Amino Acid Metabolism

LAGAMAYO, Linear Larie B.CCE - 4Amino Acid MetabolismAmino Acid Breakdown for EnergyAmino Acid CatabolismImportancePotential Source of Energy and especially during fasting of glucoseIncomplete metabolism of a number of amino acids results in the accumulation of toxic amino acid breakdown intermediatesFasting StateWithin one hour of a meal the blood glucose level begins to fall.The insulin concentration also begins to fall and the glucagon concentration begins to rise. These changes in hormone concentrations trigger the release of metabolic fuels from the body stores.

Glucagon stimulates glycogenolysis, the break down of liver glycogen to maintain the blood glucose concentration.

Glucagon also stimulates gluconeogenesis, synthesizing glucose from lactate, alanine, and glycerol. As fasting progresses, gluconeogenesis becomes more important to maintain the blood glucose concentration.

After 30 hours of fasting, the liver glycogen stores are depleted, and gluconeogenesis is generating all of the blood glucose. Only the glycerol moiety of triacylglycerols can be used as source of glucose. Glucagon also stimulates the break down of proteins and the catabolism of amino acids. This generates ammonia which the liver converts into urea to be excreted. Fasting StateDuring fasting the triacylglycerols stored in the adipose tissue become the major energy source. The glycerol component is used for gluconeogenesis and the fatty acids released are oxidized by tissues such as muscle and heart tissue. The liver absorbs these fatty acids and converts them into ketone bodies which are released in the blood. These ketone bodies can fuel the muscles and heart saving glucose for the brain. DegradationTwo Classes of DegradationGlucogenic (production of glucose)KetogenicCarbon FamiliesC3 Family Amino Acids convertible to pyruvateAlanine

Serine

Side Note: Ethanol and AcetaldehydeEthanol and Acetaldehyde

Carbon FamiliesCysteine

Threonine

Carbon FamiliesC4 Families Amino Acids convertible to oxaloacetateAsparagine

Aspartate

Carbon Families

Carbon FamiliesArginineProlineHistidineCarbon Families

Branched Chain Amino AcidsLeucineValineIsoleucineBranched Chain Amino Acids

Aromatic Amino AcidsPhenylalanineTyrosineTryptophanAromatic Amino Acids

Aromatic Amino AcidsTryptophanLysine

Methionine

SummaryAmino acids are a major energy source, especially during conditions in which glucose availability is limited.Amino acids may be either glucogenic or ketogenic.The C3 family (alanine, serine, glycine,threonine, and cysteine) can all be converted to the 3-carbon a - ketoacid pyruvate, although some are converted to other molecules under appropriate conditions.

The C4 family (aspartate and asparagine) are converted to the 4-carbon compounds oxaloacetate or fumarate.The C5 family (glutamate, glutamine, histidine, proline, and arginine) are all converted to glutamate, and then to a - ketoglutarate.SummaryThe branched-chain amino acids (leucine, isoleucine, and valine) are broken down by a series of common enzymes into coenzyme A derivatives. These are then metabolized by separate pathways depending on the structure of the original compound. Leucine is converted into HMG-CoA, the substrate for ketone body production, and is exclusively ketogenic. Valine is converted to propionyl-CoA and is exclusively glucogenic. Isoleucine is converted to acetyl-CoA and propionyl-CoA, and is therefore both ketogenic and glucogenic.

The aromatic amino acids phenylalanine and tyrosine result in formation of acetoacetate and fumarate, while tryptophan results in formation of acetoacetate and (via alanine) pyruvate. Each of these compounds is therefore both ketogenic and glucogenic.SummaryLysine is broken down by a pathway that is related to that for the tryptophan indole ring, and forms acetoacetate.Methionine is used to produce a variety of biosynthetic intermediates; it can be converted into propionyl-CoA, and is therefore glucogenic.Amino Acid BiosynthesisAmino Acid BiosynthesisNitrogen source is needed.In animals, glutamate and glutamine play the pivotal roles.Glutamate is synthesized from ammonia and -ketoglutarate by the action of glutamate dehydrogenase

Amino Acid BiosynthesisThe regulation of glutamine synthetase plays a crucial role in controlling nitrogen metabolism. The dynamic duo of glutamate dehydrogenase and glutamine synthetase are found in all living organisms.