Unit 11
Amino Acid Metabolism

Overview

The main use of amino acids in vivo is to build proteins. However, they can also be used (after carbohydrate and lipid reserves have been exhausted) as fuel molecules. In addition, amino acids are the precursors of other nitrogen-containing biomolecules, such as nucleotides, enzyme cofactors, neurotransmitters, and the heme group.

Unlike carbohydrates and lipids, amino acids contain nitrogen (situated in the amino group). Since it is only the carbon-hydrogen portion which acts as a fuel, two processes have evolved to handle the nitrogen part of the amino acid. “Transamination” transfers the amino group from the amino acid to a keto-acid “holding molecule.” The amino group can then be recycled or eliminated. The urea cycle is the mammalian process for eliminating amino groups. The deaminated amino acid (i.e., the carbon-hydrogen portion) enters the citric acid cycle.

Unit 8 is divided into five lessons:

  1. Deamination of Amino Acids
  2. Elimination of Nitrogen—Urea Cycle
  3. Deaminated Amino Acids as Metabolic Fuels
  4. Amino Acids as Biosynthetic Precursors
  5. Nitrogen Fixation

Objectives

After completing this unit, you should be able to

  1. explain the deamination of amino acids, and describe the fate of the removed amino group.
  2. describe the variety of catabolic paths for the non-nitrogen portions of an amino acid.
  3. list some of the uses of amino acids as biosynthetic precursors.
  4. describe the biochemical process that reduces elemental nitrogen to ammonia.
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Glossary

deamination enzymatic removal of the amino group from an amino acid
essential amino acid amino acid that cannot be synthesized by a given species
glucogenic amino acid amino acid whose carbon-hydrogen skeleton can be converted to a TCA intermediate
α-ketoacid amino acid minus the amino group
ketogenic amino acid one whose carbon-hydrogen skeleton can be converted to acetyl-CoA or an acetyl-CoA derivative
porphyria set of diseases caused by a genetic defect in one of the heme biosynthetic enzymes
transamination transfer of an amino group from an amino acid to α-ketoglutaric acid
vasodilator chemical which decreases blood pressure by increasing blood vessel diameter
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