Metabolic Disorders Exam #2

Congenital Enterokinase Deficiency

• cause: low trypsin as a result of a dysfunctional enterokinase

  • enterokinase is used to active trypsinogen into trypsin

• symptoms: failure to thrive, chronic diarrhea, low serum protein, and generalized edema

  • diagnosed at infancy, grew to adults leading normal lives

• treatment: isolated enterokinase is added, trypsin activity returns to normal

  • some with this deficiency have residual enterokinase activity

  • thus, trypsinogen can be self-activated at a slow rate. but once it is activated, it leads to a positive feedback loop

How are some adults able to live with congenital enterokinase deficiency, while infants have profound issues related to malabsorption?

• the self-activated trypsinogen mechanism is sufficient for adults but not infants because infants have a higher demand for protein digestion

describe how protein in ingested.

adults needs 60-100g of protein a day

• protein is digested into amino acids

• the amino acids are absorbed by the enterocyte

• enter the amino acid pool

  • the pool is derived from the diet and proteolysis of proteins

describe the characteristics of amino acids

• 20 amino acids

  • each differ in their R-group

  • R-group determines the chemical properties of the amino acid

    • charged side chain

    • uncharged side chain

    • hydrophobic side chains

    • special cases

what is the difference between synonymous SNP and non-synonymous SNP?

synonymous SNP

• change in the base, but not the codon

  • ex: ACU and ACC both code for Thr

synonymous SNP and non-synonymous SNP

• change in the base and the codon

  • in severe cases, it can result in a stop codon

• disruptions can affect enzyme activity, interactions with other proteins, or ligand-binding

  • ex: arg —> leu

    • results in a conformational change in the structure of the protein

what are the 6 phases of amino acid digestion and absorption?

1. mechanical breakdown

• the physical breakdown of food in the mouth

2. digestion that starts in the stomach

• gastric hydrolysis of the peptide bonds in proteins

3. digestion continues in the small intestines

• pancreatic proteases further breaks down the proteins into smaller peptides

  • the proteases are secreted as zymogens and activated in the small intestines

• brush border membrane peptidases further hydrolyzes peptide linkages

4. transport of amino acids

• amino acids, dipeptides, and tripeptides cross the brush border via enterocytes

5. digestion of dipeptides and tripeptides

• cytoplasmic peptidases in the enterocyte further breakdown the peptides

6. transport across the basolateral membrane

describe the gastric phase

location: the stomach

• plays a minor role in overall digestion

• gastric HCl and pepsins

  • gastric chief cells release pepsinogens (inactive)

  • the pepsinogens are activated in the presence of acid into pepsin

    • pepsin acts as an endopeptidase to target peptide bonds

purpose:

• prepare polypeptides for digestion and absorption within the small intestine

describe the small intestinal luminal phase

location: small intestine

• mucosal cells release cholecystokinin (CCK)

• CCK reaches the pancreas and binds to acinar cells to stimulate release of zymogens via the pancreatic duct and common bile duct

  • trypsinogen

    • enterokinase cleaves 8 amino acids off of trypsinogen to produce trypsin

    • trypsin activates the other zymogens and forms a pool of activated proteases

purpose:

• further breakdown peptides into amino acids

describe the mucosal phase

location: small intestines

• after pancreatic hydrolysis, free amino acids, dipeptides, tripeptides, and oligopeptides remain

  • oligopeptides must be hydrolyzed by membrane-bound enzymes in the brush border

  • dipetides and tripeptides must be hydrolyzed by cystolic aminopeptidases

purpose:

• further breakdown peptides into amino acids

what are amino acid transporters?

purpose: amino acids transporters moves protein in and out of the cell

• belong to solute carriers superfamily

describe the X- and x- amino acid transporters.

purpose: transports anionic amino acids

• X- AG: aspartate and glutamate

• x- C: acceptance of cysteine

describe the Y+ and y+ amino acid transporters.

purpose: transports cationic amino acids

• ex: arginine and lysine

describe the B and b amino acid transporters.

purpose: transports neutral amino acids

describe the T amino acid transporters.

purpose: transports aromatic amino acids

• such as tyrosine and tryptophan

describe the N amino acid transporters.

purpose: transports neutral amino acids with a selectivity for amino acids with N in the side chain

describe the IMINO amino acid transporters.

purpose: transports proline and hydroxyproline

which amino acid transporters are sodium-dependent and sodium-independent?

