Amino Acid Synthesis and Degradation Flashcards
why is the catabolism of amino acids important to note?
intermediate products like oxaloacetate, pyruvate, α-ketoglutarate, fumarate, succinyl-CoA, acetyl-CoA, and acetoacetate are formed and can enter the pathways of intermediary metabolism resulting in the synthesis of glucose, lipid, or production of energy through oxidation to CO2 by the TCA cycle
of the 20 amino acids how many can humans make and what are these amino acids called?
11; non essential amino acids
how many of the 20 amino acids is essential? name them?
9; PVT TIM HLL which are obtained from the diet because the body cannot synthesize them
where are the non esssential amino acids synthesized from?
glycolytic intermediates, TCA cycle intermediates, or from existing amino acids.
how are the carbon skeletons of the amino acids classified after they are degraded?
they are classified as glucogenic (precursor for glucose) or ketogenic (precursor of ketone bodies)
Defects in amino acid degradation pathways can lead to disease.
in terms of synthesis from alpha keto acids, what amino acid is synthesized from pyruvate?
alanine
in terms of synthesis from alpha keto acids, what amino acid is synthesized from oxaloacetate?
aspartate
in terms of synthesis from alpha keto acids, what amino acid is synthesized from alpha ketoglutarate?
glutamine
how many nitrogens does glutamine have that makes it different from glutamate?
2 nitrogens
how many nitrogens does asparagine have that makes it different from aspartate?
2 nitrogens
certain types of tumor cells (leukemic) require this amino acid for growth; how can we prevent this from happening?
asparagine; asparagine synthetase can be inhibited to prevent the synthesis of asparagine and asparaginase can also be used to make aspartate
in terms of synthesis by amidation, glutamate is used to produce what? aspartate?
glutamine; asparagine
T/F, proline and arginine are interchangeable?
T
T/F, serine, glycine and cysteine are interchangeable?
T
too much homocysteine is indicated for what medical disorders?
cardiovascular disorders
this medical condition is marked by the presence of cystathionine in the urine?
Cystathioninuria
what enzyme is affected in Cystathioninuria? what types of patients is this commonly found and how are adults affected?
deficiency in cystathionase so cysteine is not present what backs up cystathionine B-synthase causing an increase in homocysteine levels
common in premature infants (As they mature enzyme levels rise and urine cystathionine levels decline)
Adults: Caused by genetic cystathionase deficiency or dietary deficiency of pyridoxine (vitamin B6)
what is the medical condition in which there is abnormal elevation of homocysteine and its metabolites in the blood and urine?
Homocystinuria
what is the enzyme affected in Homocystinuria?
deficiency in cystathionine β-synthase or dietary deficiency of Vitamin B6 (lack of cofactor for PLP to work) and this is also called Type I Homocystinuria
how many different types of Homocystinuria are there?
3
what is type II Homocystinuria?
Deficiency in the synthesis of methyl- B12 (CH3-B12 or cobalamin)
what is type III Homocystinuria?
Deficiency in the synthesis of methyltetrahydrofolate (N5- CH3-FH4), high homocysteine levels
what are both types (II and III) of Homocystinuria associated with?
elevated plasma homocysteine but not methionine
what enzyme is used to catalyze phenylalanine to tyrosine?
phenylalanine hydroxylase (PAH)
what is the medical condition called when there is a phenylalanine hydroxylase (PAH) enzyme deficiency (classic form)?
Phenylketonuria (PKU) and also from deficiencies in dihydropteridine reductase, or enzymes in the biosynthetic pathway for BH4 (malignant form)
what is the medical condition when there is a deficiency in dihydropteridine reductase (DHPR)?
Malignant Phenylketonuria (PKU) or hyperphenylalanemia
what is the function of dihydropteridine reductase (DHPR)?
enzyme required for regeneration of BH4, a cofactor of PAH causing an indirect increase in Phe levels
Less frequently, DHPR activity is normal but a defect in the biosynthesis of BH4 exists
what is the function of BH4?
a cofactor of PAH causing an indirect increase in Phe levels
BH4 is also a cofactor for 2 other hydroxylation’s required in the synthesis of neurotransmitters in the brain [TrpSerotonin and TyrCatecholamines]
as amino acids are getting degraded what happens to their carbons?
