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Which are the only proteins that can be absorbed by intestines w/o degradation?
Maternal antibodies in breast milk
What happens in stomach during protein digestion? (2)
- 1. HCl denatures proteins for subsequent hydrolysis by proteases
- 2. Pepsin - degrades proteins into peptides
(acid-stable endopeptidase - pepsinogen) secreted as inactive zymogen from chief cells. Activated to pepsin by HCl or autocatlyitically by other (activated) pepsin molecules.
What is HCl's role in protein digestion? 2
Denatures proteins for subsequent hydrolysis by proteases and activates pepsinogen to pepsin
Where do proteolytic enzymes come from? (3)
- stomach (pepsin)
- pancreas (pancreatic proteases - specific for R groups)
- SI (enteropeptidase converts trypsinogen --> trypsin)
How are pepsin (2) and trypsin (2) activated? How are all other pancreatic zymogens activated?
HCl, enteropeptidase, enteropetidase (via trypsin).
Pepsinogen -- HCl --> Pepsin or autocatalytically by other activated pepsin molecules.
trypsinogen-- enteropeptidase (enzyme on intestinal mucosal cells of brush border --> trypsin; autocatalytically too.
Trypsin = common activator of all pancreatic zymogens, so ENTEROPEPTIDASE essentially unleashes cascade of proteolytic activity.
What two hormones mediate release & activation of pancreatic zymogens?
What kind of hormones are they (steroid, peptide, etc).
CCK & Secretin - both peptides.
- CCK (mucosal cells in lower duodenum/jejunum) in response to lipids/partially digested proteins -->
- causes gallbladder to release bile & decreases gastric motility
- Secretin (other intestinal cells) in response to low pH of chyme. Acts on liver/pancreas to release soln rich
- in bicarbonate to neutralize pH for optimal pancreatic enzyme activity.
Where are CCK and Secretin made?
What are they activated by? (2,1)
What do they do? (2,1)
What organs do they act on? (1, 2)
CCK: made in mucosal cells of lower duodenum/jejunum; Secretin: other intestinal cells
Activated by (CCK): partially digested lipids/proteins; (secretin): low pH of chyme
CCK causes gallbladder to release bile AND decreases gastric motility.
Seretin acts on liver and pancreas to release soln rich in bicarbonate to neutralize pH for optimal pancreatic enzyme activity.
What transport system do free AAs use in enterocytes? What about di/tripeptides?
Free AAs - Na+ linked secondary transport system
Di/tri peptides - H+-linked transport system.
What happens to di/tri peptides after uptake? Where in cell does this take place? Then what happens? (3) 2- has two options.
- 1. Di/tripeptides are hydrolyzed into free amino acids in the CYTOSOL
- 2. Free AAs enter portal system
- 3. Go to liver for metabolism OR general circulation (i.e. branched AAs are not metabolized by liver instead sent from liver to muscle via blood).
- 4. ATP-driven uptake into cells via active transport (lower AAs in EC fluid vs IC cells creating gradient maintained by active transport systems driven by ATP).
Which two organs have common transport systems?
Small intestine & kidneys
What are possible fates of amino acid metabolism? (6)
- 1. Metabolism to proteins
- 2. Urea synthesis
- 3. NH3 used for synthesis of AAs, nucleotides, and biological amines or excreted in urine.
- 4. GNG
- 5. Ketogenesis (only lysine & leusine are solely ketogenic)
- 6. Oxidation during starvation - reduced C skeleton is oxidized to CO2 + H2O to produce ATP.
Which AAs are solely ketogenic? (2) Which are GNG and ketogenic? (5; TTTPI) mnemonic
1. Lysine & leucine
2. GNG & Ketogenic
- Three ties try phat ice
- Threonine, tyrosine, tryptophan, phenylalanine, isoleucine.
What is the only wholly catabolic fate of AAs?
Oxidation of reduced C skeleton into CO2 + H2O thereby producing ATP.
What are the two steps involved in deamination of AAs of proteins?
Name substrates, products, and enzymes.
