Download Biochem ch 37a [2-9

Biochemistry
Protein Degradation and Amino Acid Absorption
Chapter 37
I. Protein Digestion
-begins in stomach, ends in intestines
-enzymes are released as [inactive] zymogens (larger than their active forms)
-cleaved in digestive tract
-no single enzyme can digest a protein, must act as a group
-broken into a.a’s and small peptides; cleaved by peptidases of intestinal epithelium
A. Digestion of Proteins in the stomach
-Chief cells- release pepsinogen
-activated by HCl to cleave itself into pepsin
-process is “Autocatalytic”
-pepsin not denatured at stomach pH
-works as endopeptidase, cleaving bonds within protein chain
-Gastric parietal cells- release HCl
-activates pepsinogen
-denatures and inactivates dietary proteins
B. Digestion of Proteins by enzymes from the Pancreas
-secreted in inactive (zymogen) form; active forms digest each other and the pancreas itself
-secretions from exocrine pancreas:
Amylase- for starch digestion
Lipase and Colipase- for dietary triacylglycerol digestion
Bicarbonate- neutralizes stomach acid pouring into the intestine; raises pH so that pancreatic proteases can be active
-trypsinogen cleaved into trypsin (active form) by enteropeptidase  activates pancreatic zymogens
-chymotrypsinogenchymotrypsin
-less specific, but favors hydrophobic side chains
-proelastaseelastase
-cleaves elastin, and other proteins at bonds in which carboxyl group is given by Ala, Gly, Ser
-the above 2 are serine proteases that act as endopeptidases
-procarboxypeptidasescarboxypeptidases
-this is an exopeptidase made by the pancreas, look below
-enteropeptidase (used to be called enterokinase)- secreted by brush border cells of the small intestine (activates trypsin)
-Trypsin-plays large role in digestion because it cleaves proteins, and activates proteases
-most specific cleaver; cleaves bondsin which carbonyl group is provided by Lys or Arg
-Once the above endopeptidases have done their work, exopeptidases take over:
-they cleave 1 a.a. at a time off of the carboxyl end of the strand
-carboxypeptidase A- cleaves mostly hydrophobic a.as
-carboxypeptidase B- cleaves mostly basic a.a’s (Arg, Lys)
-act in Brush border, and within the cell
-secreated by pancreas
Elastase is also found in Neutrophils, which frequently act in the lungs. Elastase is sometimes released into the lungs; normally
blocked by -1-antitrypsin (protease inhibitor secreted from liver)
-Mutation exists where inactive -1-antitrypsin is released instead. Causes Emphysema from proteolytic destruction of lung tissue.
-1-antitrypsin deficiency measured by “rate nephelometry” using a dried blood spot.
Pancreas makes a trypsin inhibitor that keeps the zymogens from activating inside of the pancrease and digesting eachother and the
organ itself.
C. Digestion of Proteins by Enzyme from Intestinal Cells
Aminopeptidases- type of exopeptidase secreted at the brush border of intestinal cells; cleave one amino acid at a time from the
amino end of peptides
-Intracellular peptidases act on small peptides absorbed by the cells
-Together the enzymes secreted by stomach, pancreas, and intestines cleave all proteins into a.a’s
Biochemistry
Protein Degradation and Amino Acid Absorption
Chapter 37
-Dietary proteins, the enzymes themselves, and the proteins of the cells sloughed off in the tract are all broken down and
digested. So it is possible to absorb more protein than consumed
II. Absorption of Amino Acids
A. Cotransport of Amino Acids and Sodium
-primary transport mechanism is the creation of a Na+ gradient
-Secondary transport process is coupling of a.a’s to the influx of sodium
Steps:
1) a.a’s cotransported with Na+ into cell on Na+ dependent transport porteins
-driven by Na gradient- so this step is passive
2) Na+ gradient made by pumping Na+ out of cell with Na+, K+ -ATPase on baslar membrane
-active, uses ATPase
3) a.a’s are transported out of cell into interstitial fluid
-by facilitated transporters in baslar membrae
-at least 6 a.a. transporters exist in the apical membrane of brush border cells
- have overlapping specificity for a.a’s; most a.a’s are transported by more than one type
1) neutral a.a’s
2) proline and hydroxyproline
3) acidic a.a’s
4) basic a.a’s (Lys, Arg, and ornithine [from Urea cycle]) & Cystine (2 cysteine residues linked by a disulfide bond)
5 & 6) not listed.. wtf.
