Protein Digestion, Amino Acid Absorption and Intertissue Trafficking Flashcards
amino acid utilization
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)
intestinal digestion of epithelial proteins
Intestinal epithelial cells can absorb only amino acids or dipeptides.
Secretions:
- Pancreatic secretion: HCO3-, zymogens
- trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase
- Gut epithelial cell enzymes: Enteropeptidase (proenzyme activation)
- Epithelial cell enzymes: aminopeptidase, dipeptidase
Note: bc of 1 &2: active enzymes (duodenum): trypsin, chymotripsin, elastase, and carboxypeptidase
Proenzymes
digestive proteases are secreted as proenzyme
Proenzyme activation
- initiation
- reactions
- location
by proteolytic cleavage
- Enteropeptidase activates trypsinogen and produces trypsin.
- Trypsin activates other enzymes
- chymotrypsibnogen
- proelastase
- procarboxypeptidase
- All take place in small intestine (duodenum)
Pepsin activation
H+ (stomach PH ~ 2) activates pepsinogen
Nitrogen balance
Normal
Positive
Negative
- Nintake = Nexcretion
- Nintake > Nexcretion
- Growth
- recovery
- Nintake > Nexcretion
- Nintake < Nexcretion
- illness
- malnutrition
- Nintake < Nexcretion
Nintake is provided by dietry proteins
diagnostic indicator of untreated malabsorption/ malnutrition caused by loss of digestive proteases:
Treatment:
Low serum protein level
Albumin is a diagnostic marker
Tx: pancreatic enzyme suplmt
Causes
A- Cystic fibrosis
B- Chronic Pancreatitis (chronic alcoholism)
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
Conditions wiht loss of digestive proteases
- CF
- Chronic alcoholism (chronic pancreatitis)
Uptake of dipeptides from the intestinal lumen
co-transport with H+ and cleaved inside the cell
dietry amino acid uptake from the intestinal lumen
§ Mainly through co-transport with Na+
§ Multiple transporters exist with overlapping specificity.
Release of aa from the serosal side of intestinal cells into the circulation:
- Release
- Direction
Uptake of dietry aa from circulation
1- Release
Through facilitated transport.
Bidirectional
Allows amino acid uptake during fasting ►energy
2- Uptake
Mainly through co-transport with Na+
Amino acid absorption in the gut vs Amino acid reabsorption in the kidneys
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
Clinical diagnostics of amino acid absorption disease
elevated levels of aa in urine
Hartnup disease
Cause
Dx marker
Sx
Tx
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)
Common causes of pellagra-like symptoms in harnup disease
Combination of:
- lack of Tryptophan
- Niacin dificiency (B3)
- brought on by poor diet
Treatment of Hartnup disease
- 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:
- High protein diet to increase the amt of dipeptides that can be taken up.
- Niacin rich diet
Cystinuria
Cause:
lab findings/ Sx:
Tx:
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
Pathways of interacellular protein digestion
- Lysosomal degradation
- Ub-proteosome degradation
lysosomal degradation is used for:
Endocytosed proteins (turnover of the extracellular matrix)
Phagocytosed extracellular particles (innate immune defense)
Autophagy (degradation of intracellular structures).
Pathway for myosin degradation
Hint: same degradation pathway for TFs
Ubiquitin-Proteasome degradation pathway
Ubiquitin-Proteasome degradation pathway for:
Misfolded/damaged intracellular proteins
Turnover of intracellular proteins (i.e myosin degradation or degradation of transcription factors
Cathepsins
are the major lysosomal proteases
matrix metalloproteinases
- 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).
Macro-auphagy and lysosomal protein degradation is ideal for:
Organelles: mt, ER, Peroxisomes, nuc. env., etc
Protein aggregates: aggregates or misfolded proteins
intracellular bacteria
lysosomal acid hydrolases
Ubiquitin-mediated proteasomal degradation of cytoplasmic proteins:
Mechanism
location of the machinery
- The proteins that are destined for degradation have to be tagged.
- The tag is ubiquitin (a small protein) – post-translational modification.
- The machinery is present in the cytosol and the nucleus
Pharmacological correlation: Proteasome inhibitors
Bortezomib
- Approved to treat multiple myeloma (a plasma cell leukemia).
- Prevents the degradation of proteins that promote cell death.
Protesome
A cylindrical protein complex
The core subunits have proteolytic activity.
Accessory (CAP) proteins guide the target into the pore of the proteasome.
Angelman Syndrome
Ub disorder
- 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
Von Hippel-Lindau (VHL) syndrome
Ubiquitination disorder
- Cause
- Mechanism
- blood level values
- Clinical manifestation
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)
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)
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
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- Angelman syndrome
B- Von Hippel-Lindau (VHL) syndrome
Storage and utilization of amino acids
- Locaiton and form of storage
- major site
- utilization
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.
Primary function of the blood amino acid pool
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).
AA utilization
Fed State
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.
AA utilization
Fasting
- Muscle: Protein degradation ► AA
- Liver: AA conversion to Glu and KB (energy for other cells)
- Ala major glucogenic AA
Kidney AA utilization
- Energy
- Purposes
- Provided to the AA pool
- Gln
- Gln - balance urine PH
- Ala
Skeletal muscle AA utilization
- Energy
- Purposes
- Provided to the AA pool
- BCAA
- All
- All (maj: Gln and Ala)
Intestine AA utilization (Fed)
- Used for energy
- Other purposes
- Provided to the AA pool
- Gln, Asp, Glu
- Gln (synthesis of citruline & ornithine)
- Ala
Intestine AA utilization (fasting)
- Used for energy
- Other purposes
- Provided to the AA pool
- Gln, BCAA
- Gln (synthesis of Citruline and Ornithine)
- Ala
Liver AA utilization
- Used for energy
- Other purposes
- Provided to the AA pool
- All
- Ala mainly (gluconeogenesis)
- non-essential AA (de novo synthesis)
Brain AA utilization
- Used for energy
- Other purposes
- Provided to the AA pool
- -
- many (NT)
- Gln
Glutamine utilization by the intestine during fasting and fed states (common to both states)
- 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
Tissues that provide Ala to the AA pool
- Kidney
- Skeletal muscle (provide all AA but Ala and Gln are major)
- Intestine-Fed
- Intestine- Fasting
- Liver (Ala is one of the 11 non-essential amino acids provided via de novo synthesis)
Tissues that provide Glutamine (Gln) to the AA pool
- Skeletal muscle (provides all, but Ala and Gln are mjor)
- Brain (Only Gln)
- Liver via de novo syntheis (non-essential aa)
Tissues capable of utilizing BCAA for energy
- Skeletal muscle
- Intestine (fasting)
Normal use of AA in brain
NT synthesis: Astroglial cells and neurons
- 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.
- The most important ones are BCAAs, Phe, Tyr, Trp, His.
- Astroglial cells: BCAA(transamination) → Glu → Gln
- Gln transported to neurons
- Neurons: Gln → Glu & GABA
A tissue that is the major user of the blood glutamine pool and its purpose
Kidney (to generate ammonia)
The use of Glutamine by Kidney
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.
Glutamine usage in acidosis
- Kidney uses even more glutamine for ammonia production.
- This leads to more HCO3- release back to the circulation helping to balance blood pH.
Glutamine and fast-dividing cells
lymphocytes
Epithelial cells of the gut and tumor
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)
AA utilization during generalized acute infection
Sepsis
Requires new protein synthesis bc:
- Hepatocytes produce acute phase proteins
- Immune cells rapidly divide to fight infectious pathogen
