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Medical Biochemistry > Protein Digestion, Amino Acid Absorption and Intertissue Trafficking > Flashcards

Protein Digestion, Amino Acid Absorption and Intertissue Trafficking Flashcards

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USMLE® Step 1 flashcards
1
Q

amino acid utilization

A

protein synthesis

degradation/ energy source

§Direct NADH/FAD(2H) production

§TCA cycle intermediates

§Gluconeogenesis

§Ketone bodies

synthesis of other biomolecules

§Purines, pyrimidines

§Phosphatydilserine, sphingosine

§Thyroxine, epinephrine, melatonin

§ Acetylcholine, GABA

§Heme, histamine, creatine, carnitine, etc

cell signaling (Gly-Glu)

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2
Q

intestinal digestion of epithelial proteins

A

Intestinal epithelial cells can absorb only amino acids or dipeptides.

Secretions:

  1. Pancreatic secretion: HCO3-, zymogens
    • trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase
  2. Gut epithelial cell enzymes: Enteropeptidase (proenzyme activation)
  3. Epithelial cell enzymes: aminopeptidase, dipeptidase

Note: bc of 1 &2: active enzymes (duodenum): trypsin, chymotripsin, elastase, and carboxypeptidase

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3
Q

Proenzymes

A

digestive proteases are secreted as proenzyme

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4
Q

Proenzyme activation

  • initiation
  • reactions
  • location
A

by proteolytic cleavage

  • Enteropeptidase activates trypsinogen and produces trypsin.
  • Trypsin activates other enzymes
    • chymotrypsibnogen
    • proelastase
    • procarboxypeptidase
  • All take place in small intestine (duodenum)
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5
Q

Pepsin activation

A

H+ (stomach PH ~ 2) activates pepsinogen

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6
Q

Nitrogen balance

Normal

Positive

Negative

A
  • Nintake = Nexcretion
    • Nintake > Nexcretion
      • Growth
      • recovery
    • Nintake < Nexcretion
      • illness
      • malnutrition

Nintake is provided by dietry proteins

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7
Q

diagnostic indicator of untreated malabsorption/ malnutrition caused by loss of digestive proteases:

Treatment:

A

Low serum protein level

Albumin is a diagnostic marker

Tx: pancreatic enzyme suplmt

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8
Q

Causes

A- Cystic fibrosis

B- Chronic Pancreatitis (chronic alcoholism)

A

A- CF (chloride channel deficiency): dried secretion blocks the pancreatic duct

B- Loss of enzyme secreting pancreatic cells

Both leads to loss of digestive proteases

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9
Q

Conditions wiht loss of digestive proteases

A
  1. CF
  2. Chronic alcoholism (chronic pancreatitis)
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10
Q

Uptake of dipeptides from the intestinal lumen

A

co-transport with H+ and cleaved inside the cell

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11
Q

dietry amino acid uptake from the intestinal lumen

A

§ Mainly through co-transport with Na+

§ Multiple transporters exist with overlapping specificity.

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12
Q

Release of aa from the serosal side of intestinal cells into the circulation:

  • Release
  • Direction

Uptake of dietry aa from circulation

A

1- Release

Through facilitated transport.

Bidirectional

Allows amino acid uptake during fasting ►energy

2- Uptake

Mainly through co-transport with Na+

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13
Q

Amino acid absorption in the gut vs Amino acid reabsorption in the kidneys

A

The amino acid transport system is the same in the small intestine and in the proximal tubules of the kidney.

If an amino acid cannot be absorbed from the intestines, it cannot be reabsorbed from the glomerular filtrate, either → urine AA Accumulation

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14
Q

Clinical diagnostics of amino acid absorption disease

A

elevated levels of aa in urine

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15
Q

Hartnup disease

Cause

Dx marker

Sx

Tx

A

Disease of AA transport (autosomal recessive)

  • Defect in the absorption of neutral amino acids (hydrophobic) at the brush border.
  • Lack of tryptophan coupled with poor diet (niacin (vitamin B3) deficiency) can lead to pellagra-like symptoms.
  • Tryptophan and vitamin B3 are both precursors for NAD+.
  • Elevated neutral amino acids in urine.

Symptoms:

§Mostly normal clinically.

§Some develop photo-sensitivity, tremors, ataxia, nystagmus

Treatment:

§Niacin-rich diet

§High protein diet (increases the amount of dipeptides that can be taken up)

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16
Q

Common causes of pellagra-like symptoms in harnup disease

A

Combination of:

  1. lack of Tryptophan
  2. Niacin dificiency (B3)
    • brought on by poor diet
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17
Q

Treatment of Hartnup disease

A
  • Is a disorder of aa transport. Specifically, inability to absorb neutral (hydrophobic) amino acids at the intestinal BB.
  • Lack of Trp and B3 causes the pellagra-like symptoms.