• sodium-dependent

  • capital letter abbreviations

• sodium-independent

  • lower letter abbreviations

  • system L, system T, and proton-coupled AATs

Cystinuria

cause: defective amino acid transporter on the apical membrane of the small intestines and renal proximal tubules

• results in amino acids such as L-cystine, lysine, arginine, and ornithine not being transported across the membrane

  • leads to hyperexcretion of these amino acids in the urine because they are unable to be reabsorbed

  • this results in high concentrations of cystine, which forms crystals in the kidneys and bladder and causes kidney stones

• mutations in either the amino acid transport or the functional unit can lead to cystinuria

symptoms: nausea, hematuria, flank pain, kidney stones, and frequent urinary tract infections

• screening can be due to (1) family history of cystinuria (2) recurrent kidney stones (3) development of stones at an early age

• testing stone and determining that it consists of cystine

treatment:

1. initial approach -- prevention of stone formation

• drinking more fluids or decreasing sodium intake

• reducing animal protein rich in methionine

  • methionine is broken down into cystine in the body

2. drug therapies -- thiol agents to form mixed disulfides that are more soluble than cystine, lowering free cystine levels

• penicillamine & tiopronin

Cystinosis

cause:

• cysteine enters the lysosome, forming cystine

• cystinosin (CTNS) is a membrane transporter that removes cystine from the lysosome

• BUT, in cystinosis, CTNS is not functional, leading to a build up of cystine and causes cystine crystals inside the lysosome

  • this can cause damage to the kidneys, and other organs as it progresses

    • ex: upper GI tract, liver, and lower GI tract

symptoms:

• dehydration, polyuria, polydipsia, metabolic acidosis, hypokalemia, etc.

treatment:

• cystine-depleting therapy -- breaks cystine into cysteine and cysteine-cysteamine -- creates mixed disulfides

  • these products are able to leave the lysosome independently of the cystinosin transporter

  • cysteine leaves via the cysteine transporter and cysteine-cysteamine leaves via the PQLC2 transporter

what are the 3 types of amino acids?

3 types of amino acids:

• essential

  • cannot be synthesized in the body, and must be obtained from the diet

• non-essential

  • can be synthesized in the body

    • nitrogen is required

• conditionally essential

  • only synthesized if essential amino acids precursors are provided

what are the 3 major metabolic fates of amino acids?

1. protein synthesis

2. precursors

3. catabolism

where does the nitrogen come from when synthesizing non-essential amino acids?

• nitrogen is provided by the diet in the form of alpha-amino groups

  • however, the body has a limited ability to incorporate inorganic nitrogen into amino acids

what is transamination?

Transamination:

• removing an amino group and adding it to a keto-acid to form a different amino acids

  • catalyzed by transaminase enzymes

    • these enzymes are particularly found in the liver, heart muscle, skeletal muscle, and kidney

  • transaminase requires vitamin B6 as a cofactor

    • a vitamin B6 deficiency can decrease the rate of transamination in tissues

• the preferred amino acid/keto-acid is:

  • alpha-ketoglutarate/glutamate

what amino acids do not participate in transamination?

• threonine

• lysine

• proline

Hypervalinemia & Hyperleucine-isoleucinemia (HVLI)

cause:

• elevated levels of valine, leucine, and isoleucine in the blood and urine

  • results from a transaminase enzyme deficiency - specifically the BCAT enzyme

    • the BCAT enzyme is the 1st step to break down valine, leucine, and isoleucine

    • if not broken down, it can lead to build up of these amino acids in the brain and blood

  • BCAT 1 - cystolic form of the enzyme

    • rarely found

  • BCAT 2 - mitochondrial form of the enzyme

    • most common cause of hypervalinemia

symptoms:

• present at birth

• protein intolerance, metabolic acidosis, vomiting, failure to thrive, coma

• delayed mental and physical development

treatment:

• diet low in valine, leucine, and isoleucine

  • returns valine levels to normal

• vitamin B6 supplementation

  • restore normal valine levels in patients with BCAT2 mutations

what is deamination?

Deamination:

• the process by which the amino group is removed to form a keto-acid and ammonia

  • the ammonia is excreted as urea

• the most common deamination is the release of ammonia by glutamate dehydrogenase (GDH)

  • converting glutamate into alpha-ketoglutarate, ammonia, and NAD(P)H

  • found in the mitochondria

  • present at high levels in the liver, kidney cortex, and brain

• examples of deamination reactions:

  • glutamate dehydrogenase deficiency

  • histidinemia

Glutamate Dehydrogenase Deficiency

cause:

• glutamate is converted into alpha-ketoglutarate by glutamate dehydrogenase

  • alpha-ketoglutarate is important for energy production (krebs cycle) and protein metabolism (transamination & deamination)

• during a deficiency, an build-up of ammonia and excess of glutamate

  • build-up of ammonia: if no more alpha-ketoglutarate, this stalls the process that clears nitrogen from the body

    • leads to a build-up of ammonia

  • excess glutamate: unable to regulate the glutamine and glutamate cycle

    • leads to a build-up of glutamate, and overexcites the brain

      • glutamate is an important neurotransmitter

symptoms:

• mimics Parkinson’s disease

• seizures, brain fog, memory issues

Histidemia

cause:

• deficiency in histidase prevents the conversion of histidine into trans-urocanic acid, resulting in ammonia not being released

