of malyl-CoA to regenerate acetyl-CoA helps in formation of glyoxylate. The
assimilation of glyoxylate requires a second cycle (Berg et al. 2010; Herter et al.
2002).
11.4.5 Hydroxypropionate–Hydroxybutyrate Cycle
This cycle is found in aerobic crenarchaeota and sulfolobales (Strauss and Fuchs
1993). In this pathway, various intermediates and the carboxylation reactions are
similar to the 3-hydroxypropionate cycle (Berg et al. 2007). In this pathway, several
enzymes are involved but the key enzymes are acetyl-CoA carboxylase, propionylCoA carboxylase, methylmalonyl-CoA mutase and 4-hydroxybutyryl-CoA
dehydratase and CO 2 fixing enzymes acety-CoA and propionyl-CoA carboxylase
(Fig. 11.4).
This pathway is found in anaerobic bacteria like crenarchaeal orders
thermoproteales and desulfurococcales (Berg et al. 2010). In these pathways, several
enzymes are involved but the enzymes are 4-hydroxybutyryl CoA dehydratase and
CO 2 fixing enzymes are pyruvate synthase and PEP carboxylase.
Fig. 11.2 Framework of the Reverse Krebs cycle for autotrophic CO 2 fixation
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