- 1. Transamination (transfer of NH3 from AA to glutamate) makes pyruvate ALT
- 2. Deamination (transfer of NH3 from glutamate to OAA via glutamate dehydrogenase). makes a-ketoglutarate and aspartate
Which are the essential AAs and purpose of each? (3) Which two CAN become essential? (2)
- 1. Arginine - insuff syn to meet growth needs, mostly used for urea synthesis
- 2. Methionine - req in large amounts to produce cysteine (if not enough cysteine in diet)
- 3. Phenylalanine - needed in large amounts to form tyrosine (if not enough tyrosine in diet).
Tyrosine & cysteine can become essential.
What is special about phenylalanine and diet? What is the major result of this?
PKU can be treated via diet by restricting phenylalanine. The major result of this is that tyrosine becomes essential.
What are essential amino acids? (2)
Amino acids that cannot be (1) synthesized in suffient amounts or (2) at all by humans.
How do turnover rates of proteins vary (timepoints)? How many g of proteins are turned over daily? How many g of protein are lost daily and to what (2)?
- Minutes to weeks
- 400 g turned over daily
- 20 g lost daily via enzyme secretion & sloughing off of gut epithelial cells.
Define with equations:
1. Steady state
2. + N balance - caused by? (3)
3. - N balance caused by? (2)
- 1. Steady state: Rate of protein syn = rate of protein degradation
- 2. + N balance: Rate of protein syn > rate of protein degradation
- 3. - N balance: Rate of protein syn < rate of protein degradation
- + N: caused by growth, pregnancy, recovery from injury
- - N: illness, starvation,
How can proteins be degraded? (2)
- 1. Lysosomes (autophagy)
- 2. Ubiquitination
How do lysosomes degrade proteins? 4 steps When are lysosomes selective?
- 1. Induction - after external/internal stimuli (nutrient depletion, rapamycin) mTOR is inhibited.
- 2. Autophagosome formation - cytosolic proteins are sequestered by a double-membraned vesicle
- 3. Docking and fusion with lysosome
- 4. Breakdown of autophagic vesicle and release contents for reuse by cell.
During fasting (not for healthy cells).
Differences between lysosome degradation and ubiquitination? (4)
- 1. Lysosomes don't require ATP
- 2. Ubiquitin is programmed cell death
- 3. Lysosomes degrade EC proteins, while proteasomes degrade endogenous proteins
- 4. Lysosome's selectivity changes depending on external environment
ATP, programmed cell death, EC vs. IC, selectivity
What are the steps of the mechanism of ubiquitination degradation of proteins? (4)
where does ubiquitination take place? which step requires atp?
- 1. Ubiquitin tags proteins
- 2. Proteasomes recognize ubiquitin-tagged protein
- 3. Proteasome denatures protein, deubiquitinates it, and sends to proteolytic core for processing. REQUIRES ATP
- 4. Released peptide fragments are digested into free AAs by non-specific proteasomes in the cytosol.
Cytosol. Proteasome digesting protein step.
How is autophagy regulated?
What is autophagy's regulatory protein regulated by? 2 each.
It is inhibited by mTOR.
mTOR is inhibited by rapamycin and fasting (lack of insulin/amino acids)
Activated by insulin & amino acids.
What is the function of autophagy? (2) what does it break down? (2) What does it ensure? (2) What is specifically targeted during autophagy?
1. Degrades cellular proteins & organelles to recycle nutrients or break down toxic/damaged material.
2. Ensures vital processes can continue and prevents cell from wasting nutrients/energy.
3. Cytosolic proteins containing Lys-Phe-Glu-Arg-Gln pentapeptide sequence.
What is special about arginine? What is it vital for? How does it regulate stuff? Where is it seen in autophagy?
- Arginine = essential amino acid needed for proper growth and development.
- It is vital for the urea cycle.
- Arginine (from a protein rich meal) activates synthesis of N-acetylglutamate which is vital/allosteric activator of carbamoyl phosphate synthetase I - regulatory enzyme for urea cycle.
It's seen in the Lys-Phe-Glu-Arg-Gln sequence (pentapeptide that is targeted by lysosomes during autophagy)
Is phenylalanine ketogenic or glucogenic? What is it the precursor for? When body can't use phenylalanine what disease is that called? Is it essential? Where is it seen in autophagy?
Phenylalanine is both ketogenic & glucogenic.
It's the precursor for tyrosine.
PKU, yes it's essential.
It's seen in the pentapeptide sequence that is targeted during autophagy: Lys-Phe-Glu-Arg-Gln