CO2
Compounds that produce glucose in the liver ( pyruvate, α-KG, succinyl Co-A, fumarate, oxaloacetate)
Ketone bodies or their precursors (Acetyl CoA, acetoacetate)
in the degradation of amino acids, what happens to the glucogenic amino acids?
carbon skeletons can be converted to pyruvate or one of the intermediates of the TCA cycle
These intermediates are substrates for gluconeogenesis and therefore, can give rise to the net synthesis of glucose in liver and kidney
in the degradation of amino acids, what happens to ketogenic amino acids?
carbon skeletons can be converted to acetoacetate or one of its precursors (acetyl-CoA or acetoacetyl-CoA)
Acetoacetate is one of the ketone bodies, which also include 3-hydroxybutyrate and acetone
Lysine and leucine are the only exclusively ketogenic amino acids
what are the amino acids that form oxaloacetate?
asparagine
aspartate
what happens to asparagine when it is used to form oxaloacetate?
Asparagine is hydrolyzed by asparaginase, liberating ammonium (NH4+) and aspartate
what happens to aspartate when it is used to form oxaloacetate?
Aspartate loses its amino group by transamination to form oxaloacetate
what is the effect of asparaginase for patients who have leukemia?
This makes asparagine an essential amino acid for these cells, which therefore requires asparagine from the blood.
Asparaginase, which hydrolyzes asparagine to aspartate can be administered systemically to treat leukemic patients.
Asparaginase lowers the level of asparagine in the plasma, thereby depriving cancer cells of a required nutrient.
what are the nonessential amino acids that form α-ketoglutarate via glutamate?
glutamine
proline
arginine
how is glutamate used to form α-ketoglutarate?
Glutamine is hydrolyzed to glutamate and ammonium by the enzyme glutaminase which is converted to α-ketoglutarate by transamination or through oxidative deamination by glutamate dehydrogenase
how is proline used to form α-ketoglutarate?
Proline is oxidized to glutamate which is transaminated or oxidatively deaminated to form α-ketoglutarate.
how is arginine used to form α-ketoglutarate?
Arginine is hydrolyzed by arginase to produce ornithine (and urea) which is subsequently converted to α-ketoglutarate, with glutamate semialdehyde as an intermediate
what is the essential amino acids that forms α-ketoglutarate via glutamate?
Histidine is oxidatively deaminated by histidase to urocanic acid which subsequently forms N-formiminoglutamate (FIGLU)
The subsequent reactions transfer 1 carbon of FIGLU to the tetrahydrofolate (FH4) pool and release NH4+ and glutamate.
Glutamate is transaminated or oxidatively deaminated to form α-ketoglutarate.
what is the medical condition when there is a histidine deficiency?
Histidinemia
rare genetic disorder caused by histidase deficiency. Causes elevated histidine level in blood. Generally considered benign.
what are the nonessential amino acids that form pyruvate?
alanine
serine
glycine
cysteine
how does alanine form pyruvate?
Alanine loses its amino group by transamination to form pyruvate
major gluconeogenic amino acid
how does serine form pyruvate?
Serine can be converted to glycine and N5,N10-methylene-FH4. Serine can also be converted to pyruvate by serine dehydratase
how does glycine form pyruvate?
Glycine can be converted to serine by the reversible addition of a methylene group from N5,N10-methylene-FH4 or oxidized to CO2 and NH4+ and serine proceeds through the conversion process to oxaoloacetate
in terms of glycine forming pyruvate, what happens when there is a deficiency of the transaminase?
causes overproduction of oxalate, the formation of oxalate stones, and kidney damage (primary oxaluria type 1 or PH1)
what are essential glucogenic and ketogenic amino acids?
threonine
how is cysteine used to form pyruvate?
