Catabolic pathways generate the energy required for anabolic reactions. Catabolic pathways produce intermediates for other pathways, and some intermediates and products of anabolic pathways provide intermediates for catabolic pathways. The Krebs cycle produces energy to drive amino acid synthesis, and several Krebs cycle intermediates are substrates for amino acid biosynthesis. For example, oxaloacetic acid is a precursor for half a dozen amino acids, and alpha-ketoglutaric acid is a precursor for about four additional amino acids. The pentose phosphate pathway produces the five-carbon sugar required for nucleotide synthesis, and various amino acids are metabolized to produce the nitrogenous bases in nucleotides. The Krebs cycle produces intermediates
