Alkaloids derived from Krebs cycle

Alkaloids Derived from the Krebs Cycle (Lysine and Ornithine Derived) As shown previously (Scheme 1.2), the Krebs cycle is a crucial metabolic process, which leads to a-ketoacids (oxaloacetic and 2-oxoglutaric acids). Their enzymatic transamination affords the two amino acids— aspartic acid and glutamic acid—which are the direct biosynthetic precursors of amino acids lysine and ornithine, respectively. These in turn produce cadaverine, a CjN unit, and putrescine, a C N unit, which are major components for the biosynthesis of important alkaloids, as will be discussed later (Scheme 1.6). Additionally, ornithine is a precursor of arginine, another important amino aeid. 

In addition to acetyl-CoA, shikimic acid, mevalonic acid, and deoxyxylulose phosphate, other building blocks based on amino acids are frequently employed in natural product synthesis. Peptides, proteins, alkaloids, and many antibiotics are derived from amino acids, and the origins of the most important amino acid components of these are briefly indicated in Figure 2.1. Intermediates from the glycolytic pathway and the Krebs cycle are used in constructing many of them, but the aromatic amino acids phenylalanine, tyrosine,... 

AcCoA is the building block of fatty acids, polyketides, and mevalonic acid (MVA), a cytosolic precursor of the C5 isoprene units for the biosynthesis of terpenes in the C,5 and Cj series (mind it is different from the MEP pathway, in product, and in cell location). Finally, AcCoA enters the citric acid or Krebs cycle, which leads to several precursors of amino acids. These are oxaloacetic acid, precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C N linear unit and methionine as a methyl supplier), and 2-oxoglutaric acid, precursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C N linear unit). All these amino acids are key precursors in the biosynthesis of many alkaloids. 

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