Protein And AA Metabolism

Question 1

What are inborn errors of metabolism?

Answer 1

Conditions associated with dysfunctional metabolism of amino acids and derivatives

Question 2

What is an amino acid pool?

Answer 2

Population of free amino acids supplied by protein turnover, digested food and de novo synthesis (of NE aa)

Question 3

What is the amino acid pool depleted by?

Answer 3

Production of body protein, synthesis of N containing compounds and degradation

Question 4

Which amino acids are considered conditionally essential?

Answer 4

Arg (prematurity), Cys, Gly, Gln, Pro and Tyr

Question 5

What is the major source of nitrogen?

Answer 5

Dietary protein

Question 6

What are the major nitrogen excretory compounds?

Answer 6

Urea, ammonia, creatine (muscle), uric acid (purine breakdown product) and urobilinogen (heme metabolism)

Question 7

How many g of protein is needed to maintain the amino acid pool?

Answer 7

30g of protein per day

Question 8

Characteristics of Hartnup and Cystinuria

Answer 8

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

Question 9

Describe Hartnup disease**

Answer 9

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

Question 10

Describe Cystinuria

Answer 10

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

Question 11

What can overcome the deficiency of transport of neutral amino acids in most patients with Hartnup disease?

Answer 11

A high protein diet

Question 12

Biosynthesis of aromatic amino acids Try and Trp

Answer 12

Phe —> Tyr

Ribose 5 P —> His

Question 13

Biosynthesis of serine

Answer 13

3PG —> Ser —> Cys or Gly

Question 14

Biosynthesis of aspartate

Answer 14

OAA —> Asp —> Asn

Question 15

Biosynthesis of glutamate

Answer 15

Alpha ketoglutarate —> Glu —> Gln, Pro and Arg

Question 16

The biosynthesis of amino acids involves what type of reaction?

Answer 16

Transamination reaction from an alpha keto acid

Question 17

What are the three protein degradation pathways?

Answer 17

Proteasome, lysosome and autophagosome

Question 18

Describe the lysosomal/autophagy pathway of protein degradation

Answer 18

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)

Question 19

What is the common denominator in the targeting of substrates in all three protein degradation pathways?

Answer 19

Ubiquitin

Question 20

Describe proteasomal degradation

Answer 20

Large proteasome cytoplasmic complexes cleave polyubiquitinated proteins —> ubiquitin pathway

Question 21

What is the signal for proteolysis in the proteasome?

Answer 21

Ubiquitin

Question 22

Structure of the proteasome

Answer 22

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

Question 23

What is the role of isopeptidase in the proteasomal degradation pathway?

Answer 23

It cleaves ubiquitin in the regulatory unit for reuse so the cell doesn’t have to make more Ubq

Question 24

What is the nucleophile in the proteasomal degradation pathway?

Answer 24

OH of threonine acts as a nucleophile to attach carbonyl of peptide bonds degraded to peptides ranging from 7-9 amino residues

Question 25

Describe extracellular proteolytic control

Answer 25

Proteolytic enzymes are secreted as inactive zymogens as needed Activated by proteolytic cleavage Enterokinases are embedded in the intestinal mucosa

Question 26

Example of extracellular proteolytic control

Answer 26

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

Question 27

Amino acids can be classified by what?

Answer 27

By what they are metabolized to

Question 28

Describe ketogenic amino acids

Answer 28

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)

Question 29

Which amino acids are ketogenic only?

Answer 29

Leu and Lys

Question 30

Which amino acids are both ketogenic and glucogenic?

Answer 30

Ile, Trp, Phe, Tyr and Thr

Question 31

Which amino acids are glucogenic only?

Answer 31

Val, His, Arg, Asn, Gln, Met, Ala, Asp, Glu, Gly, Pro, Ser, Cys

Question 32

Glucogenic amino acids are eventually converted into what?

Answer 32

Glucose

Question 33

What are the three most important reactions/enzymes in amino acid metabolism?

Answer 33

1. Aminotransferase/transaminase 2. Glutamate dehydrogenase (oxidative deamination) 3. Urea cycle

Question 34

True or false: Every amino acid has a conjugate keto acid

Answer 34

True

Question 35

OAA is the keto acid of which amino acid?

Answer 35

Aspartate

Question 36

The amine group of amino acids is shuffled into the

Answer 36

Liver as other amino acids then repackaged as urea in the urea cycle and excreted

Question 37

Describe the shuffling of amino acids through transamination

Answer 37

Amino group is transferred to an alpha keto acid Coupled reactions Enzymes = transaminases/aminotransferases

Question 38

Describe transaminases (aminotransferases)

Answer 38

Require coenzyme pyridoxal 5’-phosphate (PLP) which is a derivative of vitamin B6 Clinical relevance: ALT and AST

Question 39

Describe how amino transferases are used in a clinical setting

Answer 39

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

Question 40

Metabolism of Gln, His, Arg, Pro and Glu

Answer 40

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

Question 41

Metabolism of Met, Thr, Ile and Leu

Answer 41

Methionine pathway —> homocystinuria | Cystathionine beta-synthase needs PLP coenzyme

Question 42

Clinical manifestations in the Met cycle (and also folate cycle since THF is used as a methyl carrier) includes what?