-Renal epithelium have same transporters
-In intestine, baslar transporters are bi-directional; during starvation, epithelial cells take a.a’s from blood into cells for use -just like
the other cells of the body.
Trace amts of polypeptidases pass into the blood. Problem for premature infants, can lead to allergies caused by proteins in their
food.
B. Transport of a.a’s into cells
-Trasport from blood to cell- mostly by Na+ dependent cotransporters (like intestines) and some facilitated transporters
-not like Glucose (intest. and renal=cotransport; all else=facilitated)
-the Na+ dependence allows the cells to concentrate the a.a’s
-the transporters in the rest of the body have diff. genetic bases, a.a comp, and diff specificities than those in the intestinal
lumen. Also, vary among tissues. (brush border=bb)
Sys name
A
Na+ dependent?
Yes
ASC
N
Yes
Yes
Specificity
Small and polar neutral a.a (Ala, Ser, Gln, Gly, Pro, Cys, Asn, His,
Met
Small a.a (Ala, Ser, Cys)
Gln, Asn, His
Tissues expressed
Many
Many
Liver, basolateral membrane
of Kidney
L
No
Branched and aromatic a.a. (His, Met, Leu, Ile, Val, Phe, Tyr, Trp)
Many++
B0,+
Yes
Basic a.a.
Intestine (bb) and kidney+++
B0
Yes
Zwitterionic a.a. (monoamino, monocarboxylic a.a’s)
Intestine and Kidney**
XAGYes
Anionic a.a. (Asp, Glu)
Intestine (bb) and kidney
Imino
Yes
Pro, hydroxyproline, Gly
Intestine (bb) and kidney
Not all are listed; ++ this system exploited in PKU treatment; ++most likely issue in Cystinuria; **most likely issue in Hartnup dz
III. Protein Turnover and Replenishment of the intracell. a.a. pool
-Only 6% of the proteins that enter the digestive tract each day are excreated, the rest is recycled
-Some proteins half-life is 5-20 minutes, others hours or days
-proteins continuously synth’d and degraded:
-Hemoglobin
-RBCs live 120 days, so every day, 3x1011 RBCs die and their proteins recycled
Biochemistry
Protein Degradation and Amino Acid Absorption
Chapter 37
-Muscle protein
-degraded during fasting, used for gluconeogenesis
-after ingestion of protein, muscle protein is rebuilt
-cannot increase amt of muscle by excess protein intake. Excess is converted to glycogen, then
triacylglycerols, then stored
-digestive enzymes- discussed earlier, break eachother down after secretion
-proteins of cell sloughed in digestive lumen- discussed earlier
-about ¼ of cells lining GI are lost, broken down, and recycled each day
-also recycled within cells
A. Lysosomal Protein Turnover
Lysosomes= autophagy= intracellular components are surrounded by membranes that fuse with lysosomes and endocytosis.
Cytoplasm is sequestered and delivered to lysosomes.
-in lysosomes: cathepsin family of proteases= proteins a.a’sback to cytoplasm
-starvation triggers this process
mTOR=mammalian target of rapamycin
Rheb-GTP= (ras homolog enriched in brain, bound to GTP)
AMPK=AMP-activated protein kinase
TSC1/TSC2= Tuberous sclerosis complexes 1 & 2
Akt= activated by growth factors (like insulin); phos’s TSC 1 & 2 and site different from that of AMPK
(high E available)Rheb-GTPactivates mTORinhibts autophagy
(low E available) AMP activated phos’s TSC1 & 2activates Rheb-GTPaseinactive Rheb-GTP… no longer able to inhibit
autophagy favored self-proteolysis and inhibition of protein synth
(Akt activation) phos’d TSC1 & 2 inhib’d Rheb-GTPase Rheb active for extended periods autophagy inhib’d for
extended periods of time
B. The Ubiquitin-Proteasome Pathway
PEST= Proline (P), Glutamate(E), Serine(S), Threonine (T) lots of these in a protein=short ½ life
Ubiquitin= tiny (76a.a’s), highly conserved in evolution (yeast & humans have almost identical versions)
Polyubiquination= done by a 3-enzyme system=adds a chain of ubiquitin; targets protein for degradation
-attaches to Lys residues
Proteasome= protease complex; degrades targeted protein; releases ubiquitin intact to be reused
-simple version= cylindrical 20S protein complex with multiple internal proteolytic sites
-ATP used to unfold protein and to push it into the complex
-Regulation- 19S cap complexes, bind targeted enxyme (ATP req’d) and deliver to 20S complex
-Enzymes with areas containing lots of PEST regions use this system of degradation
Classification
Cathepsins
Caspases
Matrix
metalloproteinases
Mechanism
Cysteine proteases
Cysteine proteases, which cleave after aspartate
Require zinc from catalysis
Proteasome
Calpains
Serine proteases
Large complex that degrades ubiquitin-tagged proteins
Calcium-dependent cysteine proteases
Active-site serine in a catalytic triad with histidine and
aspartic acid
Role
Lysosomal enzymes
Apoptosis; activated from procaspases
Model extracellular matrix components;
regulated by TIMPs (tissue inhibitors of
matrix metalloproteinases
Protein turnover
Many cellular roles
Digestion and bld clotting; activated usually
from zymogens
Biochemistry
Protein Degradation and Amino Acid Absorption
Chapter 37
Clinical Correltions!