Thus, treatment includes:

  1. High protein diet to increase the amt of dipeptides that can be taken up.
  2. Niacin rich diet
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18
Q

Cystinuria

Cause:

lab findings/ Sx:

Tx:

A

Cause:

  • Cystinuria (autosomal recessive) caused by deficient BB transport of COAL- basic aa: Cys, Ornithine, Arg, Lys.

Lab/Sx:

  • Hyperaminoaciduria of COAL
  • Cystine stones in kidneys, ureter and bladder

Tx:

  • High fluid intake
  • Meds to increase urine PH
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19
Q

Pathways of interacellular protein digestion

A
  1. Lysosomal degradation
  2. Ub-proteosome degradation
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20
Q

lysosomal degradation is used for:

A

Endocytosed proteins (turnover of the extracellular matrix)

Phagocytosed extracellular particles (innate immune defense)

Autophagy (degradation of intracellular structures).

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21
Q

Pathway for myosin degradation

Hint: same degradation pathway for TFs

A

Ubiquitin-Proteasome degradation pathway

22
Q

Ubiquitin-Proteasome degradation pathway for:

A

Misfolded/damaged intracellular proteins

Turnover of intracellular proteins (i.e myosin degradation or degradation of transcription factors

23
Q

Cathepsins

A

are the major lysosomal proteases

24
Q

matrix metalloproteinases

A
  • endocytosis and lysosomal protein degradation
  • turnover of the ECM
  • Proteases: metalloproteinase (main)
  • metalloproteinase requires Zn

Note:The main proteases involved in extracellular matrix turnover

are matrix metalloproteinases (require zinc ion for activity).

25
Q

Macro-auphagy and lysosomal protein degradation is ideal for:

A

Organelles: mt, ER, Peroxisomes, nuc. env., etc

Protein aggregates: aggregates or misfolded proteins

intracellular bacteria

lysosomal acid hydrolases

26
Q

Ubiquitin-mediated proteasomal degradation of cytoplasmic proteins:

Mechanism

location of the machinery

A
  1. The proteins that are destined for degradation have to be tagged.
  2. The tag is ubiquitin (a small protein) – post-translational modification.
  3. The machinery is present in the cytosol and the nucleus
27
Q

Pharmacological correlation: Proteasome inhibitors

A

Bortezomib

  • Approved to treat multiple myeloma (a plasma cell leukemia).
  • Prevents the degradation of proteins that promote cell death.
28
Q

Protesome

A

A cylindrical protein complex

The core subunits have proteolytic activity.

Accessory (CAP) proteins guide the target into the pore of the proteasome.

29
Q

Angelman Syndrome

Ub disorder

A
  • Caused by a defective E3 subunit (UBE3A) in brain neurons.
  • Prevents the degradation of certain proteins in hippocampus and cerebellum.
  • The levels of these proteins in neurons will be higher than the homeostatic range.
  • Leads to a neurodevelopmental disease.
      • delayed development and intellectual disability
      • unusual happy demeanor, frequent laughing
      • speech impairment
      • movement and balance disorders
      • seizures
30
Q

Von Hippel-Lindau (VHL) syndrome

Ubiquitination disorder

  1. Cause
  2. Mechanism
  3. blood level values
  4. Clinical manifestation
A

Caused by a defective E3 subunit (VHL).

Prevents the degradation of hypoxia inducible factor subunits (HIF1a and HIF2a) that promote hypoxic responses such as blood vessel formation.

The level of HIFs will be high even under normoxic conditions.

Leads to tumor/cancer formation.

  • Vascular tumors in brain, spinal cord amd retina (hemangioblastomas)
  • Kidney carcinoma
  • Adrenal gland tumor (pheochromacytoma)
31
Q

A from of ubiquitination disorder

Increased hypoxic responses (i.e. blood vessel formation)

High levels of HIF even during normoxic conditions

Ultimately, cancer, tumor formation (i.e. vascular tumors)

A

Von Hippel-Lindau (VHL) syndrome

defective E3 subunit → high levels of HIFs

functional E3 subunit is responsible for degradation of HIFs

* HIF: hypoxia inducible factor subunits

32
Q

Ubiquitination disorders caused by:

A- defective E3 subunit (UBE3A) in brain neurons.

elevated levels of undesired protein s in hippocampus and cerebellum (higher levels than homeostatic range)

B- defective E3 subunit (VHL)

elevated HIF

A

A- Angelman syndrome

B- Von Hippel-Lindau (VHL) syndrome

33
Q

Storage and utilization of amino acids

  1. Locaiton and form of storage
  2. major site
  3. utilization
A

There is no specialized storage for amino acids in the human body. Instead, amino acids are stored dynamically in functional proteins.

