Protein And AA Metabolism Flashcards
What are inborn errors of metabolism?
Conditions associated with dysfunctional metabolism of amino acids and derivatives
What is an amino acid pool?
Population of free amino acids supplied by protein turnover, digested food and de novo synthesis (of NE aa)
What is the amino acid pool depleted by?
Production of body protein, synthesis of N containing compounds and degradation
Which amino acids are considered conditionally essential?
Arg (prematurity), Cys, Gly, Gln, Pro and Tyr
What is the major source of nitrogen?
Dietary protein
What are the major nitrogen excretory compounds?
Urea, ammonia, creatine (muscle), uric acid (purine breakdown product) and urobilinogen (heme metabolism)
How many g of protein is needed to maintain the amino acid pool?
30g of protein per day
Characteristics of Hartnup and Cystinuria
Protein transporters mediate transport of amino acids in and out of cells
Both are autosomal recessive conditions that are associated with defects in these transporters
Describe Hartnup disease**
Defective transport of nonpolar or neutral amino acids (e.g. tryptophan) leading to concentrated level in the urine
Transporter located in the kidney and SI
Manifests in infancy with failure to thrive, nystagmus, tremor, intermittent ataxia and photosensitivity
Describe Cystinuria
Defective transport of dimeric cystine and dibasic amino acids Arg, Lys and ornithine
Formation of cystine crystals in the kidneys (renal calculi)
Pts present with renal colic, which is abdominal pain that comes in waves and is liked to the formation of kidney stones
What can overcome the deficiency of transport of neutral amino acids in most patients with Hartnup disease?
A high protein diet
Biosynthesis of aromatic amino acids Try and Trp
Phe —> Tyr
Ribose 5 P —> His
Biosynthesis of serine
3PG —> Ser —> Cys or Gly
Biosynthesis of aspartate
OAA —> Asp —> Asn
Biosynthesis of glutamate
Alpha ketoglutarate —> Glu —> Gln, Pro and Arg
The biosynthesis of amino acids involves what type of reaction?
Transamination reaction from an alpha keto acid
What are the three protein degradation pathways?
Proteasome, lysosome and autophagosome
Describe the lysosomal/autophagy pathway of protein degradation
Lysosomes sequester >50 hydrolase-type intracellular proteolytic enzymes that are active at pH of 5 (in the lysosome) and inactive at pH 7 (cytoplasmic)
Non-selective
3 types: macroautophagy, macroautophagy and chaperone mediated autophagy (CMA)
What is the common denominator in the targeting of substrates in all three protein degradation pathways?
Ubiquitin
Describe proteasomal degradation
Large proteasome cytoplasmic complexes cleave polyubiquitinated proteins —> ubiquitin pathway
What is the signal for proteolysis in the proteasome?
Ubiquitin
Structure of the proteasome
Catalytic core = 20s
Each ring consists of 7 subunits that are stacked to form a barrel
Active sites are hidden inside the barrel so something must be ushered into the catalytic core
19s regulatory subunit attached to both ends fo the catalytic subunit and contains ubiquitin receptors
What is the role of isopeptidase in the proteasomal degradation pathway?
It cleaves ubiquitin in the regulatory unit for reuse so the cell doesn’t have to make more Ubq
What is the nucleophile in the proteasomal degradation pathway?
OH of threonine acts as a nucleophile to attach carbonyl of peptide bonds degraded to peptides ranging from 7-9 amino residues
Describe extracellular proteolytic control
Proteolytic enzymes are secreted as inactive zymogens as needed
Activated by proteolytic cleavage
Enterokinases are embedded in the intestinal mucosa
Example of extracellular proteolytic control
Inactive trypsinogen and chymotrypsinogen are released into the SI lumen
Trypsinogen is activated by an enterokinase, enteropeptidase
Trypsin activates chymotrypsinogen and other molecules of trypsinogen
Amino acids can be classified by what?
By what they are metabolized to
Describe ketogenic amino acids
Can be deaminated to produce alpha keto acids and KBs —> fatty acids
Alpha keto acids and KBs cannot be metabolized into glucose (both carbonyl carbons are metabolized to CO2 in the TCA cycle)
Which amino acids are ketogenic only?
Leu and Lys
Which amino acids are both ketogenic and glucogenic?
Ile, Trp, Phe, Tyr and Thr
Which amino acids are glucogenic only?
Val, His, Arg, Asn, Gln, Met, Ala, Asp, Glu, Gly, Pro, Ser, Cys
Glucogenic amino acids are eventually converted into what?
Glucose
What are the three most important reactions/enzymes in amino acid metabolism?
- Aminotransferase/transaminase
- Glutamate dehydrogenase (oxidative deamination)
- Urea cycle
True or false: Every amino acid has a conjugate keto acid
True
OAA is the keto acid of which amino acid?
