17.1 and Some 17.2 Flashcards
What is ammonia assimilation?
Process of incorporating NH4+ into the amino acids Gln and Glu
What 3 enzymes mediate ammonia assimilation?
- Glutamine synthetase
- Glutamate synthase
- Glutamate dehydrogenase
What reaction does glutamine synthetase catalyze?
Conversion of Glu to Gln (2-step reaction)
1. Requires ATP and involves the formation of a phosphoryl intermediate, γ-glutamyl phosphate
2. NH4+ replaces the phosphate group to form glutamine
What reaction does glutamate dehydrogenase catalyze?
Interconversion of α-ketoglutarate and Glu in the presence of NH4+
What type of reactions do aminotransferases catalyze?
Reversible reactions that transfer the amino group of amino acids to α-keto acids
What happens in the 2-stage reactions of aminotransferases?
-
α amino group of the amino acid is transferred to an enzyme-linked PLP group
- Results in formation of pyridoxamine phosphate and release of the corresponding α-keto acid - Amino group from PLP is transferred to an incoming α-keto acid (α-ketoglutarate or oxaloacetate) to generate the amino acid product
What is the mechanism of the aminotransferase reaction?
Ping pong mechanism
What is the ping pong mechanism?
First product leaves the active site before the second substrate enters
What is nitrogen balance?
When an organism’s daily intake of nitrogen equals the amount of nitrogen it excretes
A normal healthy adult needs about ___ g of protein per day to maintain nitrogen balance
60 g
What type of nitrogen balance (positive or negative) do young children and pregnant women have? Why?
Positive nitrogen balance because they accumulate nitrogen in the form of new protein (needed to support tissue growth)
In what case would someone have a negative nitrogen balance?
- Sign of disease or starvation
- Occurs in individuals with elevated rates of protein breakdown (loss of muscle tissue) or an inability to obtain sufficient amounts of amino acids in their diets
Where does protein digestion in humans take place?
Stomach and small intestine
When food enters the stomach, it stimulates the release of ___.
Gastrin
What is gastrin?
Peptide hormone secreted by mucosal cells in the stomach lining
What does gastrin do?
Triggers the release of gastric juices containing hydrochloric acid from parietal cells and the secretion of pepsinogen from chief cells
- Increases acidity of stomach (pH ~2)
What is the inactive state (zymogen) of pepsin?
Pepsinogen
What does the increased acidity in the stomach do?
- Denatures dietary proteins, resulting in greater exposure of peptide bonds for hydrolysis
- Kills most bacteria contained in the food
- Activation of the protease by autocatalytic cleavage of pepsinogen to expose the protease active site (now in the active form: pepsin)
At what pH is pepsin maximally active?
pH ~2
What does the esophagus do?
Delivers food to the stomach
What does the esophageal sphincter do?
Prevents acid reflux
What does the stomach do?
Has a low pH that denatures proteins and activates pepsin
What does the pyloric sphincter do?
Functions as a valve and closes off the stomach
What does the duodenum do?
Secretes the hormones secretin and cholecystokinin, as well as enteropeptidase
What is enteropeptidase?
Intestinal protease released into the duodenum that activates several pancreatic proteases
What is the role of secretin?
Stimulates the pancreas to secrete HCO3- to neutralize chyme
What does the activated trypsin enzyme do?
Cleaves pancreatic zymogens chymotrypsinogen, proelastase, and procarboxypeptidase, as well as trypsinogen itself to amplify the proteolytic cascade
Small intestine contains ___ and ___ that degrade peptides into amino acids.
Aminopeptidases and dipeptidases
What are aminopeptidases?
Group of enzymes that remove amino acids from the amino terminus of proteins and peptides
What are dipeptidases?
Group of intestinal enzymes that hydrolyze dipeptides into individual amino acids
What does the pancreas do?
Secretes HCO3- to neutralize chyme and stimulates enteropeptidase to cleave trypsinogen to generate trypsin
What are the two pathways for protein degradation?
