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Questions and Answers
Which enzyme is secreted as an inactive zymogen form in the stomach?
Which enzyme is secreted as an inactive zymogen form in the stomach?
What is the mol. wt. of the inactive zymogen form of pepsin secreted in the stomach?
What is the mol. wt. of the inactive zymogen form of pepsin secreted in the stomach?
In which cells of the stomach is pepsin synthesized?
In which cells of the stomach is pepsin synthesized?
What percentage of pepsin is secreted in the blood stream?
What percentage of pepsin is secreted in the blood stream?
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Which enzyme catalyzes the rate-limiting step of the urea cycle?
Which enzyme catalyzes the rate-limiting step of the urea cycle?
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Which form of CPS uses ammonia as its nitrogen donor?
Which form of CPS uses ammonia as its nitrogen donor?
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Which enzyme transfers the carbamoyl group of carbamoyl phosphate to ornithine?
Which enzyme transfers the carbamoyl group of carbamoyl phosphate to ornithine?
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In which cellular location does the transcarbamoylase reaction occur?
In which cellular location does the transcarbamoylase reaction occur?
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What is the endergonic process that facilitates the reduction of the glutamate γ-carboxyl group to an aldehyde?
What is the endergonic process that facilitates the reduction of the glutamate γ-carboxyl group to an aldehyde?
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Which unstable product is presumed to be the substrate for the reduction that follows in the conversion of glutamate to proline?
Which unstable product is presumed to be the substrate for the reduction that follows in the conversion of glutamate to proline?
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What directly transaminates to yield ornithine in a reaction catalyzed by ornithine-𝛅amino transferase?
What directly transaminates to yield ornithine in a reaction catalyzed by ornithine-𝛅amino transferase?
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What is formed from the glycolytic intermediate 3-phosphoglycerate in a three-reaction pathway?
What is formed from the glycolytic intermediate 3-phosphoglycerate in a three-reaction pathway?
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What is the selective pathway in lysosomes activated by?
What is the selective pathway in lysosomes activated by?
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Which sequence is selectively lost from tissues that atrophy in response to fasting?
Which sequence is selectively lost from tissues that atrophy in response to fasting?
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Which eukaryotic process involves ubiquitin and is independent of lysosomes?
Which eukaryotic process involves ubiquitin and is independent of lysosomes?
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What is the essential cellular function of ubiquitin suggested by its highly conserved nature?
What is the essential cellular function of ubiquitin suggested by its highly conserved nature?
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Study Notes
Proteins from diet or body during fasting can be utilized through various pathways in amino acids and protein metabolism. Amino acids metabolism involves digestion and transport processes in the body.
Digestion and transport:
- Digestion begins in the mouth where no proteolytic enzymes are present.
- Food reaches the stomach and meets gastric juice containing proteolytic enzymes like pepsin, rennin, gastriscin, and gelatinase.
- Pepsin is a potent proteolytic enzyme secreted as inactive zymogen pepsinogen. It is synthesized in chief cells and secreted into gastric juice.
- Lysosomes have a selective pathway that imports and degrades cytosolic proteins containing the pentapeptide sequence Lys-PheGluArg-Gln (KFERQ) or a closely related sequence.
- Protein breakdown in eukaryotic cells also occurs independently of lysosomes, involving the monomeric protein ubiquitin.
Ubiquitin and protein degradation:
- Ubiquitin is a highly conserved eukaryotic protein that marks proteins for degradation by covalently linking them to ubiquitin.
- Ubiquitination occurs in three steps: activation, conjugation, and chain elongation.
- Ubiquitin participates in the urea cycle, which is involved in nitrogen metabolism, specifically in the conversion of ammonia to urea.
Urea cycle:
- The urea cycle involves five enzymes: carbamoyl phosphate synthetase I (CPS I), ornithine transcarbamoylase, argininosuccinate synthetase, argininosuccinase, and arginase.
- CPS I catalyzes an essentially irreversible reaction that is the rate-limiting step of the urea cycle.
- The ornithine transcarbamoylase reaction occurs in the mitochondrion, so ornithine, which is produced in the cytosol, must enter the mitochondrion via a specific transport system.
- The conversion of glutamate to proline involves the reduction of the γ-carboxyl group to an aldehyde.
Nonessential amino acids biosynthesis:
- Serine is formed from the glycolytic intermediate 3-phosphoglycerate in a three-reaction pathway.
- Ornithine is derived from glutamate via a branch from proline biosynthesis.
- Serine, cysteine, and glycine are derived from 3-phosphoglycerate.
- Ornithine is converted to arginine through the reactions of the urea cycle.
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Description
Test your knowledge of the enzymes involved in urea biosynthesis and pyrimidine biosynthesis, including carbamoyl phosphate synthetase and ornithine transcarbamoylase. Explore the pathways in amino acids and protein metabolism in eukaryotes.
