phosphate dehydrogenase, 3, 3-phosphoglycerate kinase; phosphoglycerate mutase, enolase, and pyruvate kinase; 4, pyruvate decarboxylase; 5, alcohol dehydrogenase. (b) Fermentation of glucose to ethanol and CO ₂ by Zymomonas mobilis. 1, hexokinase; 2, glucose-6-phosphate dehydrogenase; 3, 6-phosphogluconate dehydratase; 4, 2-keto-3-deoxy-6-phosphogluconate (KDPG) aldolase; 5, 3-phosphoglycerate kinase; 6, phosphoglycerate mutase, enolase and pyruvate kinase; 7, pyruvate decarboxylase; 8, alcohol dehydrogenase.

Figure 8. Acetone-butanol fermentation as carried out by C. acetobutylicum.1, reactions as in Figure 2; 2, pyruvate: ferredoxin oxidoreductase, 3, hydrogenase; 4, acetoacetyl-CoA: acetyl transferase (thiolase); 5, acetoacetyl CoA: acetate CoA transferase; 6, acetoacetate decarboxylase; 7, L(+)-β-hydroxybutyryl-CoA dehydrogenase, crotonase, and butyryl-CoA dehydrogenase; 8, butyraldehyde dehydrogenase; 9, butanol dehydrogenase.

Figure 9. Mixed acid fermentation. 1, enzymes of the Embden-Meyerhof-Parnas pathway as in Figure 2; 2, pyruvate kinase; 3, pyruvate-formate lyase; 4, formate-hydrogen lyase; 5, phosphotransacetylase; 6, acetate kinase; 7, acetaldehyde dehydrogenase; 8, alcohol dehydrogenase; 9, lactate dehydrogenase; 10, PEP carboxylase; 11, malate dehydrogenase, fumarase, and fumarate reductase.