Plant Biochemistry

• The Kennedy pathway (first delineated by Eugene Kennedy in 1956) is the canonical eukaryotic route for de novo biosynthesis of triacylglycerols (TAG).
• The sequential metabolic steps and bona fide intermediates of the Kennedy pathway are:
• $\text{sn-Glycerol-3-phosphate} + \text{Acyl-CoA} \xrightarrow{\text{GPAT}} \mathbf{\text{Lysophosphatidic acid (LPA)}} + \text{CoA}$
• $\text{LPA} + \text{Acyl-CoA} \xrightarrow{\text{AGPAT}} \mathbf{\text{Phosphatidic acid (PA)}} + \text{CoA}$
• $\text{PA} \xrightarrow{\text{PAP / Lipin}} \mathbf{\text{1,2-Diacylglycerol (DAG)}} + P_i$
• $\text{DAG} + \text{Acyl-CoA} \xrightarrow{\text{DGAT}} \text{Triacylglycerol (TAG)} + \text{CoA}$
• Glyceraldehyde-3-Phosphate (G3P) is a phosphorylated triose intermediate of glycolysis and the Calvin cycle, not a direct intermediate in the Kennedy lipid acylation cascade.
• Therefore, Glyceraldehyde-3-Phosphate is not an intermediate of the Kennedy pathway (Option 4).

Stepwise reactions of the mitochondrial β-oxidation spiral:

• Oxidation / Dehydrogenation: Acyl-CoA dehydrogenase (D) oxidizes acyl-CoA to trans-Δ2-enoyl-CoA.
• Hydration: Enoyl-CoA hydratase (C) adds water across the double bond to form L-β-hydroxyacyl-CoA.
• Oxidation: β-hydroxyacyl-CoA dehydrogenase (A) oxidizes L-β-hydroxyacyl-CoA to β-ketoacyl-CoA.
• Thiolysis: Thiolase (B) cleaves β-ketoacyl-CoA with CoA-SH to release acetyl-CoA and a 2-carbon shortened acyl-CoA.

Cardiolipin (diphosphatidylglycerol) structure and biosynthesis:

• Cardiolipin is a unique tetra-acyl phospholipid consisting of two phosphatidyl groups linked across a shared central glycerol bridge (1,3-bis(phosphatidyl)glycerol).
• In bacterial cardiolipin synthase, two phosphatidylglycerol molecules condense with the release of one free glycerol to yield cardiolipin.
• Cephalin is phosphatidylethanolamine, ceramide is a sphingosine linked to a fatty acid, and gangliosides contain complex oligosaccharide headgroups with sialic acid.