80% of the body’s proteins are in the skeletal muscle.

When the body needs amino acids, muscle proteins (mainly myosin) break down, and the amino acids are delivered by the circulation to sites of utilization.

34
Q

Primary function of the blood amino acid pool

A

The primary function of the blood amino acid pool is to traffic amino acids between tissues for the production of biomolecules, energy or waste products (urea).

35
Q

AA utilization

Fed State

A

The vast majority of amino acids are delivered to the liver by the hepatic portal vein.

Amino acids are also transported to the peripheral tissues to produce proteins.

Excess amino acids are converted to glycogen and triglycerides.

36
Q

AA utilization

Fasting

A
  1. Muscle: Protein degradation ► AA
  2. Liver: AA conversion to Glu and KB (energy for other cells)
  3. Ala major glucogenic AA
37
Q

Kidney AA utilization

  1. Energy
  2. Purposes
  3. Provided to the AA pool
A
  1. Gln
  2. Gln - balance urine PH
  3. Ala
38
Q

Skeletal muscle AA utilization

  1. Energy
  2. Purposes
  3. Provided to the AA pool
A
  1. BCAA
  2. All
  3. All (maj: Gln and Ala)
39
Q

Intestine AA utilization (Fed)

  1. Used for energy
  2. Other purposes
  3. Provided to the AA pool
A
  1. Gln, Asp, Glu
  2. Gln (synthesis of citruline & ornithine)
  3. Ala
40
Q

Intestine AA utilization (fasting)

  1. Used for energy
  2. Other purposes
  3. Provided to the AA pool
A
  1. Gln, BCAA
  2. Gln (synthesis of Citruline and Ornithine)
  3. Ala
41
Q

Liver AA utilization

  1. Used for energy
  2. Other purposes
  3. Provided to the AA pool
A
  1. All
  2. Ala mainly (gluconeogenesis)
  3. non-essential AA (de novo synthesis)
42
Q

Brain AA utilization

  1. Used for energy
  2. Other purposes
  3. Provided to the AA pool
A
  1. -
  2. many (NT)
  3. Gln
43
Q

Glutamine utilization by the intestine during fasting and fed states (common to both states)

A
  • Used for energy
  • Used for the synthesis of Citruline and ornithine

Note*

Fed: I_n addition to Gln,_ intestine uses Asp and Glu for energy

Fasting: In addition to Gln, intestine uses BCAA for energy

44
Q

Tissues that provide Ala to the AA pool

A
  1. Kidney
  2. Skeletal muscle (provide all AA but Ala and Gln are major)
  3. Intestine-Fed
  4. Intestine- Fasting
  5. Liver (Ala is one of the 11 non-essential amino acids provided via de novo synthesis)
45
Q

Tissues that provide Glutamine (Gln) to the AA pool

A
  1. Skeletal muscle (provides all, but Ala and Gln are mjor)
  2. Brain (Only Gln)
  3. Liver via de novo syntheis (non-essential aa)
46
Q

Tissues capable of utilizing BCAA for energy

A
  1. Skeletal muscle
  2. Intestine (fasting)
47
Q

Normal use of AA in brain

A

NT synthesis: Astroglial cells and neurons

  1. Transporterd through BBB by transporters
    • The most important ones are BCAAs, Phe, Tyr, Trp, His.
      • Tyr, Trp, His and Met are important for synthesizing neurotransmitters in the brain.
  2. Astroglial cells: BCAA(transamination) → Glu → Gln
  3. Gln transported to neurons
  4. Neurons: Gln → Glu & GABA
48
Q

A tissue that is the major user of the blood glutamine pool and its purpose

A

Kidney (to generate ammonia)

49
Q

The use of Glutamine by Kidney

A

To generate ammonia

Kidney is the major user of the blood Gln pool

Renal tubular cells produce ammonia to maintain urine pH (neutralize excreted protons) and this process is coupled to bicarbonate release to the circulation.

50
Q

Glutamine usage in acidosis

A
  • Kidney uses even more glutamine for ammonia production.
  • This leads to more HCO3- release back to the circulation helping to balance blood pH.
51
Q

Glutamine and fast-dividing cells

lymphocytes

Epithelial cells of the gut and tumor

A

Fast dividing cells use Gln as energy source

Gln A→ GluB → a-KetoglutarateC ⇒ TCA

A- glutaminase

B- TA or Glutamate dehydrogenase

C- aketoglutrate dehydrogenase (makes NADH and Succinyl Co-A)

52
Q

AA utilization during generalized acute infection

Sepsis

A

Requires new protein synthesis bc:

  • Hepatocytes produce acute phase proteins
  • Immune cells rapidly divide to fight infectious pathogen

Medical Biochemistry (12 decks)

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