124 Chapter 5
Figure 5.17 Pathways by which amino acids can be catabolized for energy. These pathways are indirect for some
amino acids, which first must be transaminated into other amino acids before being converted into keto acids by deamination.
Citric acidCitric acid cycleNH 3 UreaNH 3 UreaUrea NH 3Urea NH 3NH 3 UreaNH 3 UreaAlanine, cysteine,
glycine, serine,
threonine, tryptophanLeucine,
tryptophan,
isoleucineIsoleucine,
methionine,
valinePhenylalanine,
tyrosineArginine, glutamate,
glutamine, histidine,
prolineAsparagine,
aspartatePyruvic acidAcetyl CoA훂–Ketoglutaric acidOxaloacetic acidSuccinic acidFumaric acidFigure 5.18 The interconversion of glycogen, fat, and protein. These simplified metabolic pathways show how glycogen,
fat, and protein can be interconverted. Note that while most reactions are reversible, the reaction from pyruvic acid to acetyl CoA is
not. This is because a CO 2 is removed in the process. (Only plants, in a phase of photosynthesis called the dark reaction, can use CO 2
to produce glucose.)
Amino acidsUreaGlucosePhosphoglyceraldehydePyruvic acidAcetyl CoAGlycerolFatty acidsTriacylglycerol
(triglyceride)Ketone
bodiesC 5C 6
C 4GlycogenProteinLactic acidCitric
acid
cycle