Citrate synthase, one of the enzymes in the series of enzyme-c...

Question

Citrate synthase, one of the enzymes in the series of enzyme-catalyzed reactions known as the citric acid cycle, catalyzes the synthesis of citric acid from oxaloacetic acid and acetyl-CoA. If the synthesis is carried out with acetyl-CoA that contains radioactive carbon (¹⁴C) in the indicated position, the isomer shown here is obtained.
a. Which stereoisomer of citric acid is synthesized: R or S?

a. Which stereoisomer of citric acid is synthesized: R or S?

Reaction diagram showing the synthesis of citric acid from oxaloacetic acid and acetyl-CoA by citrate synthase.

Accepted Answer

Hey, everyone. Let's do this problem. It says the following synthesis is carried out using the green yards re agent labeled with the radioactive carbon carbon 14 to get the ice is shown below, identify which stereo ice merge and the products is S and which one is our. So when it mentions S and R, we're talking about the configuration of the Cairo carbon in the ice climbers. So our synthesis shows in a Seattle to hide compound reacting with our green yard re agent C 14 H three MGB R and an acid work up step. And we're producing these two stereo ice climbers that have each have one Cairo center, one Cairo carbon and attached to that Cairo carbon is four different groups. A methyl group, an alcohol group, A methyl group with carbon 14 and a hydrogen and the same groups are on the other stereo ice um are just in a different orientation. So if we're determining the configuration R or S for the stereo customers, we're going to use the con angled pre log nomenclature rules. So let's review those rules before we start applying them. So first, as we mentioned, a cargo center will have four different groups. So we want to assign the priorities of those four different groups. And we do this by atomic mass. So the higher the atomic mass of the atom, the first Adam attached to the carroll center, then the higher the priority. Well, what if the first atom is the same? So maybe the whole group is different, but the first atom is the same and we have a tie, then we simply look at the adjacent atoms in that group and compare those with the same priority rubric. What if we have a double bond or a triple bond? Well, then we count those atoms as being attached multiple times. So say for a double bond that's two times a triple bond, that is three times. So if we have a carbon double bonded to oxygen, we treat that as if the carbon has two single bonds to two oxygen atoms. Okay. Now, when we're assigning R or S configuration, we look at the orientation of the lowest priority group group number four. So if that group is wedge or dash, we will follow a different rubric. So when we're assigning our s we can we connect groups 12 and three. And if those groups connect counterclockwise, that will have a different orientation than if they connect clockwise. So if group number four is wedge, then counterclockwise is our configuration clockwise is as configuration if group number four is dash, counterclockwise is S configuration clockwise is our configuration. Usually We learn that group number four being dashed or in the back or into the page is the standard. So I would memorize this orientation, this rubric and then just remember that the other one is the opposite. Okay. So if I have the option, I always make number four dash and that's simply because our orientation will be going towards the right clockwise, right. So it's easiest to remember that are to the right or to the right. But we have a little curveball in this question when we go to assign priorities, notice that our hydrogen atom which is always group number four is not wedge or dash, right? So we have two different options. In this case, we can either draw an eyeball and use that perspective to make group number four in the page. So our eyeball would be over here, right? If we look at this compound from the side now that h that hydrogen is going to be going away from me or we can rotate and redraw the atoms. Okay. So I'll show you both options. So if we redraw stereo I summer A, I'm going to rotate them but keeping their orientation the same. Okay. So we need our spatial awareness here is very important. So let me redraw, the methyl group is staying the same. Now, we're going to redraw these three groups, the hydrogen, the C 14 method and the alcohol and I'm going to send that hydrogen to where the alcohol is. So we're basically rotating it this way back into the page if we're looking straight down from where we are now. So the hydrogen will be dashed, that alcohol is going to come forward and be wedged, which will send that C14 method group in line with the plane C H three. Okay. So these are the same thing stereo A summer B, we can do the same thing rotate these three groups so that the hydrogen is dashed, That C14 Methyl group is going to come to the front and be wedge and that alcohol group is going to go into the plane. Okay. So now let's assign the priorities of our stereo is members of the groups on the carol carbons for our stereos. So starting with stereo is a, we have our lowest priority group hydrogen, highest priority group alcohol. Now we have two methyl groups but carbon usually has an atomic mass of 12. So the carbon 14 method is going to have higher priority, which gives the regular methyl group CH three with carbon have the third priority. So if we're using our eyeball, the CH three group, carbon 14 CH three is sort of on the right hand side, the alcohol group is on the left hand side and this methyl group is on the bottom. So connecting 12 and three were going clockwise and since our group four would be in the back. That is our configuration, right? So let's double check with our other drawing, make sure it's the same our priorities. 1234, connect 12 and three. That is also clockwise. So that is our configuration. Does that make sense? So let's apply the same for stereo B Well, we know it's going to be s right, because it says one product is S and one is our. So let's double check that that is true. So we have our priorities. Alcohol is one, it's actually the same four groups, right? So same four priorities, one, 234 using our eyeball connecting 12 and three that is going from the top to the left to the bottom up around to the right. So that is counter clockwise. And since group four would be in the back here, that is as configuration. And let's double check with our rotated drawing. We have priorities connecting 12 and three. Yep, that's going counterclockwise. So that is S configuration. So that makes a, the correct answer for this problem. That's it for this one. I hope this video is helpful and thanks for watching.

Key Concepts

Citrate Synthase Function

Stereochemistry and Isomerism

Radioactive Tracing in Organic Chemistry

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