Week 9 Metabolism Of Amino Acid Groups

Question 1

TRANSAMINATION

Answer 1

First step in amino acid metabolism:
NH4+ from one amino acid is transferred to α-ketoglutarate to make glutamate.

Effect: NH4+ from many different amino acids are “collected” as glutamate.
Glutamate acts as NH4+ donor for biosynthetic/excretory pathways.

Catalysed by aminotransferase (transaminase) enzymes. • Different aminotransferases for different amino acids.

Question 2

AMINOTRANSFERASE COENZYME

Answer 2

PLP is derived from vitamin B6.

PLP function: carrier of amino groups at the active site of aminotransferases.

PLP is covalently bound to the enzyme at the active site.

Exists as two forms:
Aldehyde (with CHO group) called pyridoxal phosphate Amine (with NH3+ group) called pyridoxamine phosphate

Aldehyde form is aminated when it accepts an NH3 group from the amino acid substrate.

Question 3

TRANSAMINATION MECHANISM

Answer 3

1 moving towards each other’s
2 bind amino group to, water is being spliced off to create bond between amino acid and transferase
3 a-keto acid is spliced of and is used else where
4 a ketoglutarate comes up to amino transfer
5 ketoglutarate binds with transferase
6 water is being used = forms glutamate

Question 4

TRANSAMINATION IS AN EXAMPLE OF A “PING-PONG” REACTION

Answer 4

1. Amino acid substrate donates its amino group to pyridoxal phosphate at the active site of aminotransferase
2. That deaminated amino acid (= α-keto acid) then leaves the active site, which now has pyridoxamine phosphate as a coenzyme
3. The second substrate, α-ketoglutarate, enters the active site
4. The amino group on pyridoxamine phosphate is then donated to α- ketoglutarate forming glutamate, which can then dissociate from the active site
5. This leaves behind pyridoxal phosphate and an unoccupied active site ready to catalyse the next reaction

Question 5

EXAMPLE: ASPARTATE TRANSAMINASE

Answer 5

Aspartate transaminase (AST) = aspartate aminotransferase (AAT) = glutamate-oxaloacetate transaminase (GOT)
Blood levels measured in diagnosis of liver damage (through drug use, alcohol abuse or infection)
Tissue damage means enzymes leak into blood

Question 6

GLUTAMATE UNDERGOES OXIDATIVE DEAMINATION IN THE LIVER

Answer 6

In hepatocytes glutamate is transported from the cytosol to the mitochondria where it undergoes oxidative deamination.
Enzyme: Glutamate dehydrogenase
Coenzyme: Either NAD+ or NADP+ (only known enzyme that can use both)
The NH4+ released can enter the urea cycle
The α-ketoglutarate formed can enter the citric acid cycle or
be used for glucose synthesis
Note: The effect of an aminotransferase and glutamate dehydrogenase combined is called transdeamination.

Question 7

AMMONIA IS TRANSPORTED IN BLOOD AS GLUTAMINE

Answer 7

Excess ammonia from other tissues is transported to the liver as glutamine
Enzyme glutamine synthetase aminates glutamate in a two-step process
In the mitochondria of hepatocytes the NH4+ is removed by glutaminase
• Glutamine is deaminated to glutamate in liver mitochondria. The NH4+ can enter the urea cycle (see notes from BMS1050)

Ammonia always binds to Glutamate or glutamine as its highly toxic

Question 8

SUMMARY OF REACTIONS SO FAR

Answer 8

Amino acid -> transaminases > a-keto acid/ carbon Skelton

A- ketoglutarate > glutamate dehydrogenase > glutamate

NAD+ + H20> NADH + NH4+ (GLUTAMINASE) > urea

Question 9

UREA AND CITRIC ACID CYCLE ARE LINKED

Question 10

ASPARTATE-ARGININE-SUCCINATE SHUNT OF CITRIC ACID

Answer 10

Urea cycle and citric acid cycle are independent, but they communicate via intermediates:
• Malate and glutamate transported into mitochondrial matrix
• Aspartate transported into cytosol