Download biochem 37 [4-20

Chapter 37
Learning Objectives
1. What cells secrete pepsinogen? What are the conditions necessary for conversion of pepsinogen
to pepsin? What does it target?
 chief cells
 acid allows autocatalytic conversion
 Pepsin targets the peptide bonds of aromatic and acidic amino acids, on the carbonyl side
i. i.e. Phe, Tyr, Glu, & Asp
[Fen Tear, Glue Asp]
2.
What are the conditions necessary for conversion of tripsinogen to trypsin? What does it do?
What other enzyme acts on similar AAs?
 Trypsinogen is cleaved by enteropeptidase, which comes from the brush border cells
 Trypsin is a serine protease
 Trypsin activates other zymogens and digests basic AAs (Lys & Arg)
 Carboxy-peptidase B also digests Basic AAs, but from the carboxyl end of the chain
(exopeptidase)
3. Can you describe activation and function of chymotrypsin, carboxy-peptidase A, and elastase?
 They are all activated by trypsin
 Chymotrypsin digests hydrophobic AAs (Phe, Tyr, Trp, & Leu)
 Carboxy-peptidase A also digests hydrophobic AAs, but is an exopeptidase (only end AAs)
 Elastase targets AAs with small R groups (Ala, Gly, & Ser)
4.
Why does cystic fibrosis lead to problems in the GI system?
 The chloride channel defect of CF causes thickening and drying (inspissation) of pancreatic
secretions, eventually causing obstruction
5. How are amino acids transported into all cells (2)?
 Primarily by Na+ co-transport, just like for carbohydrates
 Secondarily by fascilitated diffusion via one of at least six carrier proteins
6. What organ produces the molecule trypsin inhibitor? Why?
 pancreas
 prevents the stored enzymes from prematurely activating
7. What causes cysteinuria? Symptoms?
 Defect in transport of cysteine and basic amino acids (Lys, Arg, & ornithine) in both the
intestines and renal tubules
i. System B0,+
 kidney stones are the most serious problem, AAs besides Lys can be synthesized (it is unclear
why no Lys deficiency occurs)
8. How is Hartnup disease similar to Cystinuria? Symptoms?
 Transport defect of neutral AAs (Iso, Leu, Phe, Thr, Try, & Val) in both intestines and renal
tubules
i. system B0
[Nupnup, mmm….turkey--tryptophan]

A rare pellagra-like condition is due to a deficiency of tryptophan
9. How is the transport of glucose different from Amino Acid tranpsort?
 only intestinal and kidney cells use the Na+ system when transporting glucose
10. Describe the ubiquitin-proteasome pathway (4 stages). How is its specificity modified? Can you
give some examples? For which AAs is the pathway most utilized?
 1) An enzyme system tags a target protein with one or more ubiquitin proteins
 2) ATP hydrolysis is used to unfold the target and then to push it into the 20S core of the
proteasome cylinder
 3) Different cap complexes alter specificity
i. the PA28 cap complex targets only short proteins
ii. PA700 is used for ubiquinylated proteins
 4) The ubiquitin is recycled
 Pro-GST: proline, glutamate, serine, and threonine predominant proteins, which have short
half lives; “PEST sequences”
11. Essential AA recap:
 on the HILL, Met Thre(e) Phe(asants) Try(ing) to be Val(iant)
 histidine, isoleucine, leucine, lysine, methionine, threonine, phenylalanine, tyrosine, valine
12. Why might a kid with kwashiorkor have a fatty liver?
 The liver continues to synthesize fatty acids from dietary carbohydrate or fat sources, but the
very low density lipoproteins cannot be assembled because apolipoproteins can’t be made