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Biochemistry > Week 9: Nitrogen > Flashcards

Week 9: Nitrogen Flashcards

1
Q

Why can’t nitrogen be stored?

A

Ammonia is toxic

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2
Q

How are amino acids obtained?

A
  1. The diet
  2. De novo synthesis
  3. Normal protein degradation
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3
Q

What are the 3 routes of AA depletion?

A
  1. Synthesis of body protein
  2. Consumption of aa as precursors of essential nitrogen-containing small molecules
  3. Conversion of AA to glucose, glycogen, fatty acids and ketones or oxidation to CO2 and H2O
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4
Q

What is nitrogen balance?

A

Input amino acids in the amino pool is balance with amino acid output

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5
Q

What is protein turnover?

A

Proteins are constantly being synthesized and degraded but the total amount of protein in the the body remains constant in healthy peopel

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6
Q

Which proteins have longer half-lives?

A

Structural

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7
Q

What are the mechanisms that degrade proteins?

A
  1. Digestion in stomach and small intestine
  2. Ubiquitin-proteasome proteolytic pathway
  3. Lysosomal digestion by acidic hydrolases
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8
Q

Where does protein digestion occur?

A

Stomach and small intestine

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9
Q

What is a proteases?

A

Cleave peptide bonds to release amino acids from peptides

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10
Q

What is stimulates with food enters the stomach?

A

Release of gastrin

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11
Q

What is gastrin?

A

Triggers the release of gastric juices (HCl and pepsin)

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12
Q

How is the purpose of a low pH in the stomach?

A

Denature proteins

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13
Q

What is chyme?

A

Acidic slurry of digested food moves from the stomach into the duodenum

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14
Q

What hormones are triggered when chyme enters the small intestine?

A

Secretin and cholecystokinin

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15
Q

What is the purpose of secretin?

A

Stimulates the release of bicarbonate from the pancreas to neutralize chyme

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16
Q

What is the purpose of cholecystokinin?

A

Stimulates the secretion of bile and digestive enzymes

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17
Q

What is the purpose of protease enteropeptidase?

A

Activates several proteolytic zymogens that digest the proteins into free aa that are absorbed by electrolytes

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18
Q

Describe the absorption of dietary proteins?

A
  1. Free amino acid are taken into enterocytes by sodium-linked secondary transport system
  2. Di and tripeptides are taken up by a proton-linked transport system
  3. Amino acids exit and are taken to the liver by the portal vein
  4. Amino acids are metabolized by the liver or released into the blood
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19
Q

Where are branched chain aa go?

A

Not metabolized by the liver but are sent to the muscles through the blood

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20
Q

What is acute pancreatitis?

A

Disease caused by an obstruction of duct that pancreatic secretions move from the pancreas to the small intestine

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21
Q

What occurs during acute pancreatitis?

A
  1. Pancreatic zymogens are prematurely converted into their catalytically active form inside the pancreas
  2. Proteases attack pancreatic tissue
  3. Damage to pancreas causing extreme pain
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22
Q

What occurs during the Ubiquitin-Proteasome Degradation Pathway?

A
  1. Protein selected for degradation is tagged with ubiquitous
  2. Ubiquinated proteins are recognized by the proteasome.
  3. Proteases in cytosol then nonspecifically degrades fragments into amino acids
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23
Q

Where is proteasome located?

A

Cytosol

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24
Q

How does the proteasome process proteins?

A
  1. Removing ubiquitin from the protein and ubiquitin is recycled
  2. Protein is unfolded and chopped into fragments
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25
Q

Which protein degradation mechanism is ATP dependent?

A

Ubiquitin-Proteasome Degradation Pathway

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26
Q

Which protein degradation mechanism is ATP independent?

A

Lysosomal degradation

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27
Q

What occurs during lysosomal degradation?

A

Proteins are broken down inside the lysosome

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28
Q

What is in lysosome?

A

Acidic proteases that non-selectively degrade protein particles

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29
Q

What is autophagy?

A

Degradation of intracellular proteins

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30
Q

What extracellular proteins are degraded by lysosomes?

A
  1. Proteins from endocytosis
  2. Plasma proteins
  3. LDL
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31
Q

What occurs during amino acid catabolism?

A
  1. Recycled into new proteins
  2. Oxidized for energy but the removal of A-amino group from the urea cycle
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32
Q

What happens to the carbon skeleton of the aa when amino group is removed?

A

Glycolysis, CAC

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33
Q

Describe the removal of the amino group on aa?

