(see figure 6.1). For example, the important precursor metabolite pyruvate can be converted to the amino acid alanine.

Many organisms, including Escherichia coli, can make all of their cell components, including proteins, lipids, carbohydrates, and nucleic acids using only a dozen or so precursor metabolites. The precursor metabolites used by E. coli are listed in table 6.3. Recall from chapter 4 that E. coli can grow in glucose salts medium, which contains only glucose and a few inorganic salts. Some organisms, however, are not as versatile as E. coli with respect to their biosynthetic capabilities. Any essential compounds that a cell cannot synthesize from the appropriate precursor metabolite must be provided from an external source.■ glucose salts medium, p. 93

Scheme of Metabolism

Three key metabolic pathways, called the central metabolic pathways, are used to gradually oxidize glucose, the preferred energy source of many cells, completely to carbon dioxide (figure 6.9). In the step-wise process, the pathways provide cells with energy in the form of ATP, reducing power, and the precursor metabolites needed to synthesize the cells' building blocks. The central metabolic pathways include:

■ Glycolysis

■ Pentose phosphate pathway

■ Tricarboxylic acid cycle (TCA cycle)

Table 6.3 Precursor Metabolites, Their Source, and Their Use in Biosynthesis in E. coli

Precursor Metabolite

Pathway Generated

Biosynthetic Role

Glucose 6-phosphate



Fructose 6-phosphate



Dihydroxyacetone phosphate


Lipids (glycerol component)



Protein (the amino acids cysteine, glycine, and serine)



Protein (the amino acids phenylalanine, tryptophan, and tyrosine)



Proteins (the amino acids alanine, leucine, and valine)

Ribose 5-phosphate

Pentose phosphate cycle

Nucleic acids and proteins (the amino acid histidine)

Erythrose 4-phosphate

Pentose phosphate cycle

Protein (the amino acids phenylalanine, tryptophan, and tyrosine)


Transition step

Lipids (fatty acids)


TCA cycle

Protein (the amino acids arginine, glutamate, glutamine, and proline)


TCA cycle

Protein (the amino acids aspartate, asparagine, isoleucine, lysine, methionine, and threonine)

Some organisms use succinyl-coA as a precursor in heme biosynthesis; E. coli uses glutamate

Some organisms use succinyl-coA as a precursor in heme biosynthesis; E. coli uses glutamate

136 Chapter 6 Metabolism: Fueling Cell Growth

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