Protein Metabolism: Oxidation and Urea

Question 1

what are the 4 fates of dietary amino acids?

Answer 1

• protein synthesis
• energy production (CAC)
• biosynthesis
• urea excretion

Question 2

what are the 3 divisions of protein oxidation?

Answer 2

• normal synthesis and degradation
• protein rich diet
• starvation or diabetes mellitus

Question 3

draw out the 4 fates of dietary amino acids

Answer 3

Question 4

dietary protein stimulates the release of ___ from gastric mucosa in the stomach. This then stimulates the release of ___ from parietal cells, and ___ from chief cells.

Answer 4

• gastrin
• HCl
• pepsinogen

Question 5

what does pepsinogen/pepsin do?

Answer 5

• it is a protease
• hydrolyzes phenylalanine (Phe), tryptophan (Trp), and tyrosine (Tyr) polypeptide bonds
• **pepsinogen changes to it’s active form (pepsin) in the stomach due to low pH (1.0-2.5)

Question 6

what does secretin do?

Answer 6

stimulates the release of bicarbonate from the pancreas

Question 7

what does cholecystokinin do?

Answer 7

stimulates zymogen release from the pancreas

Question 8

what are the 3 main pancreatic zymogens (proteases) released from the pancreas, and what are they precursors to?

Answer 8

• trypsinogen → trypsin
• chymotrypsinogen → chymotrypsin
• procarboxypeptidase A and B → carboxypeptidase A and B
• important to remember that zymogens are enzyme precursors that require a biochemical change (usually cleavage) to become active

Question 9

where does amino group catabolism occur?

Answer 9

hepatocytes in the liver

Question 10

what is the basic strategy of amino group catabolism in the liver?

Answer 10

separate the amine group, leave the carbon chain

Question 11

what is the source of most amino acids?

Answer 11

diet

Question 12

what are the 4 metabolically important amino acids for amino group catabolism? what do they do?

Answer 12

• glutamate
• glutamine
• aspartate
• alanine
• amine group carriers; precursors and common metabolites; entry and exit molecules from the CAC

Question 13

describe the functions of glutamate, glutamine, and alanine in amino group catabolism

Answer 13

Question 14

ammonia is toxic. what are some effects of too much ammonia?

Answer 14

• reduction in learning
• reduction in movement
• hyperacusis
• imbalance
• gait ataxia
• seizures
• loss of consciousness
• death

Question 15

how does ammonia prevent GABA inhibition?

Answer 15

• crosses blood brain barrier and disrupts astrocyte K+ uptake
• high extracellular K+ will prevent GABA inhibition
• causes neuronal hyperactivity, seizures, oxidative stress, and death

Question 16

how is ammonia toxicity controlled?

Answer 16

amine groups are stabilized as urea (vertebrates) or uric acid (birds and reptiles) for excretion

Question 17

what are the 2 major amino acid catabolism reactions?

Answer 17

• transaminase reactions
• one-carbon transfers

Question 18

describe transaminase reactions

Answer 18

• transfer amine groups
• PLP = pyroxidal phosphate
  
  • common coenzyme
  • carrier of amino groups

Question 19

how is glutamate transported to the liver via glutamine transport in the blood?

Answer 19

• glutamine is transported to the liver
• glutamine is non-toxic
• most cells have glutamine synthetase
• common synthetic precursor

Question 20

how it glutamine transported to the liver via the glucose-alanine cycle?

Answer 20

• cycle allows proteins to function as energy sources
• cycle occurs in anaerobic states
• G-A cycle and Cori cycle occur simultaneously

Question 21

how is intracellular ammonia buffered?

Answer 21

by converting glutamate to glutamine

Question 22

what does the urea cycle excrete?

Answer 22

liver nitrogen

Question 23

where does the urea cycle occur

Answer 23

hepatocytes in the liver

Question 24

describe the 4 major steps of the urea cycle

Answer 24

• forms urea, a stable amine-rich molecule that can safely excrete nitrogen
• 4 steps, 5 structural changes
• requires enzymes within the mitochondria and cytoplasm
• ATP-dependent
• ornithine is absolutely essential

Question 25

what are the 2 feeder molecules for the urea cycle?

Answer 25

carbamoyl phosphate and aspartate

Question 26

draw out the urea cycle beginning with carbamoyl phosphate and aspartate

Answer 26

remember, ornithine is super important

Question 27

describe the percentages of NH3 that enter the liver, and the percentages of urea that is excreted as urine vs. ammonia excreted in fecal matter

Answer 27

Question 28

what are the 2 levels of urea cycle regulation?

Answer 28

allosteric N-acetylglutamate regulation * increased synthesis of: * ornithine transcarbomoylase * arginosuccinate synthetase * arginosuccinase * arginase * allosteric carbamoyl phosphate synthetase I regulation * responsible for production of carbamoyl phosphate

Question 29

describe allosteric n-acetylglutamate regulation

Answer 29

Question 30

describe how the citric acid cycle and urea cycle can be linked

Answer 30

* dependent upon mitochondrial transporters * aspartate nitrogen donation is important * amino acid synthesis * urea cycle

Question 31

what are the energy substrates produced by amino acid catabolism?

Answer 31

THESE ARE ALL REVERSIBLE * glucogenic - amino acids that can be converted to glucose * ketogenic - amino acids that can be converted to ketone bodies * transaminase and one-carbon transfer reactions * fuels 10-15% of cellular energy production \*don't need to know which amino acids are keto- and gluco-genic

Question 32

name the nonessential amino acids

Answer 32

* glutamate * aspartate * alanine * asparagine * serine

Question 33

what are the conditionally essential amino acids?

Answer 33

* glutamine * glycine * arginine * proline * tyrosine * cysteine

Question 34

what are the essential amino acids?

Answer 34

* phenylalanine * valine * threonine * tryptophan * isoleucine * methionine * histidine * leucine * lysine

Question 35

which 6 amino acids are degraded to pyruvate?

Answer 35

* tryptophan * alanine * cysteine * serine * glycine * threonine * they are degraded to pyruvate, which then feeds into the CAC or gluconeogenesis

Question 36

which 7 amino acids are degraded to acetyl-coa?

Answer 36

* tryptophan * lysine * phenylalanine * tyrosine * leucine * isoleucine * threonine * acetyl-coa feeds directly into the CAC

Question 37

what are 3 important one-carbon transfer reaction cofactors?

Answer 37

1. biotin 2. s-adenosylmethionine 3. tetrahydrofolate * one-carbon group donors or recipients * often methyl groups * vitamins

Question 38

which 5 amino acids are degraded to alpha-ketoglutarate?

Answer 38

* glutamate * glutamine * proline * arginine * histidine * alpha-ketoglutarate is part of the CAC * less exergonic because we don't produce an NADH from the isocitrate -\> alpha-ketoglutarate * remember that whenever you have glutamate, you have alpha-ketoglutarate

Question 39

which 4 amino acids are degraded to succinyl-coa?

Answer 39

* methionine * isoleucine * valine * threonine * succinyl-coa is part of the CAC * less exergonic because we don't produce NADH from prior 2 steps in CAC

Question 40

which 2 amino acids are degraded to oxaloacetate?

Answer 40

* asparagine * aspartate * oxaloacetate is part of the CAC * least exergonic because it it towards the end of the CAC, so we are not producing any of the NADHs, FADH2s, or GTPs, from previous steps