mod 11

Question 1

dietary protein per day needed to maintain amino acid pool

Answer 1

70-100g dietary protein

Question 2

proteolysis

Answer 2

process whereby peptide bonds within proteins are cleaved - hydrolytic rxn - proteolysis begins in the stomach and continues on in the small intestine

Question 3

stomach secretes ____

Answer 3

gastric juice: pH of 2 (equivalent to 100mM HCl) - includes gastrin and pepsin

Question 4

2 purposes of low pH of gastric juice

Answer 4

• denaturation/unfolding of proteins - makes the peptide bonds accessible to proteases
• serves as an antiseptic - destroys bacteria and viruses

Question 5

gastrin

Answer 5

peptide hormone - primary role is to stimulate acid secretion into stomach

Question 6

pepsin

Answer 6

protease - cleaves peptide bonds following a hydrophobic amino acids

Question 7

pancreatic juice

Answer 7

secreted by pancreas into small intestine - neutralizes digestive slurry coming in from stomach to pH 7 with HCO3-

Also contains proteases that degrade proteins into free amino acids that are then absorbed by endothelial cells and then into the portal vein where they are transported throughout the body for utilization.:

• trypsin
• chymotrypsin
• elastase
• carboxypeptidases and aminopeptidases

Question 8

trypsin cleaves ____

Answer 8

trypsin cleaves bonds following arginine or lysine

Question 9

chymotrypsin cleaves _____

Answer 9

chymotrypsin cleaves bonds following aromatic amino acids

Question 10

elastase cleaves ____

Answer 10

elastase cleaves bonds after amino acids that have smaller, hydrophobic side chains

Question 11

carboxypeptidases and aminopeptidases cleave ____

Answer 11

carboxypeptidases and aminopeptidases cleave bonds working from either end of peptides with little specificity

Question 12

pepsinogen to pepsin

Answer 12

cleave 44 amino acid masking sequence to transform to active form - sel-activated process called autocatalysis - triggered by low pH of gastric juice

can also self-inactivate

Question 13

GERD

Answer 13

heartburn - over-active proton pump (K+/H+ pump) - gastrin and histamine bind to receptors on parietal cell –> activation of protein kinase A –> phosphorylates pump leading to more acid in the stomach and low pH)

Question 14

GERD treatments

Answer 14

• anatacids absorb protons (TUMS - calcium carbonate)
• inhibit histamine H2 receptor
• PPIs (proton pump inhibitors): bind directly to pump

Question 15

amino groups eventually secreted in the form of _____ ; carbon skeletons converted into _____

Answer 15

amino groups eventually secreted in the form of urea; carbon skeletons converted into alpha-keto acids (can be oxidized for energy production or converted to glucose via gluconeogenesis)

Question 16

transamination

Answer 16

amino groups from 17 amino acids (except glutamate, lysine, threonine) funnelled into glutamate - catalyzed by aminotransferase (pyridoxal phosphate)

• in cytosol
• amino acid + alpha ketoglutarate = alpha ketoacid + glutamate
• reverse rxn possible (depending on relative concentrations of substrates and products)

Question 17

aminotransferases require ____ as a coenzyme

Answer 17

pyridoxal phosphate (derived from vitamin B6)

Question 18

alanine aminotransferase

Answer 18

alanine + alpha ketoglutarate = pyruvate + glutamate

Question 19

aspartate aminotransferase

Answer 19

aspartate + alpha ketoglutarate = oxaloacetate + glutamate

Question 20

oxidative deamination

Answer 20

amino group from glutamate released as ammonium and alpha-ketoglutarate - catalyzed by glutamate dehydrogenase (in the mitochondria of the liver) - reversible

• removal of amino group + oxidation of glutamate (produce NADH or NADPH)
• in mitochondria

ammonium will be used to synthesize urea in the liver

Question 21

ammonium vs ammonia

Answer 21

ammonium: odorless; most common form of free amino groups at physiological pH

• produced in the mitochondria from oxidative deamination; toxic if levels are too high so rapidly converted into urea within mitochondria to prevent spread and accumulation

ammonia: strong smell; used as cleaning agent

Question 22

ammonium conversion to urea in tissues other than liver (urea synthesis only in liver)

Answer 22

two ways:

• most tissues: free ammonium incorporated into GLUTAMINE by GLUTAMINE SYNTHESTASE - glutamine can get out of cells, enters bloodstream, transported to liver - GLUTAMINASE in mitochondria of liver uses water to cleave ammonium off of glutamine – ammonium released converted to urea; glutamate can be can be acted upon by glutamate dehydrogenase to release a second amino group - keeps free ammonium levels in peripheral tissues low
• glucose-alanine cycle

Question 23

glucose-alanine cycle

Answer 23

in muscle:

excess amino groups collected in teh form of glutamate –> transfers them to pyruvate, which then becomes alanine (transamination reaction by alanine aminotransferase) –> alanine takes amino groups to liver through blood, where alanine transferase converts it back to pyruvate and gives amino groups back to alpha-ketoglutarate to form glutamate –> glutamate is acted upon by glutamate dehydrogenase to rpoduce ammonium, which will become urea

• pyruvate released from this reaction enters gluconeogenesis to produce glucose which goes back to muscle and gets metabolized to pyruvate and enters this cycle again

Question 24

carrier of ammonium from peripheral tissues to liver

Answer 24

glutamine

• free ammonium incorporated into glutamine by glutamine synthetase
• glutaminase in liver mitochondria cleaves ammonium off using water - glutamine gets converted back to glutamate and is acted upon by glutamate dehydrogenase to release another amino group
• glutamate + ammonium = glutamine

Question 25

precursors for urea

Answer 25

NH4+, CO2, aspartate

Question 26

carbamoyl phosphate synthetase 1 obligate allosteric activator

Answer 26

N-Acetylglutamate (synthesized from glutamate and acetyl CoA by N-acetylglutamate synthase)

• stimulated by arginine and glutamate

Question 27

glutamate provides amino groups used in urea synthesis in two ways

Answer 27

• free ammonium is cleaved off of glutamate by glutamate dehydrogenase to form free ammonium which is used to form carbamoyl phosphate
• glutamate transfers its amino groups to oxaloacetate to form aspartate, which is used later to add the second amino group onto urea

Question 28

three cytosolic enzymes of part two of urea synthesis

Answer 28

argininosuccinate synthetase, argininosuccinase, and arginase (clustered together in a complex)

Question 29

urea cycle’s links to citric acid cycle

Answer 29

fumarate and aspartate

Question 30

energy cost of urea cycle

Answer 30

3 ATP (4 high energy bonds) per urea molecule

Question 31

overall rxn urea cycle

Answer 31

Aspartate + NH4+ + CO2 + 3 ATP → urea + fumarate + 2 ADP + AMP + 2Pi + PPi

Question 32

Ammoniotelic

Answer 32

Organisms that eliminate excess nitrogen as ammonium rather than urea (ureotelic)