Synthesis And Degradation Of Amino Acids

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1

pyridoxal phosphate

derived from vitamin B6
quintessential coenzyme for transamination and certain reactions involving carbon skeleton

2

tetrahydrofolate

FH4
required in pathways to either accept or donate one carbon group

3

tetrahydrobiopterin

BH4
required for one ring dehydroxylation

4

derived from glycolysis intermediates

glycine, serine, cysteine, alanine

5

glycine

synthesized from serine in majority and a small amount from threonine
40% oxaloacetate in the liver comes from glycine metabolism

6

kidney stones

glycine is synthesized from serine in majority and a small amount from threonine
once glyoxylate is formed from glycine degradation, it can be oxidized to oxalate, which is sparingly soluble and tends to precipitate in kidney tubules

7

alanine

produced from pyruvate by a transamination reaction catalyzed by alanine aminotransferase (ALT) or from tryptophan by side chain cleavage
major gluconeogenic amino acid

8

related to TCA intermediates

amino acids from oxaloacetate synthesis; aspartate, asparagine
amino acidss from alpha-ketoglutarate synthesis; glutamine, glutamate, proline, arginine
degradation of amino acids converts them into 4 TCA cycle intermediates

9

glutamate

derived from alpha-ketoglutarate by transamination or by glutamate dehydrogenase reaction

10

arginine

produced by urea cycle; ornithine formed from transamination of glutamate semialdehyde is a major source
quantities generated by the urea cycle are adequate only for adult and are insufficient to support growth; therefore, during periods of growth, this becomes an essential amino acid

11

histidine

essential amino acid; five of its carbons form glutamate when it is degraded

12

aspartate

Carbons can form fumarate in the urea cycle; generated cytosolic fumarate is converted to malate for transport into the mitochondria for oxidative or anaplerotic purpose
analogous sequence of reactions occurs in the purine nucleotide cycle

13

phenylalanine and tyrosine

phenylalanine is converted to tyrosine by a hydroxylation reaction; tyrosine is oxidized to form acetaldehyde and fumarate
conversion of fumarate to malate allows the carbons to be used for gluconeogenesis

14

form succinyl CoA

essential amino acids methionine, valine, isoleucine, and threonine are degraded to for propionyl CoA which is converted to succinyl CoA via the following reactions

15

threonine

dehydratase converts threonine to ammonia and a-ketobutyrate
dehydrogenase converts it to 2-amino-3-ketobutyrate and glycine C-acetyltransferase (2-3-KB-CoA ligase) converts 2-3-KB to glycine and acetyl CoA
aldolase converts it to glycine and acetaldehyde (not in human)

16

valine and isoleucine

branched-chain amino acids are universal fuels
muscle carries out the highest level of branched-chain amino acid oxidation
degradation generates energy (NADH and FADH2) and provide TCA cycle intermediates; leucine is strictly ketogenic

17

form aceyl CoA and acetoacetate

tryptophan, threonine, isoleucine, leucine, lysine, phenylalanine, tyrosine; leucine and lysine are strictly ketogenic

18

phenylketonuria

deficiency of PAH
can be treated with low-phenylalanine-diet with protein supplements
subset of disease caused by a deficiency in dihydropteridine reductase (DHPR), an enzyme required to synthesize cofactor BH4 of PAH, develops into progressive neurologic disorders and the patients usually die within the first 2 years of life

19

tyrosinemia II

caused by a deficiency of TAT and can be treated with a low tyrosine, low-phenylalanine diet
tyrosinemia I is caused by a genetic deficiency of FAH; the acute form is associated with liver failure and death within the first year of life

20

tryptophan

since NAD and NADP are produced from tryptophan degradation, tryptophan can be substituted for vitamin niacin

21

isoleucine

produces both succinyl CoA and acetyl CoA

22

leucine

purely ketogenic and produces hydroxymethylglutaryl CoA (HMGCoA), which is cleaved to form acetyl CoA and acetoacetate

23

lysine

degraded by a complex pathway, during which NADH and FADH2 are generated for energy