Specific enzymes/processes

Question 1

Aromatic amino acid synthesis (tryptophan, tyrosine and phenylalanine)

Answer 1

DAHP synthase catalyses joining of phosphoenolpyruvate to erythrose-4-phosphate (forming DAHP)

Chorismate intermediate – tryptophan
Later prephenate intermediate - phenylpyruvate — phenylalinine
OR – hydroxyphenylphyruvate – tyrosine

Chorismate and prephenate are inhibited by their respective AA (sequential inhibition)

Question 2

Purine biosynthesis

Answer 2

Amidophosphoribosyl transferase = catalyses step one, AMP and GMP synergistically inhibit

Question 3

Aspartokinase - isoenzymes forming threonine, methionine or lysine from aspartate in E. coli

Answer 3

AspI - inhibited by lysine
AspII - by gene expression responding to cellular Met levels (high = repressed, course control)
AspIII - threonine inhibits

Ile - Thr = linear pathway
Thr - Asp = sequential pathway
AspI-III = nested inhibition

Question 4

Glycogen phosphorylase

Answer 4

Converts glycogen to digestible glucoe-1-phosphate
Dimer with interconvertible forms, active a and inactive b
Phosphorylation of Ser14 residue on each half of the dimer activates, from adrenaline
AMP allosterically activates the enzyme as well

Question 5

Glutamine synthetase

Answer 5

Done in notes, for transport of nitrogen

Question 6

Lactic acid dehydrogenase

Answer 6

Isoenzymes in heart, muscle and liver had differing half- lives, involved in carbohydrate processing

Question 7

Acetyl CoA carboxylase

Answer 7

Lipid synthesis, degraded more rapidly in fasted conditions

Question 8

HMG-CoA reductase

Answer 8

Cholesterol metabolism, has a bipartite structure with a catalytic cytoplasmic region and a membrane domain to sense degradation signals

Sterols bind the sensing domain.
E3 gp78 binds in the cytosol, and ubiquitinates Lys residues 89 and 248
Blockage of first step of sterol synthesis (in response to high dietary intake??)

Course control – 15-16 hours half life normally, reduced to 2 in this state