- ATP-independent process that degrades proteins inside lysosomes, which are intracellular vesicles derived from Golgi membranes
- ATP-dependent process that degrades proteins containing a polymer of ubiquitin protein
What is ubiquitin?
Small protein that tags proteins for destruction in a proteasome
What is a proteasome?
Large protein complex in cells consisting of an inner chamber lined with proteases that degrade ubiquitinated proteins targeted to it
What are lysozomes?
Low-pH (∼5) compartment filled with digestive enzymes that function nonselectively
What does a proteasome consist of?
- 20S core particle
- Two 19S regulatory particles
- Serve as caps to regulate protein entry into and exit from the proteolytic core
- Contain binding sites for ubiquitinated proteins and also encode ATP hydrolyzing enzymes that function in protein unfolding
What are the three classes of proteins involved in uqbiquitination? What does each one do?
- E1 enzymes: attach ubiquitin to E2 enzymes
- E2 enzymes: attach ubiquitin to target proteins
- E3 enzymes: facilitate ubiquitination of target proteins by forming heterotrimeric complexes with E2 enzymes and target proteins
What is another name for the E3 enzymes?
Ubiquitin ligases
What are the steps of ubiquitination?
- Ubiquitin is covalently attached to a cysteine residue in the E1 active site through an ATP-dependent reaction
- E1 associates with an E2 enzyme and transfers the ubiquitin to a cysteine residue in the E2 active site
- Each E2–ubiquitin complex then binds to an E3 enzyme to form an E2–ubiquitin–E3 complex
- Complex attaches ubiquitin to target proteins recognized by the complex
- Ubiquitin is either first transferred from E2 to E3 before the target protein is ubiquitinated or the target protein is ubiquitinated directly by the E2 subunit in the complex
- Minimum of 3 more ubiquitins must be attached by Gly76–Lys48 linkages before the protein is recognized by the 19S complex of the proteasome
- Sequential attachment of ubiquitin subunits occurs within the same E3–target protein complex through continual of E2-ubiquitin moieties
How is protein ubiquitination regulated? (2 most common mechanisms)
- Biochemical changes in target proteins
- Biochemical changes in E3 ligases
Target proteins that have a Phe/Leu/Asp/Lys/Arg residue at the N terminus are __ and ___ by ___.
Recognized and ubiquitinated by certain E2-E3 complexes, which follow the N-end rule of protein degradation
What is the N-end rule?
Half-life of a protein in the cytosol is greatly determined its N terminal amino acid residue
What does the deamination of amino acids generate?
NH4+
- Used for synthesis of other nitrogen-containing compounds or excreted as urea
- Remaining carbon skeletons are used as metabolites in energy conversion pathways
What amino acids are the primary nitrogen carriers?
Glutamate and glutamine
What are the nitrogens in urea derived from?
- NH4+ released when Glu and Gln are deaminated
- Asp (formed when oxaloacetate is transaminated by aspartate aminotransferase
What are the 3 sources of the amino acids that are transported to the liver, where the nitrogen is removed and used for urea synthesis?
- Amino acids derived from the digestion of dietary proteins
- Gln, which is generated from Glu and NH4+ in peripheral tissues by glutamine synthase
- Ala, which is formed by the alanine aminotransferase reaction
idk man i guess just know this figure (17.27)
What is carbamoyl phosphate?
Small molecule that transfers NH4+ into the urea cycle by transferring a carbamoyl group to ornithine to form citrulline
What is the alanine-glucose cycle?
Mechanism for transporting amino groups from muscle to liver in the nontoxic form, alanine
What problem does the alanine-glucose cycle solve?
Muscle protein is degraded during exercise –> excess nitrogen from amino acid catabolism must be removed to avoid cell toxicity
Where does the urea cycle occur?
Almost exclusively in the liver
What does the urea cycle accomplish for the organism?
Provides an efficient mechanism to remove excess nitrogen from the body
