Biochemistry of lipid accumulation

The key enzyme that oleaginous microorganisms possess, and which is not present in the non-oleaginous ones, is ATP citrate lyase (ACL) (see reaction (9.4)). This enzyme serves to provide acetyl-CoA from citrate in the cytosolic compartment of eukaryotic cells and the abundant 

When the microorganism exhausts all fixed nitrogen from the culture medium, an intracellular scavenging process for N then takes place. This takes the form of activating the enzyme AMP deaminase which catalyses reaction (9.1). 

As a result of the enzyme s action, the concentration of AMP within the cell rapidly drops (Boulton and Ratledge, 1983a). The amount of NH4 that is released is quickly consumed and the cells have only a brief respite from nitrogen deprivation. 

Citrate exits from the mitrochondrion and is immediately cleaved by ACL into acetyl-CoA and oxaloacetate (reaction (9.4)). 

Acetyl-CoA produced from citrate is the primer for fatty acid biosynthesis via acetyl-CoA carboxylase (to give malonyl-CoA) and the fatty acid synthetase complex. However, for fatty acid biosynthesis to occur, reducing equivalents in the form of NADPH are also required. These are produced in whole or in part by the subsequent metabolism of oxaloacetate also arising from citrate cleavage (see reaction (9.4)). 

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