Aspartate
The amine group of amino acids is shuffled into the
Liver as other amino acids then repackaged as urea in the urea cycle and excreted
Describe the shuffling of amino acids through transamination
Amino group is transferred to an alpha keto acid
Coupled reactions
Enzymes = transaminases/aminotransferases
Describe transaminases (aminotransferases)
Require coenzyme pyridoxal 5’-phosphate (PLP) which is a derivative of vitamin B6
Clinical relevance: ALT and AST
Describe how amino transferases are used in a clinical setting
Located in mitochondria, and cytoplasm of liver, kidney, intestine and muscle cells
+aminotransferase indicative of tissue damage
Alanine transferase (ALT): increases in viral hepatitis, liver cell necrosis, prolonged circulatory collapse
Aspartate transferase (AST): increases 6-8 hours after MI, biliary cirrhosis, liver cancer, pancreatitis, mono, alcoholic cirrhosis and strenuous exercise
ALT more specific to liver disease vs AST
Metabolism of Gln, His, Arg, Pro and Glu
Hyperammonemia: primary cause of neurological disorders
Glutamate vs glutamine —> N trap mechanism
Glutamine synthetase: Glu —> Gln using NH4+ and ATP (N trap via energy consumption)
Glutaminase: acts as a hydrolase to convert Gln to Glu and NH4+
Reversible reactions whose direction will be dependent on N levels
Metabolism of Met, Thr, Ile and Leu
Methionine pathway —> homocystinuria
Cystathionine beta-synthase needs PLP coenzyme
Clinical manifestations in the Met cycle (and also folate cycle since THF is used as a methyl carrier) includes what?
Homocystinuria
Describe the role of homocysteine methyltransferase enzyme in homocystinuria
Defect in this enzyme can cause secondary homocytisinuria
Uses cobalamin/vit B12 as coenzyme and THF as methyl carrier
Converts homocysteine to methionine
Describe the role cystathionine beta synthase (CBS) enzyme plays in homocytinuria
Converts homocysteine —> cystathionine
Needs PLP as a coenzyme (active form of vitamin B6)
Mutations in this enzyme are the most common cause of homocystinuria
Consequences of homocystinuria and homocysteinemia can affect four organ systems including
Ocular: ectopia, lentis and high myopia
Skeletal: limbs grow out of proportion with trunk, anterior chest wall deformities, osteoporosis and altered facial appearance
CNS: dementia
Vasculature: stroke and thrombosis
What are hyperhomocysteinemia and homocystinuria?
Vitamin deficiencies such as B6, B12 and folic acid or genetic defects in enzymes (CBS) respectively that cause defective metabolism of homocysteine
Hyperhomocysteinemia is a risk factor for what?
Atherosclerotic heart disease and stroke and can result in neuropsychiatric illness (vascular dementia, Alzheimer’s disease)
Also lens dislocation in the eyes, osteoporosis and mental retardation
What can normalize plasma homocysteine levels in some cases?
Vitamin supplementation (with B vitamins)
Describe the metabolism of branched chain amino acids (BCAs)
Metabolism produces both ketogenic and glucogenic intermediates
Branched chain alpha keto acid dehydrogenase performs oxidative decarboxylation
Needs CoA, FAD (B2), lipoic acid, NAD (B3) and TPP (B1)
Deficiencies in the BCA metabolism pathway lead to what condition?
Maple syrup urine disease (MSUD)
Describe maple syrup urine disease
Rare autosomal diseases resulting from deficient branched chain alpha keto acid dehydrogenase complex (BCKD) activity which results in branched chain ketoaciduria
BCAAs in urine give hallmark maple syrup smell
Also accumulate in blood causing toxic effects on brain function and eventually mental retardation
Tx includes a synthetic diet limiting BCAAs (Val, Leu and Ile)
BCKD activity may be restored with thiamine supplementation
Which population presents with higher risk of MSUD?
Mennonite, Amish and Jewish populations
Asparagine metabolizes to what?
Aspartate by the removal of an amine group by asparaginase
What is the best known clinical presentation of deficiencies in metabolism of Phe?
