Citrate -> Isocitrate and optical activity

[nfg05]

In the citric acid cyle, citrate (a symmetrical intermediate) is converted to isocitrate (an asymmetrical intermediate). Which of the following best describes the effect the conversion will have on optical activity.

A. The transition results in an optically inactive compound gaining the ability to rotate the plane of polarized light +24 degrees.

I thought it was impossible to product an optically active product from optically inactive reactants?

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[HawkeyePostOp]

I thought it was impossible to product an optically active product from optically inactive reactants?

Well you gotta start somewhere.

 

[nfg05]

Perhaps it would help if I clarified where I'm coming from?

http://www.google.com/search?q=optically+inactive+reactants+yield+optically+inactive+products

Is it that the material that helps with the conversion is chiral? Something has to be chiral for this to happen, and it isn't citrate (per the answer explanation I have citrate is not optically active)

 

[sleepy425]

Perhaps it would help if I clarified where I'm coming from?

http://www.google.com/search?q=optically+inactive+reactants+yield+optically+inactive+products

Is it that the material that helps with the conversion is chiral? Something has to be chiral for this to happen, and it isn't citrate (per the answer explanation I have citrate is not optically active)

That statement is correct. in general, optically inactive reactants will yield optically inactive products. but what that's really saying is that, in general, you get racemization, so you still don't have optical activity.

however, if the catalyst for the reaction is chiral such that it prefers formation of one stereoisomer over the other, you can generate an optically active product.

in this case, aconitase (the enzyme that converts citrate to aconitate to isocitrate) is the catalyst. aconitase is an enzyme, and enzymes are chiral. thus, you get an optically active product.

 

[sleepy425]

oh, and just to show you what that general statement means:

the two steps of the reaction catalyzed by aconitase are as follows:

1. beta-elimination of water (condensation)

2. alpha-addition of water

if you think back to ochem, obviously elimination of water will give you a double bond (it's an elimination rxn). the next step is the addition of water across this double bond. now, if you tried to add water across a double bond using basic ochem reactions (acid catalyzed hydration), you'd get a racemic mixture of products because the water can add from both faces of the molecule. thus it would be optically inactive. that's the generality, that since aconitate (the double bond-containing intermediate) is achiral, adding water across the double bond with an achiral catalyst (like an inorganic acid) will give you an optically inactive product. but by using a chiral catalyst (the enzyme aconitase), you only form one isomer, so it's optically active.

 

[nfg05]

Thanks for the explanation sleepy, very helpful!

 

[sleepy425]

np, good luck! lemme know if you have any questions. also I fixed a typo in the last sentence of my previous post but it shouldn't change the meaning much (I wrote aconitate where I meant to write aconitase)