Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

MABS Biochemistry > The Urea Cycle > Flashcards

The Urea Cycle Flashcards

1
Q

T/F, Unlike fats and carbohydrates, amino acids are not stored in the body – rather there are free “pools” of amino acids in cells and in the blood.

A

T

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

where must amino acids be obtained from?

A

Diet
Synthesized de novo
Produced from normal protein degradation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what happens to the amino acids in excess?

A

they are rapidly degraded

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

what is the first molecule that starts of the Urea Cycle?

A

Carbamoyl phosphate from NH3 and CO2 ultimately leading to Urea leaving via the urine

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

How many phases of amino acid catabolism is there?

A

2 phases

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

what is the first phase of amino acid catabolism?

A

removal of the α-amino groups forming ammonia and α-keto acids (carbon skeletons)

most of the ammonia produced is used to synthesize urea

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

what is the second phase of amino acid catabolism?

A

Carbon skeletons of α-keto acids are converted to common intermediates of energy producing metabolic pathways

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

how does nitrogen enter the body?

A

in a variety of compounds present in food, the most important source being amino acids contained in dietary protein

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

how does nitrogen leave the body?

A

urea, ammonia, and other products derived from amino acid metabolism

other products being uric acid, creatinine, ammonia

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

what are the sources that supply the amino acid pool?

A

degradation of endogenous (body) proteins

exogenous (dietary) protein

Nonessential amino acids synthesized from simple intermediates of metabolism

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

what depletes the amino acid pool?

A

Synthesis of body protein (translation)

Consumption of amino acids as precursors of essential nitrogen-containing small molecules (e.g. hormones, heme, neurotransmitters)

Conversion of amino acids to glucose, glycogen, fatty acids, and ketone bodies, or oxidation to CO2 + H2O

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

what is protein turnover?

A

The simultaneous synthesis and degradation of protein molecules

input=output in healthy individuals, thus individual is in nitrogen balance

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

what is negative nitrogen balance?

A

The amount of nitrogen excreted is greater than the amount consumed.

Associated with burns, tissue injury, wasting diseases, fevers, periods of fasting. Can be used as part of a clinical evaluation of malnutrition.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

what is positive nitrogen balance?

A

The amount of nitrogen excreted is less than the amount consumed.

Associated with periods of growth, hypothyroidism, tissue repair, and pregnancy.
Typical of growing children: lots of amino acid and protein synthesis.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

what is the ultimate fate of the nitrogen from amino acid?

A

urea synthesis

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

where does the conversion of the nitrogen from amino acid occur?

A

liver

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

what are the sources of nitrogen that feed into the urea cycle?

A

aspartate, glutamate, free ammonium

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

how do transamination reactions work?

A

The amino group from the original amino acid is transferred to α-ketoglutarate, forming glutamate, whereas the original amino acid is converted to its corresponding α-keto acid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

all amino acids except thesis have the ability to undergo transamination?

A

lysine and threonine

20
Q

what catalyzes transamination reactions? are these reversible?

A

transaminases or aminotransferases; yes

21
Q

what is the required cofactor for all aminotransferases?

A

pyridoxal phosphate (PLP)

22
Q

what are the pairs in the transamination reactions?

A

aspartate and oxaloacetate

α-ketoglutarate and glutamate

23
Q

what are the two important aminotransferases of particular diagnostic value?

A

ALT – alanine aminotransferase (abundant during gluconeogenesis)
AST – aspartate aminotransferase (abundant in the first bypass)

24
Q

what do elevated aminotransferases indicate?

A

damage to cells rich in these enzymes (e.g. physical trauma or disease)

Plasma AST and ALT are elevated in nearly ALL liver diseases

25
Q

what is the purpose in the oxidative deamination of amino acids?

A

the liberation of the amino group from glutamate as free ammonia catalyzed by glutamate dehydrogenase. Direction of reaction depends on the relative concentrations of glutamate, α-ketoglutarate, ammonia, and the ratio of oxidized to reduced coenzymes

Ingestion of high protein meal leads to ↑glutamate levels.. Reaction proceeds in direction of amino acid degradation

or

↑ammonia levels…. Reaction proceeds in direction to glutamate synthesis

26
Q

why is ammonia excreted with urea?

A

to reduce toxicity of ammonia, because ammonium is positively charged; processing of nitrogen or nitrogen build up in liver lead to high levels of ammonia and patient becomes toxic

27
Q

how is ammonia transported from the peripheral tissues to the liver for its conversion to urea?

A

glutamine cycle and the glucose alanine cycle

28
Q

how does the glutamine cycle work?

