Lecture 60

Question 1

nitrogen metabolism

Answer 1

carbon skeleton can be completely broken down into CO₂ and H₂O

pg 1536

Question 2

nitrogen metabolism: step 1

Answer 2

• removal of the α-amino group
• done through transamination and oxidative deamination
• only the liver can convert ammonia to urea

pg 1538

Question 3

transamination

Answer 3

• amino acid reacts with α-keto acid using aminotransferase (requires PLP) and forms another AA and other α-keto acid
• takes the amino from AA and gives to α-keto acid to form new AA, takes oxygen from α-keto acid and gives to AA to form new α-keto acid
• aminotransferases, aka transaminases, are specific to particular AAs

pg 1539-1540

Question 4

aminotransferase (transaminases)

Answer 4

• transfer amino group from an AA to produce glutamate (in all but 1 case)
• different enzymes for most of the AAs specific for one or a few amino group donors
• localized in the cytosol and mitochondria of cells throughout the body
• all of them require the coenzyme pyridoxal phosphate (PLP, a vitamin B6 derivative) that participates in the amino group exchange
• reversible reactions with constant equilibrium near 1 which allow them to function in both…
• AA degradation through removal of α-amino groups
• AND
• NEAA biosynthesis through addition of amino groups to the carbon skeletons of α-keto acids

pg 1541

Question 5

aspartate aminotransferase (AST)

Answer 5

• unique aminotransferase (does NOT form glutamate)
• requires the coenzyme PLP that participates in the amino group exchange
• reversible reaction
• during AA catabolism, AST primarily transfers amino groups from glutamate to oxaloacetate, forming aspartate, which is used as a source of nitrogen in the urea cycle

pg 1542

Question 6

hepatic diseases and aminotransferases

Answer 6

• plasma AST and ALT are elevated
• ALT very high in conditions that cause extensive cell necrosis (severe viral hepatitis, toxic injury, and prolonged circulatory collapse)
• ALT is more specific than AST for liver disease
• AST is more sensitive because liver contains larger amounts
• serial measurements of AST and ALT (liver function tests) are often useful in determining the course of liver damage
• ALT has a roughly 20x increase from normal values after ingestion of hepatic toxins

pg 1543

Question 7

non-hepatic diseases and aminotransferases

Answer 7

• aminotransferases may be elevated in nonhepatic diseases (damage to cardiac or skeletal muscle)
• however, these disorders can usually be distinguished clinically from liver disease

pg 1543

Question 8

glutamate dehydrogenase

Answer 8

• expressed at high levels in liver and kidney
• mitochondrial localization
• can use either one as a coenzyme… NAD+ in oxidative deamination (loss/release of NH3 with simultaneous oxidation of the carbon skeleton) OR NADPH in reductive amination (gain/addition of NH3 with reduction of the carbon skeleton → production of NEAAs)
• direction depends on relative concentrations of glutamate, α-ketoglutarate, and NH₃ and the ratio of oxidized to reduced coenzymes
• allosteric regulation: GTP → inhibits, GDP → activates

glutamate to α-ketoglutarate = oxidative deamination; reverse = reduction amination

pg 1544

Question 9

role of glutamate

Answer 9

• disposal of amino acids: oxidative deamination (uses NH₂ of glutamate) releases NH₃ or free ammonia which enters urea cycle
• synthesis of amino acids: reductive amination uses free ammonia to form the NH₂ of glutamate

pg 1545

Question 10

role of glutamate in N₂ disposal

Answer 10

• glutamate is product of transamination of amino acids
• glutamate then undergoes transamination to release aspartate
• glutamate releases NH4+ using GDH
• aspartate and NH4+ go through the urea cycle to produce urea which has 1 amino group from free ammonia and 1 from aspartate
• urea is highly soluble and can be excreted from the body

pg 1546, 1548

Question 11

other sources of ammonia

Answer 11

• several amino acids release their nitrogen as NH4+: His (direct deamination), Ser and Thr (direct deamination requiring PLP), Gln and Asn (R-group amides released by deamidation), glutaminase reaction important in kidney for pH balance
• in muscle and brain (NOT liver), purine nucleotide cycle allows NH4+ to be released from AAs and leave the tissues in the form of glutamine
• significant amount of NH4+ is also produced by bacteria that live in the lumen of the intestinal tract

pg 1547

Question 12

transport as glutamine

Answer 12

• glutamine synthetase (uses ATP, energy-dependent)
• free NH₃ added to glutamate to make glutamine (AA safely transported from tissues to liver in bloodstream)
• once in liver, glutaminase releases NH3+ and converts back to glutamate

pg 1549

Question 13

transport as alanine

Answer 13

• mostly done by SKM cells
• alanine aminotransferase (ALT) converts pyruvate and glutamate to alanine and alpha-ketoglutarate
• transamination of pyruvate
• in the liver, Ala is converted to pyruvate, again by transamination → the “glucose-alanine cycle”

pg 1550

Question 14

nitrogen metabolism: step 2

Answer 14

production of urea!

pg 1551