A
  1. Transfer of an a-amino group to a-ketoglutarate
  2. Produces an a-keto acid and glutamate
  3. Glutamate can be oxidatively deaminated or used as an amino group donor for nonessential amino acid synthesis
  4. Aminotransferases (transaminases) catalyze the transfer of amino groups from one carbon skeleton to another
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34
Q

What is aminotranferase?

A

Catalyze the transfer of amino group from one carbon skeleton to another

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35
Q

What is transamination?

A

The transfer of an amine to a common metabolic that can be easily converted into CAC intermediates

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36
Q

What initiates aminotransferase?

A

Coenzyme pyridoxal phosphate from vitamin B6

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37
Q

Why is transamination reversible?

A

It can be used for amino acid degradation and for biosynthesis of nonessential amino acids

38
Q

What are the 2 aminotransferases?

A
  1. Alanine (ALT)
  2. Aspartate (AST)
39
Q

What is the function of ALT?

A

Catalyzes the transfer of amino groups of alanine to a-ketoglutarate forming pyruvate and glutamate

Reversible

40
Q

What is the function of AST?

A

Transfers amino groups from glutamate to oxaloacetate forming aspartate

Reversible

41
Q

Which aminotransferases serves as a source of nitrogen in the urea cycle?

A

AST

42
Q

What happens if there is low levels of Plasma Aminotransferases?

A

Normal cell turnover

43
Q

What happens if there is elevated levels of Plasma Aminotransferases?

A

Cell damage

44
Q

Why is it important to measure AST and ALT?

A

Determine the extent of liver damage

45
Q

What happens when Plasma AST and ALT are elevated in liver disease?

A

Cell necrosis

46
Q

What is oxidative deamination?

A

Liberate amino groups as free ammonia

47
Q

Where does oxidative deamination located?

A

Liver and kidney mitochondria

48
Q

Describe the process of oxidative deamination?

A
  1. Produce a-keto acids that can enter metabolic pathways and ammonia
  2. Ammonia exists as ammonium (NH4+) in aqueous solutions.
  3. The unionized form (NH3) can cross the membranes.
  4. Glutamate undergoes rapid oxidative deamination to generate a-ketoglutarate.
49
Q

What is used for oxidative deamination?

A

NAD+

50
Q

What is used for reductive amination?

A

NADPH

51
Q

When would oxidative deamination occur?

A

Glutamate levels are higher after a meal

52
Q

What are the mechanisms that transport ammonia to the liver?

A
  1. Glutamine synthetase combines ammonia and glutamate to form glutamine
  2. Alanine is formed by the transamination of pyruvate by alanine aminotransferase (ALT)
53
Q

What occurs during glutamine synthetase?

A
  1. Glutamine serves as the nontoxic transporter of ammonia (NH3) in the blood
  2. In the liver, glutaminase cleaves glutamine to produce glutamate and free ammonia (NH3)
  3. Ammonia (NH3) is converted into urea by the urea cycle
54
Q

What occurs during alanine transamination of pyruvate?

A
  1. Alanine is transported by the blood to the liver
  2. In the liver, alanine is converted to pyruvate by transamination by ALT
  3. Pyruvate is used to synthesize glucose that can be used by the muscle (glucose-alanine cycle).
55
Q

When is ammonia produced?

A

Metabolism of all tissues

56
Q

What primary disposes of ammonia?

A

Urea cycle

57
Q

What is hyperammonemia?

A

Elevated levels of ammonia in the blood

58
Q

What happens to ammonia in the urea cycle?

A

Conversion of NH3 into urea and glutamine keeps levels of NH3 low in the blood

59
Q

What is Blood urea nitrogen (BUN)?

A

Hyperammonemia can cause tremors, slurred speech, drowsiness, vomiting, blurred vision and at high concentrations can cause coma or death

60
Q

What is the major disposal form of aa?

A

Urea

61
Q

Where is urea produced?

A

Liver

62
Q

Where does the nitrogen urea come from?

A

Free ammonia and aspartate

63
Q

What is the source of aspartate nitrogen through the transamination of OAA?

A

AST

64
Q

What is the overall purpose of the urea cycle?

A

To remove excess nitrogen from the body

65
Q

What is the overall reaction of the urea cycle?

A

NH4+ + CO2 + Aspartate + 3 ATP → Urea + Fumarate + 2 ADP + AMP + 4 Pi

66
Q

Where are the first 2 reactions of the urea cycle located?

A

Matrix

67
Q

What is step 1 of the urea cycle?

A
  1. Catalyzed by carbamoyl phosphate synthetase I (CAPS I)
  2. Requires hydrolysis of 2 ATP
  3. Joins ammonia (NH4+) with HCO3- (form of CO2) to make carbamoyl phosphate
68
Q

What are the steps of the urea cycle?