Sissy Fibrosa
6 year old female patient
-repeated bouts of bronchitis caused by Pseudomonas aeruginosa
-A defect in chloride channels
-In pancreatic secretory ducts, this defect causes inspissation (drying and thickening) of the exocrine secretions which causes
obstruction of the ducts.
-does not allow for the break down of proteins in the intestinal lumen
-pts present with foul-smelling, glistening, bulky stools, low/declining weight and height growth curves(if children), low serum
proteins albumin and prealbumin, listlessness, and irritability
-Treated with enteric-coated microspheres of pancreatic enzymes and initially, a diet plan aimed at improving their protein levels
and growth.
-Diet:
Foods vary in essential amino acid content and availability (its capacity to contribute to the growth of a child)
Eggs-97% avail.
Meats, poultry, fish- 85%-100%
Plants- 75%-90%
-Protein requirements- 0.8g of protein per kilogram of desirable body weight (adult)
much greater for growing children
Cal Kulis
Dx- Cystinuria
Hx-Renal colic (ch.6); passed kidney stone
Rx-high daily water intake + meds to increase pH of urine (make more basic): to increase solubility of large amounts of cystine t
prevent kidney stones
Cystinuria-genetic defect in transporting Cystine and the basic a.a’s (Lys, Arginine, ornithine) across brush border in intestine and
kidney (system B0,+)
No sx of a.a. deficiency b/c the body synthesizes all of these a.a’s. (except Lysine) Kidney stones are the biggest issue=severe
bleeding and pain= renal colic
(gene SLC7A9 and SLC3A1)
Hartnup dz- rare autosomal disorder. Defect in transport of neutral a.a.’s across intestinal and renal cells (System B0, gene SLC6A19,
where SLC= solute carrier family of transport proteins…. There are 55, with 362 diff genes)
Sx= caused by deficiency of essential a.a’s
Problem transporting:
monoamine, monocarboxylic acid
some essential a.a’s= Ile, leu, phe, thr, trp, val
some non essential= ala, ser, tyr
those afflicted typically have few problems if ID’d early on in newborn screening
The phenotype, when it does show, has pellagra-like sx (deficiency of B3-Niacin or tryptophan=both precursors to NAD and NADP)
-photosensitive rash, ataxia, neruopsychiatric sx
Asymptomatic pts, adnormality may be incomplete and the effects subtle
Rx- Niacin (nicotinic acid) p.o. 300 mg QD. Rash, ataxia and neuropysch issues resolve. High-protein diet may be helpful
Hyperaminoaciduria and intestinal transport issues usually do not.
Hartnup and Cystinuria have defects in 2 different transport proteins.
In both cases, issue is malabsorbtion from intestine and increased excretion from kidney.. increased conc. of a.a. in urine
Kwashiorkor, problem in 3rd world countries. Deficiency of protein in a diet with sufficient calories. Suffer from muscle wasting, dec.
[plasma protein] especially [albumin]. Causes inc. interstitial fluidedema and distended abdomen..makes children appear
Biochemistry
Protein Degradation and Amino Acid Absorption
Chapter 37
“plump”. Muscle wasting b/c muscle broken down for the a.a’s not obtained in diet. Issues compounded by inability to make enough
digestive enzymes and new intestinal cells due to lack of a.a’s
Biochemical Comments:
-glutamyl cycle needed to make glutathione= protects cells from oxidative damage. Reduces compounds like H2O2; cycle
sometimes used to transport a.a’s, but not always since not all cells have the right proteins for that fxn.