3-hydroxypropionate/malyl-CoA cycle

Hydroxypropionate / malyl-CoA cycle 10 7 NAD(P)H, but 1 FAD is reduced in the cycle Acetyl-CoA/propionyl-CoA carboxylase HCOJ Acetyl-CoA, pyruvate, succinyl-CoA Malonyl-CoA reductase, propionyl-CoA synthase, malyl-CoA lyase... 

Figure 3.4 3-Hydroxypropionate/malyl-CoA cycle, as studied and proposed in Chloroflexus aurantiacus.

The energy costs of the 3-hydroxypropionate/malyl-CoA cycle are high, with ten ATP required per triose phosphate (see Table 3.1). However, bicarbonate rather than C02 is the actual inorganic carbon species used by acetyl-CoA/propionyl-CoA carboxylase (this is discussed in Chapter 4). Moreover, as this enzyme is virtually irreversible and has a high affinity for bicarbonate, this cycle is expensive although kinetically effective. 

For this reason (similar approach of conversion), it was assumed that both enzymes are identical [38]. In both cases a C2-unit is produced the acetyl-CoA in the 3-hydroxypropionate/malyl-CoA cycle and the glyoxylate in the 3-hydroxypropionate/4-hydroxybutyrate pathway. 

The pathway can be divided into two metabolic cycles (Figure 3.4). In the first cycle, acetyl-CoA is carboxylated to malonyl-CoA, which is subsequently reduced and converted into propionyl-CoA via 3-hydroxypropionate as a free intermediate. Propionyl-CoA is carboxylated to methylmalonyl-CoA, which is subsequently converted to succinyl-CoA the latter is then used to activate L-malate by succinyl-CoA L-malate coenzyme A transferase, which forms L-malyl-CoA and succinate. Succinate is oxidized to L-malate via conventional steps. L-Malyl-CoA, the second characteristic intermediate of this cycle, is cleaved by L-malyl-CoA/P-methylmalyl-CoA lyase, thus regenerating the starting molecule acetyl-CoA and releasing gly-oxylate as a first carbon-fixation product [27]. 

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