Answer 42

Homocystinuria

Question 43

Describe the role of homocysteine methyltransferase enzyme in homocystinuria

Answer 43

Defect in this enzyme can cause secondary homocytisinuria Uses cobalamin/vit B12 as coenzyme and THF as methyl carrier Converts homocysteine to methionine

Question 44

Describe the role cystathionine beta synthase (CBS) enzyme plays in homocytinuria

Answer 44

Converts homocysteine —> cystathionine Needs PLP as a coenzyme (active form of vitamin B6) Mutations in this enzyme are the most common cause of homocystinuria

Question 45

Consequences of homocystinuria and homocysteinemia can affect four organ systems including

Answer 45

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

Question 46

What are hyperhomocysteinemia and homocystinuria?

Answer 46

Vitamin deficiencies such as B6, B12 and folic acid or genetic defects in enzymes (CBS) respectively that cause defective metabolism of homocysteine

Question 47

Hyperhomocysteinemia is a risk factor for what?

Answer 47

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

Question 48

What can normalize plasma homocysteine levels in some cases?

Answer 48

Vitamin supplementation (with B vitamins)

Question 49

Describe the metabolism of branched chain amino acids (BCAs)

Answer 49

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)

Question 50

Deficiencies in the BCA metabolism pathway lead to what condition?

Answer 50

Maple syrup urine disease (MSUD)

Question 51

Describe maple syrup urine disease

Answer 51

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

Question 52

Which population presents with higher risk of MSUD?

Answer 52

Mennonite, Amish and Jewish populations

Question 53

Asparagine metabolizes to what?

Answer 53

Aspartate by the removal of an amine group by asparaginase

Question 54

What is the best known clinical presentation of deficiencies in metabolism of Phe?

Answer 54

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

Question 55

Describe phenylketonuria (PKU)

Answer 55

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

Question 56

Secondary PKU can result from

Answer 56

``` Tetrahydrobiopterin deficiency (a cofactor for PAH) Defects in synthesis of regeneration of BH4 ```

Question 57

Tryptophan derivatives

Answer 57

Same THB/DHB cycle in Phe —> tyrosine metabolism Serotonin is the p-hdyroxylated and decarboxylated form of tryptophan Makes niacin and melatonin

Question 58

Tyrosine derivatives

Answer 58

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

Question 59

Hypothyroidism =

Answer 59

High TSH | Low T4

Question 60

Hyperthyroidism =

Answer 60

Low TSH | High T4/T3

Question 61

How are albinism and tyrosinase connected?

Answer 61

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

Question 62

Thyroglobulin and thyroid hormones

Answer 62

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

Question 63

Ammonia is removed as what?

Answer 63

Glu and Gln in the brain by glutamine synthase | As Glu in other tissues

Question 64

How is urea generated?

Answer 64

In amino acid metabolic pathways by deamination mechanisms

Question 65

Where is urea produced?

Answer 65

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

Question 66

Structure of ammonia

Answer 66

2 amino groups (NH2) | 1 C=O bond

Question 67

Increased entry of ammonia to the brain is a primary cause of what?

Answer 67

Neurologic disorders such as congenital deficiencies of urea cycle enzymes, hepatic encephalopathies, Reye syndrome, several other metabolic disorders and some toxic encephalopathies

Question 68

A blood ammonium concentration of what is associated with coma and convulsions?

Answer 68

200umol/L

Question 69

Describe the removal of excess NH4+ from the brain

Answer 69

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

Question 70

Describe the removal of NH4+ from muscle

Answer 70

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

Question 71

Pyruvate is the alpha keto acid of which amino acid?

Answer 71

Alanine

Question 72

What are the two major sources of urea cycle deficiencies?

Answer 72

Liver disease and IEM

Question 73

Defects in any of the 6 enzymes that contribute to the urea cycle can result in hyperammonemia including

Answer 73

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

Question 74

What is another example of a defective enzyme in the urea cycle?

Answer 74

Ornithine transcarbamoylase Causes hyperammonemia with orotic aciduria X linked inheritance

Question 75

Describe ammonia toxicity

Answer 75

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

Question 76

Urea cycle and the high protein diet

Answer 76

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

Question 77

THB/BH4 cycle

Answer 77

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)