[orgo814]

A hypothetical three-step metabolic pathway consists of intermediates W, X, Y, and Z and enzymes A, B, and C. Deduce the order of the enzymatic steps in the pathway from the following information:
1. Compound Q, a metabolic inhibitor of B, causes Z to build up
2. A mutant in enzyme C requires Y for growth
3. An inhibitor of enzyme A causes W, Y, and Z to accumulate
4. Compound P, a metabolic inhibitor of enzyme C, causes W and Z to build up.

My question is more with #2. Supposedly this means that Y is the product of C but how is that with the information provided? It says a mutant in enzyme C requires Y for growth... I would assume that means Y must be substrate for C in order to activate it but apparently that is wrong. Please help

[Yggdrasil]

Generally, loss of function mutants cannot be re-activated.  Instead, think about it this way.  Let's say the pathway is for the synthesis of uracil.  If any of the genes are inactivated, the cell will not be able to produce uracil and it will not be able to grow.  But, if you provide uracil in the media, the cell will be able to grow regardless of whether any of the enzymes A, B, or C are functional, since the cell no longer needs to use those enzymes in the synthesis of uracil.

[orgo814]

What confuses me is how they determined that y is the product of C based on that information

[Yggdrasil]

It does not necessarily mean that Y is the product of C. It just means that Y is downstream of C in the metabolic pathway.

[orgo814]

How do you know that?

[Yggdrasil]

Let's say you have the pathway W --> X --> Y --> Z, catalyzed by the enzymes A, B, and C in that order.  The job of enzyme A is to produce X, so that X can be used to create Y and Z.  If you supply the cell with X, Y, or Z, enzyme A is no longer required for the survival of the cell because the cell no longer needs to produce X, Y, or Z on its own.

[orgo814]

That makes sense. But how does knowing the fact that a mutant in enzyme C requires A for growth help me understand A is after? Sorry that it is taking me awhile to conceptualize this- I appreciate your patience

[Yggdrasil]

Again, let's consider the pathway W --> X --> Y --> Z, catalyzed by the enzymes A, B, and C in that order.  The cell needs compound Z in order to replicate.  Normally, you feed compound W to the cells, and enzymes A, B, and C convert compound W to compound Z, allowing the cell to survive.  If any of enzymes A, B, or C are mutated and non-functional, the cell can no longer produce compound Z and the cells will not survive.

Let's say enzyme B (which catalyzes the conversion of compound X to compound Y) is mutated.  If you feed the cells compound W, will the cell still be able to produce compound Z?  If you feed the cells compound X, will the cells still be able to produce compound Z?  If you feed the cells compound Y, will the cells still be able to produce compound Z?