Here is a portion of the methionine biosynthesis pathway inyeast: HS---->AHS----
ID: 3265 • Letter: H
Question
Here is a portion of the methionine biosynthesis pathway inyeast:HS---->AHS---->HC---->M
HS is homoserine, AHS is o-acetylhomoserine, HC is homocysteine,and M is methionine. Yeast with defects in any of the enzymesinvolved in this pathway cannot make methionine. Three yeastmutants that required methionine to survive were isolated. The ability of the mutant yeast to survive when supplemented withthe pathway intermediates is described in this table:
Mutant Supplied Intermediate
Result met-1 homoserine did not grow
met-2 homoserine
did not grow
met-3 homoserine
did not grow
met-1 homocysteine
growth observed
met-2 homocysteine
growth observed
met-3 homocysteine did not grow
met-1 o-acetylhomoserine did not grow
met-2 o-acetylhomoserine growth observed
met-3 o-acetylhomoserine
did not grow
If each mutant is defective for one of the pathway enzymes, what isthe order in which the enzymes act in this pathway? Justifyyour answer.
There was a hint given out with this question:
"The biochemical pathway proceeds in the order given (visualizerightward arrows between the steps). Each mutant represents adefective version of a normal gene encoding an enzyme thatcatalyzes one of the three steps shown. Therefore, eachmutant is blocked at one particular step and this prevents theentire pathway from proceeding. The issue here is to use thedata for rescue of the various mutants by the chemicalintermediates provided, to deduce where each block occurs. The principle involved is that a blocked step can be bypassed byproviding intermediates downstream of the point of blockage (to theright of the pathway). By showing where a given block occurs,you can infer that the enzyme product of the wildtype version ofthe gene acts at that step."
Mutant Supplied Intermediate
Result met-1 homoserine did not grow
met-2 homoserine
did not grow
met-3 homoserine
did not grow
met-1 homocysteine
growth observed
met-2 homocysteine
growth observed
met-3 homocysteine did not grow
met-1 o-acetylhomoserine did not grow
met-2 o-acetylhomoserine growth observed
met-3 o-acetylhomoserine
did not grow
Explanation / Answer
This is not an easy question if you did not understand the wholeextract. Firstly, lets review that the pathway isHS---->AHS---->HC---->M For met-1, supply of homocysteine result in growth.Orelse, there isno growth. This proves that the pathway needs homocysteine toproceed. So, the met-1 is the mutation of enzyme catalyzing AHS------> HC For met-2, supply of homocysteine and AHS resulting in growth. Thisshows that the enzyme responsible for catalying HS ----> AHS isnot functioning. So, supply of AHS or homocysteine is required inorder for the pathway to proceed. For met-3, supply of any material resulting in no growth at all.This shows that the enzyme is catalyzing the last steps. Eventhough you supply all the intermediates, yet the enzyme is notfunctional. So, there will be no methionine produced. Thus, met-3is a mutation caused in the last enzyme in the pathway. Thus, the order of enzyme action is met2-met1-met3 Hope this helps!