nitrogen balance

Question 1

differentiate between endopeptidases, carboxypeptidases and aminopeptidases

Answer 1

endopeptidases- cleave peptide bonds at specific amino acids
carboxyptpidases- cleave aa present at the C-terminus, specific per R group
aminopeptidases- cleave aa at N-terminus

Question 2

movement of amino acids into and out of enterocyte

Answer 2

in- uses Na-dependent cotransporter (there are at least 7 different types per aa class)
out- facilitated transport into portal blood

Question 3

what is the major form of nitrogen that is excreted by the kidney?

Answer 3

urea

Question 4

how is nitrogen excretion affected by the following:
increased protein intake
decreased protein intake
fasting

Answer 4

increased intake = increased N output
decreased intake = decreased N output
fasting = increased N output (due to muscle catabolism)

Question 5

scenarios that can lead to negative nitrogen balance

Answer 5

1- normal intake, increased output = metabolic stress, illness
2- decreased intake, normal output = fasting, low protein diet

Question 6

what is associated with the highest negative nitrogen balance?

Answer 6

burns

Question 7

what will lead to positive nitrogen balance?

Answer 7

input is greater than output. Will occur during growth, pregnancy, lactation and recovery from illness

Question 8

what is the average daily protein requirement for a healthy adult?

Answer 8

50 g/day

Question 9

what is the effect of low carb diets on protein intake?

Answer 9

increased protein intake is needed bc need precursors for gluconeogenesis

Question 10

how do aminotransferases work?

Answer 10

aminotransferases will take NH3 from an amino acid and put it onto an a-ketoacid, usually resulting in the formation of glutamate; use PLP as a cofactor

Question 11

Reaction catalyzed by ALT

Answer 11

ALT = alanine aminotransferase
Alanine + a-ketoglutarate –> glutamine + pyruvate
pyruvate used in gluconeogenesis
PLP is cofactor

Question 12

reaction catalyzed by AST

Answer 12

AST = asparate aminotransferase
asparate + a-ketoglutarate –> glutamate + oxaloacetate
PLP is cofactor

Question 13

why is the reaction catalyzed by AST often run in “reverse”?

Answer 13

reaction is run in reverse to generate aspartate that then feeds into the urea cycle

Question 14

what is the central substance in amino acid metabolism?

Answer 14

glutamate

Question 15

how is PLP formed?

how does it work?

Answer 15

PLP is formed from vitamin B6 using NAD+ and ATP

PLP will transfer amino groups (accepts amino group PLP –> schiff base –> PMP)

Question 16

function of glutamate dehydrogenase

Answer 16

present in the mitochondria, converts glutamate back to a-ketoglutarate and produces NH4

Question 17

interplay of transaminases and glutamine dehydrogenase

Answer 17

1- transaminases work first on aa and transfer NH3 to a-ketoglutarate to form glutamate
2- glutamate dehydrogenase will then convert glutamate back to a-ketoglutarate and release NH4+ to the urea cycle

Question 18

what is the significance of the location of glutamate dehydrogenase?

Answer 18

present in mitochondria, it releases NH4 which is used int he urea cycle, which starts in the mitochondria

Question 19

what are the sources of the 2 NH2s in urea?

Answer 19

one comes from NH4+

other comes from aspartate

Question 20

what is the regulated step in the urea cycle?

Answer 20

step 1: HCO3 + NH4 –> carbamoyl phosphate by CPS I

Question 21

regulation of CPS I

Answer 21

+ N-acetylglutamate, high degree of protein catabolism, availability of substrates