Lecture 31 Flashcards
Describe the 2 things that the low pH of the stomach causes that contribute to the breakdown of proteins in food.
The low pH denatures proteins (unfolds them)
Low pH also activates Pepsin
State the 3 enzymes in the small intestine that help break down proteins and state their functions.
Dipeptidase: breaks apart dipeptides
Enteropeptidase: cleaves Trypsinogen into it’s active form, Trypsin, in the duodenum
Aminopeptidase: Exopeptidases that chew from the N terminal on peptides
(“DEA” mnemonic)
AA’s, dipeptides, and tripeptides all have what in common?
they can all be transported into cells
What are the 2 structures within cells that degrade proteins? which of these does most of the work?
The proteasome (does most of the work) and lysosome
Describe the “N-end Rule”
N-end Rule: the N-terminal AA identity determines the rate of ubiquitination
Degradation is a form of ______.
Regulation
True or False:
Ubiquitin is Reusable. explain
True
What is the function of ubiquitin?
ubiquitin is a protein that binds to proteins to signal for the proteasome to come degrade them.
When the proteasome is interacting with a ubiquitinated protein, the ubiquitin molecule binds to the proteasome, feeds the protein into the proteasome, and is then released
Describe how ubiquitin is NOT just a degradation signal
It can play roles in the cell cycle by either recruiting factors or adding to a protein in disrupting interactions it may have otherwise had with other proteins
The proteasome is an _____, which used ATP to degrade proteins.
ATPase
The proteasome has a 20S subunit that is the_______ domain and a 19S subunit that is the _______ domain. Describe how these subunits can fit together to form different types of proteasomes
catalytic/proteolytic
Regulatory
Basically, several of the 20S and 19S subunits can come together in forming a proteasome
“The more subunits, the larger the proteasome, however the math does not add up correctly”
(ex. 20S + 19S = 26S proteasome ; 20S + 19S + 19S = 30S proteasome)
Compare the functions of Proteasomes and Cytosolic proteases
Proteasome: create AA fragments from proteins
Cytosolic proteases: create INDIVIDUAL AA’s from AA fragments
(these work in a sort of assembly line to fully degrade a protein completely into AA’s)
Describe the specific cellular function that occurs via protein degradation as they pertain to the following “3 R’s”
Reduce:
Reuse:
Recycle:
Reduce: dispose of nitrogen through the urea cycle
Reuse: Make new proteins out of AA’s created from old/ingested proteins
Recycle: Repurpose the carbon skeletons of old AA’s that used to compose proteins
State and describe the process by which AA’s are reduced to their “carbon skeleton”. Also include how PLP fits into this process.
Deamination: separating the NH3 group from the alpha Carbon, to leave behind a “carbon skeleton”
PLP (pyridoxal phosphate) is ALWAYS the coenzyme for this process
(Pyridoxine aka Vitamin B6 is also a common coenzyme for this process)
Describe the 2-enzyme process by which nearly all AA’s are deaminated. Then describe the 2 AA’s that are deaminated in a different manner.
- Aminotransferase
- Glutamate Dehydrogenase
Serine and Threonine are the 2 AA’s that are deaminated by the action of the single enzyme, Dehydratase
Which AA’s are deaminated by the dehydrogenase and which are deaminated by the dehydratase?
Most AA’s = dehydrogenase (glutamate dehydrogenase to be specific)
Serine and Threonine = Dehydratase
What 2 enzymes are uniquely specific to each AA so that they will only deaminate their specific AA for which they fit?
Aminotransferase and Dehydratase (NOT dehydrogenase)
State the 2 step process by which Serine and Threonine are DIRECTLY deaminated by Dehydratase (direct deamination). Then state the molecules that each of these AA’s re deaminated to become.
- Remove H2O (dehydration)
- Add H2O back in order to remove NH4+ (Deamination)
Serine becomes Pyruvate
Threonine becomes alpha-ketobutyrate
Describe the 2 steps of Deamination of an AA via the Glutamate reaction.
- Aminotransferase (either AST/SGOT or ALT/SGPT) uses Alpha-ketoglutarate to split an AA and make glutamate in the following reaction
(AA + alpha-ketoglutarate = Alpha-keto acid + Glutamate) - Glutamate Dehydrogenase Releases an Ammonium Ion (NH4+) from Glutamate
(Glutamate to Schiff-base intermediate to (NH4+ leaves here) Alpha-ketoglutarate)
Compare AST/SGOT and ALT/SGPT Transaminase)
AST/SGOT: Aspartate to OAA
Catalyzes the interconversion of Aspartate and OAA
(Aspartate Aminotransferase/Serum Glutamate-Oxaloacetate Transaminase)
ALT/SGPT: Alanine to Pyruvate
Catalyzes the interconversion of Alanine and Pyruvate
(Alanine Aminotransferase/ Serum Glutamate-Pyruvate
What is a toxic byproduct of AA catabolism that can be converted to urea in the liver? where does this urea then go once it is formed in the liver?
NH4+ (Ammonium)
Once NH4+ is converted to urea in the liver, it is transported to the kidneys to be excreted
Explain the pathway that the NH4+ takes to eventually be excreted when it is formed in AA catabolism in most body tissues (include it’s different forms)
NH4+ is added to Glutamate (via glutamate synthetase) to form Glutamine, which can then travel to the liver
Glutaminase then uses water to cleave the NH4+ off of the glutamine, converting it back to glutamate
NH4+ is then excreted as urea
Briefly describe the pathway NH4+ takes to be excreted when it is formed in muscle
NH4+ is added to pyruvate, forming Alanine, which can then be transported to the liver
Once in the liver, the NH4+ is added to alpha-ketoglutarate to form Glu, which reverts Alanine back to pyruvate
Describe the glucose-alanine cycle and where in the body it occurs
glucose in the muscle is converted to Pyruvate via glycolysis
Pyruvate becomes Alanine when an NH4+ is added to it so that it can take the NH4+ to the liver.
Once in the liver, the Alanine reverts back to pyruvate after delivering the Nh4+
The Pyruvate now in the liver is converted to glucose via gluconeogenesis and that glucose moves back into the muscles to be used as fuel.