A
  1. Formation of carbamoyl phosphate
  2. Formation of citrulline
  3. Formation of arginosuccinate
  4. Cleavage of arginosuccinate
  5. Cleavage of arginine
69
Q

What regulates the urea cycle?

A
  1. CAPS I
  2. Availability of aspartate, ammonia, and CO2
70
Q

What is the rate limiting step of the urea cycle?

A

CAPS 1

71
Q

What is step 2 of the urea cycle?

A
  1. Carbamoyl portion of carbamoyl phosphate is transferred to ornithine by ornithine transcarbamoylase to form citrulline
  2. High energy phosphate is cleared
  3. Citrulline is transported into the cytosol
72
Q

What are the basic amino acids that move across the mitochondrial membrane via a cotransporter?

A

Ornithine and citrulline

73
Q

Why isn’t Ornithine and citrulline not incorporated into cellular proteins?

A

No codons for these aa

74
Q

What is regenerated with each turn of the urea cycle?

A

Ornithine

75
Q

What is step 3 of the urea cycle?

A
  1. Citrulline is combined with aspartate to form arginosuccinate
  2. Catalyzed by argininosuccinate synthetase driven by the hydrolysis of ATP to pyrophosphate (PPi))
  3. a-amino group of aspartate provides one of the nitrogen’s in urea
76
Q

What is step 4 of the urea cycle?

A
  1. Arginosuccinate is cleaved into fumarate and arginine by arginosuccinate lyase
77
Q

What is the precursor for urea?

A

Arginine

78
Q

What happens to fumarate from step 4?

A
  1. Hydrated to malate
  2. Malate is oxidized to oxaloacetate producing NADH
  3. Oxaloacetate can be transaminated into aspartate by AST to feed back into urea cycle at reaction 3
79
Q

What is step 5 of the urea cycle?

A
  1. Arginine is cleaved into urea and ornithine by arginase
  2. Occurs exclusively in the liver
  3. Other tissues use enzymes of the urea cycle to synthesize arginine (kidneys)
80
Q

What is the Aspartate-Arginosucinnate Shunt?

A
  1. Converts fumarate into malate
  2. Connects the urea cycle to the CAC
81
Q

What are the fates of urea?

A
  1. Urea from liver to Urine from kidneys
  2. Urea to blood and cleaved by bacterial urease in small intestine
  3. Ammonia is reabsorbed in blood and excreted in feces
  4. Promotes the transfer of urea from the blood to the intestines
82
Q

When would hyperammonemia occur?

A

Patients with kidney failure or liver disease

83
Q

What occurs when insulin/glucagon ratio is high?

A
  1. Increased glucose and also amino acids and fatty acids in the blood stimulate insulin secretion
  2. Stimulates the entry of amino acids from the blood into cells
  3. Stimulate protein synthesis
  4. BCAA are used to synthesize muscle protein
  5. Stimulates the conversion of excess protein into storage as TAG
  6. Primarily uses glucose as fuel
84
Q

What occurs when insulin/glucagon ratio is low?

A
  1. Increased dietary amino acids and epinephrine stimulate the release of glucagon
  2. Muscle protein breakdown
  3. Increase aa uptake
  4. Glucogenic aa is used for gluconeogenesis
  5. Ketogenic amino acids for ketogenesis
  6. Uses glycogen stores, fatty acids, and ketone bodies as fuel
85
Q

Describe the degradation of aa?

A
  1. Transaminations can produce carbon skeletons that can feed into the CAC or ketogenesis.
  2. Glucogenic amino acids make gluconeogenesis intermediates
  3. Ketogenic amino acids make ketone bodies
86
Q

What amino acids are both glycogenic and ketogenic?

A
  1. Phe
  2. Tyr
  3. Ile
  4. Thr
  5. Trp
87
Q

Which essential aa is made by biosynthesis?

A
  1. His
  2. Trp
  3. Phe
  4. Lys
  5. Thr
  6. Met
  7. Leu
  8. Ile
  9. Val
  10. Arg
88
Q

What organisms can make all 20 aa?

A

Plants and bacteria

89
Q

How many amino acids are made by oxaloacetate and pyruvate?

A

9

90
Q

Which aa are conditionally essential?

A

His and Arg

91
Q

How are essential aa obtained?

A

Diet

92
Q

What is the difference between essential and nonessential aa?

A

Essential amino acids are generally more complex than the nonessential amino acids are require a greater number of reactions to synthesize

Nonessential amino acids have carbon skeletons very similar to common metabolic precursors and are more easily synthesized

Biochemistry (16 decks)

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