  • elevated levels of histidine levels in the blood and urine

symptoms:

• mostly asymptomatic, but some clinical symptoms were reported in patients

describe the glycine cleavage system.

purpose: uses glycine to donate a methyl to tetrahydrofolate

• this cleavage system is important because it continues to methylate tetrahydrofolate in order for the folate cycle to continue running

4 different enzymes in the cleavage system:

• GCSH, GLDC, DLD, and AMT

  • GLDC: needed for the release of CO2 and the transfer of glycine to GCSH

Nonketotic Hyperglycinemia (NKH)

cause:

• results for a GLDC not functioning in the glycine cleavage system

  • nonfunctional GLDC accounts for most cases in NKH

• leads to a build-up of glycine and accounts for most cases of NKH

symptoms:

• high levels of glycine in the brain

• neurological problems & global development delay

• coma & death

treatment:

• sodium benzoate

  • combines with glycine in the liver to promote excretion

• dextromethorphan

  • stops glycine receptors from overfiring

• diet

  • limiting gelatin -- high in glycine

  • coordination for swallowing and breathing may be needed

    • oral feeding by bottles, etc.

    • nasogastric tube or gastronomy tube

  • low glycine diet, ketogenic diet

what are the 3 types of NKH?

1. classic NKH

• severe mutations in the core GCS genes

2. variant NKH

• mutations in the genes outside of the GCS system

3. transient NKH

• short-term and resolves eventually

what is deamidation?

Deamidation:

• removing the amide group to release the ammonia

1. glutamine -- metabolized by glutaminase

2. asparagine -- metabolized by asparaginase

what is incorporation of ammonia?

• most reactions utilize nitrogen in the amino or amide groups. however, some reactions can use ammonia

  • ex: glutamate dehydrogenase -- can function in the reverse direction, and can incorporate ammonia into glutamate

Describe the urea cycle.

Purpose: clearing the waste nitrogen from protein turnover

• composed of 6 enzymes and 2 transporters

enzymes:

• 5 catalytic enzymes

  • CPS1, OTC, ASS1, ASL, and ARG1

• 1 cofactor-producing enzyme

  • NAGS

• 2 amino acid transporters

  • ORNT1, citrin

*KNOW: location of the enzymes/transporters -- mitochondria or cytoplasm

Urea Cycle Disorders

cause:

• any genetic defects in the enzymes or transporters

  • leads to high levels of ammonia which increases glutamate and depletes ATP

    • can cause brain damage

  • build-up of ammonia, which is a toxin

• enzymes at the top of the diagram are more lethal if non-functional compared to those at the bottom

symptoms:

• lethargy, anorexia, seizures, coma, etc.

  • 1st signs: drowsiness and failure to feed

• hyperammonemia

  • loss of appetite, lethargy, vomiting, behavioral abnormalities

treatment:

• low-protein diet

• medications to remove ammonia from the blood

  • sodium benzoate

  • sodium phenylacetate

• supplements

  • arginine and citrulline

what is the outcome of the urea cycle testing?

• ARG1

  • increases arginine

• ORNT1

  • increase in ornithine, and decrease in citrulline

• ASS1

  • increase in citrulline, and decrease in arginine

what is an organic acid?

• organic compounds with acidic properties -- many are carboxylic acids

  • can be found as physiological intermediates in other metabolic pathways

    • ex: sulfonic acids

• produced from the breakdown of amino acids

describe organic acidemias

• group of disorders characterized by increased excretion of non-amino acids in the urine

  • this means the body is able to remove the nitrogen from the amino acid, but cannot process the carbon skeleton to form the organic acid

• organic acidemias disturb mitochondrial energy metabolism

  • this means that they have a large range of clinical and biochemical ways to manifest in the TCA cycle

1. classic organic acidemias

• systemic effects

2. cerebral organic acidemias

• brain effects

how are organic acidemias detected during newborn screening?

• if there is a metabolic defect in the TCA cycle, this leads to acyl-CoA build-up

  • the accumulated acyl-CoA compounds are esterified with free carnitine and leads to an increase in acylcarnitines

• the acylcarnitines can be detected during newborn screening

what are the clinical manifestations of organic acidemias?

• energy deficiency

• targets brain, liver, kidney, pancreas, retina, and other organs

describe clinical signs of the 3 types of enzyme activity for organic acidemias.

1. complete absence of enzyme activity

• first few weeks of life

• severe, acute-life threatening attacks

  • provoked by a stress such as an infection, trauma, etc.

  • an attack is . . . metabolic acidosis, hyperammonemia, and hypoglycemia

• caused by: accumulation of neurotoxic organic acids and their acyl-CoA esters

2. residual enzyme activity

• symptom-free between attacks

  • provoked typically by an infection

• neurological signs, movement disorder, epilepsy, feeding difficulties, and intellectual disability

3. insidious, late-onset forms

• progressive neurological and GI symptoms

• no apparent crisis during disease progression