Cysteine undergoes desulfuration to yield pyruvate
The sulfate released can be used to synthesize 3’-phosphoadenosine-5’-phosphosulfate (PAPS), an activated sulfur donor to a variety of acceptors
Cysteine can be oxidized to its disulfide derivative, cystine.
how is threonine converted to pyruvate?
converted to pyruvate in most organisms but its a minor pathway (at best) in humans
how do both phenylalanine and tyrosine form fumarate?
Hydroxylation of phenylalanine produces tyrosine in a reaction catalyzed by BH4 requiring phenylalanine hydroxylase (PAH)
Tyrosine undergoes oxidative degradation in a pathway that produces fumarate and acetoacetate
Inherited deficiencies in the enzymes of phenylalanine and tyrosine metabolism lead to the diseases:
PKU [see slides 15-17]
Tyrosinemia
Alkaptonuria
T/F, tyrosine and phenylalanine are both glucogenic and ketogenic amino acids
T, though phenylalanine is essential and tyrosine is nonessential
what are the different types of Tyrosinemia? what causes tyrosinemia?
Transient tyrosinemia
Tyrosinemia II
Tyrosinemia I (Tyrosinosis)
deficiency in a tyrosine enzyme
what is Transient tyrosinemia?
Condition appears to be benign
Dietary restriction of protein returns plasma Tyr to normal.
Biochemical defect is most likely a low level immaturity of 4-hydroxyphenylpyruvate dioxygenase
frequently observed in newborn infants (especially premature)
what is Tyrosinemia II?
deficiency in tyrosine aminotransferase
Tyrosine accumulates in the urine.
Leads to eye and skin lesions, neurological symptoms.
Treated by restricting Tyr and Phe intake
what is Tyrosinemia I (Tyrosinosis)?
deficiency in fumarylacetoacetate hydrolase [last step]
characteristic cabbage-like odor
this medical deficiency is due to an Inherited genetic deficiency of homogentisate oxidase resulting in an accumulation of homogentistic acid (HGA) in the skin and other tissues.
Alkaptonuria
what are the symptoms of Alkaptonuria?
Infant urine may darken, arthritis (especially of spine), darkening of ear, dark spots on sclera and cornea.
what is a viable treatment option for Alkaptonuria?
high-dose of vitamin C
what is the prognosis of Alkaptonuria?
All patients will experience chronic joint pain.
what are possible complications of Alkaptonuria?
Arthritis in ~50% of older adults, heart valve replacement, coronary heart disease, and kidney stones.
what are the amino acids that form succinyl- CoA?
methionine
valine and isoleucine
threonine
what are the amino acids that form acetyl CoA or acetoacetyl CoA?
leucine isoleucine (ketogenic and glucogenic) lysine threonine tryptophan (ketogenic and glucogenic)
what are the BCAA’s?
BCAAs isoleucine, leucine, and valine are essential amino acids that are primarily metabolized by peripheral tissues (particularly muscle) rather than the liver [universal fuels]
what are the two steps in the catabolism of BCAA?
1st step is transamination by B6 requiring branched-chain amino acid aminotransferase forming an α-ketoacid
2nd step: α-keto analogs undergo oxidative decarboxylation by the branched chain α-keto acid dehydrogenase (BCKD) complex
what are the two major functions in the degradation of BCAA?
(1) energy (2) provide precursors to replenish TCA cycle intermediates
what happens when there is an error in the dehydrogenase of the second step in the catabolism of BCAAs?
Errors in the dehydrogenases of the second step leads to Maple Syrup Urine Disease
what is the product in the Branched-chain amino acids and methionine degradation?
Succinyl-CoA
what is the function of Methylmalonyl CoA Mutase (MCM) ?
MCM catalyzes the isomerization of methylmalonyl-CoA to succinyl-CoA
what cofactor does MCM require?
requires a vitamin B12-derived prosthetic group to function and is involved in key metabolic pathways
what is the effect in an MCM deficiency?
Genetic deficiency in this mutase gene causes failure to thrive, vomiting, dehydration, developmental delay, and seizures
Can also lead to methylmalonic acidemia
Alcaptonuria is a disorder related to the metabolism of?
tryptophan tyrosine glycine branched chain amino acids cysteine
tyrosine