Phenylketonuria which is a debilitating IEM that is now screened for at birth along with galactosemia and tyrosinemia
Controlled with diet
Caused by majority missense mutation in the catalytic domain of the enzyme that converts Phe to Tyr (phenylalanine hydroxylase)
Recombinant expression showed altered kinetics and decreased stability
Describe phenylketonuria (PKU)
Caused by defects in the activity of Phe hydroxylase (PAH)
Most common IEM and first one to be screened in newborns
Phe instead converted to phenylpyruvate and then to phenyllactate (causes musty odor in urine) and phenylacetate
Latter two disrupt neurotransmission and block aa transport in the brain as well as myelin formation, resulting in severe impairment in brain function
Dietary limit Phe, protein supplied with synthetic formula supplemented with Tyr
Secondary PKU can result from
Tetrahydrobiopterin deficiency (a cofactor for PAH) Defects in synthesis of regeneration of BH4
Tryptophan derivatives
Same THB/DHB cycle in Phe —> tyrosine metabolism
Serotonin is the p-hdyroxylated and decarboxylated form of tryptophan
Makes niacin and melatonin
Tyrosine derivatives
Thyroglobulin made by follicular thyroid cells (~120 Tyr residues)
T4 = coupling two di-iodotyrosine
T3 = mono-iodotyrosine + di-iodotyrosine
Tyrosine peroxidase oxidizes iodine ions to iodine atoms for addition to tyrosine residues on thyroglobulin
Also makes dopamine —> NE —> Epinephrine and melanin
Hypothyroidism =
High TSH
Low T4
Hyperthyroidism =
Low TSH
High T4/T3
How are albinism and tyrosinase connected?
Albinism is due to severe lack of melanin
Conversion of tyrosine to melanin is blocked due to defects in the enzyme tyrosinase
Of blocking of the transfer of tyrosine in the body
Results in partial or complete absence of pigmentation in the skin, hair and eyes
Thyroglobulin and thyroid hormones
Thyroglobulin is a protein made by the thyroid and is used to produce T4 and T3
Pts with hyperthyroidism are treated with agents (carbimazole and propylthiouracil) which blocks iodination of thyroglobulin to decrease the production of T4 and T3
Ammonia is removed as what?
Glu and Gln in the brain by glutamine synthase
As Glu in other tissues
How is urea generated?
In amino acid metabolic pathways by deamination mechanisms
Where is urea produced?
Primarily in the liver (kidneys to a lesser extent)
Regulated by NAG
Secreted into the blood then filtered by the kidneys to be excreted in the urine
Structure of ammonia
2 amino groups (NH2)
1 C=O bond
Increased entry of ammonia to the brain is a primary cause of what?
Neurologic disorders such as congenital deficiencies of urea cycle enzymes, hepatic encephalopathies, Reye syndrome, several other metabolic disorders and some toxic encephalopathies
A blood ammonium concentration of what is associated with coma and convulsions?
200umol/L
Describe the removal of excess NH4+ from the brain
Glutamate dehydrogenase keeps on going and keeps on churning out glutamate to use up the NH4
Which in turn depletes the pool of alpha ketoglutarate and lowers the level of ATP and leads to unconsciousness
AKG —> Glu —> Gln (travels to liver from brain) —> converted back into Glu to make NH4+ which enters the urea cycle and is excreted as urea
Describe the removal of NH4+ from muscle
Pyruvate from glycolysis is converted to alanine by ALT in the muscle which then travels to the liver
Alanine is deaminated by ALT back into pyruvate which enters gluconeogenesis in the liver
The amino group from alanine is taken up by Glu which undergoes oxidative deamination to produce alpha-KG and NH4+ which is sent to the urea cycle to be excreted in the urine as urea
Pyruvate is the alpha keto acid of which amino acid?
Alanine
What are the two major sources of urea cycle deficiencies?
Liver disease and IEM
Defects in any of the 6 enzymes that contribute to the urea cycle can result in hyperammonemia including
NAG synthase which produces N-acetylglutamate (NAG) form glutamate and acetyl CoA
Lack of NAG enzyme results in excessive accumulation of N in the form of ammonia in the blood —> hyperammonemia
Excess ammonia which is a neurotoxin travels to the CNS through the blood resulting in sx and physical findings of NAG deficiency
Sx include vomiting, refusal to eat, progressive lethargy and coma
Autosomal recessive inheritance
What is another example of a defective enzyme in the urea cycle?
Ornithine transcarbamoylase
Causes hyperammonemia with orotic aciduria
X linked inheritance
Describe ammonia toxicity
Excessive ammonia due to disorders in the urea cycle or liver failure can have highly toxic effects on the brain and CNS
NH3 is a toxic agent bc of its ability to permeate membranes
Causes pH imbalance, swelling of astrocytes which leads to cerebral edema and intracranial HTN
Postsynaptic excitatory proteins are inhibited which depresses CNS fxn
Depletion of glutamate results in disruption of its neurotransmitter activity (key reactant in formation of GABA)
Ammonia also causes mito dysfunction
Urea cycle and the high protein diet
Urea production is increased by a high protein diet and decreased by a high carb diet
Insulin and glucagon play a role in urea production
About 20-30% of urea produced is hydrolyzes in the GI tract by bacterial urease which provides a source of N for gut bacteria (salvage and reuse)
High protein diets enhance this production and hydrolysis
THB/BH4 cycle
Cofactor for hydroxylase enzymes and important in the metabolism of Phe, the Trp derivative pathway and the Tyr derivative pathway (latter two - biosynthesis of serotonin, melatonin, dopamine, NE and epinephrine)