A

glutamine synthetase used to combine ammonia with glutamate to form glutamine (nontoxic) then this is transported in the blood to the liver where it is cleaved by glutaminase to produce glutamate and free ammonia which is then converted to urea

29
Q

how does the glucose alanine cycle work?

A

in the muscle: pyruvate is produced from both aerobic glycolysis and metabolism of succinyl coenzyme A generated by the catabolism of BCAAs; Alanine is formed from the transamination of pyruvate and is transported by the blood to the liver, where it is converted back to pyruvate by transamination; the pyruvate is used to synthesize glucose which can enter the blood and be used by the muscle

30
Q

what is glutamate used for once in the liver?

A

used for its own amino acid pools
it can make glutamate which can be used
or more alpha ketoglutarate can be made by removing another nitrogen from the glutamate

31
Q

what is the major disposal form of amino groups derived from amino acids? what is the percentage that this accounts for of the nitrogen containing components of the urine?

A

urea is the major disposal form of amino groups; 90% of the nitrogen containing components of the urine

32
Q

what are the two sources of nitrogen?

A

1 nitrogen is supplied by free ammonia and 1 nitrogen is supplied by aspartate

33
Q

what is the immediate precursor of both free ammonia and aspartate nitrogen?

A

glutamate through oxidative deamination and transamination of oxaloacetate

34
Q

what is carbon and oxygen of urea derived from?

A

CO2 as HCO3-

35
Q

where is urea produced and excreted?

A

liver and excreted in the urine

36
Q

describe the reaction of the urea cycle?

A

Step 1: Formation of carbamoyl phosphate via carbamoyl phosphate synthetase I – CPS I

Step 2: Formation of citrulline by ornithine transcarbamoylase (OTC)

Step 3: Synthesis of argininosuccinate by Argininosuccinate synthetase

Step 4: Cleavage of argininosuccinate by argininosuccinate lyase

Step 5: Cleavage of arginine to ornithine and urea by arginase

37
Q

how many high energy phosphate bonds are consumed in the synthesis of each molecule of urea? and is the synthesis of urea irreversible?

A

4 high energy phosphate bonds; the synthesis of urea is irreversible (large –ΔG)

38
Q

how many nitrogens is supplied by free ammonia? what about aspartate? so the total is how many supplied?

A

1 nitrogen is supplied by free ammonia while the other one is supplied by aspartate but really both are from glutamate

39
Q

this amino acid is the intermediate precursor of both ammonia and aspartate?

A

glutamate

40
Q

what is the urea cycle regulated by? what are the other two types of regulation control?

A

substrate availability, so increase the rate of NH3 production increase the rate of urea formation, feed forward regulation

two types
Allosteric activation of CPSI by N-acetylglutamate (NAG)

Induction/repression of urea-cycle enzyme synthesis

41
Q

in terms of the regulation of the urea cycle by NAG, briefly describe how this happens?

A

high levels of arginine stimulate the production of NAG from acetyl CoA and Glutamate; this in turn stimulates the CPS I of the urea cycle to work producing carbamoyl phosphate (molecule that stimulates the cycle)

so increase NAG, increase carbamoyl phosphate production and increase ornithine and urea via arginase

42
Q

what is amino acid carbon converted too?

A

glucose

43
Q

what is acid nitrogen converted too?

A

urea

44
Q

when is urinary excretion of urea high?

A

during fasting, brain eventually begins using ketone bodies, sparing blood glucose (↓ muscle cleaved, ↓ urea)

45
Q

when liver function is compromised, due to genetic defects in urea cycle OR liver disease, blood ammonia levels can rise > 1,000µmol/l = _______? ( name this disorder and the two major types)

A

hyperammonemia

2 major types of hyperammonemia (acquired and congenital)

Hyperammonemia is a medical emergency because ammonia has a direct neurotoxic effect on the CNS

↑ plasma NH3 to ammonia intoxication to tremors, slurring of speech, somnolence (drowsiness), vomiting, cerebral edema, and blurring of vision
↑↑ plasma NH3 to coma and death

46
Q

Hepatitis, chronic alcohol consumption-metabolism, toxic substances eventually lead to portal blood being shunted into systemic circulation preventing access to the liver, therefore, the conversion of ammonia to urea is severely impaired, leading to elevated levels of ammonia. This medical condition is called?

A

Acquired Hyperammonemia

47
Q

this type of hyerammonemia is due to X-linked ornithine transcarbamoylase deficiency, mostly affecting males but female carriers are also common.

A

Congenital Hyperammonemia, note that all other urea cycle disorders follow an autosomal-recessive inheritance pattern
In each case, failure to synthesize urea leads to hyperammonemia during the first weeks following birth

MABS Biochemistry (54 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions