Argininosuccinate Synthetase

00:02 The next step in the reaction involves the enzyme argininosuccinate synthetase.

00:07 And in this reaction, you remember there's a two-step process.

00:10 In the first step of the process, ATP cleaves off a pyrophosphate.

00:15 We see that happening in that very first step in the top.

00:18 We see a little molecule in blue, PPi.

00:20 That's pyrophosphate.

00:22 That is released from the ATP.

00:24 That allows the remaining part of the ATP in the form of AMP to combine with the citrulline to make the L-citrulline adenylate complex.

00:33 In the second part of the process, aspartate displaces the AMP and that’s where we see the AMP leading also shown in blue to create the L-argininosuccinate.

00:44 The chemical mechanism of the reaction is shown below.

00:47 Now this two-step process, as I said, involves AMP attachment.

00:51 And we see the AMP attachment going here, followed by displacement of the AMP by the aspartate.

00:57 And the displacement is shown here.

01:00 The product to the reaction argininosuccinate is the substrate for the next step of the reaction.

01:05 This reaction is the rate limiting step of the cycle.

01:09 Meaning that it is the slowest and it's the step that other reactions await for it to be completed.

01:16 The gene expression of the enzyme is reduced by arginine.

01:20 Now arginine will see seized further ahead in the pathway.

01:23 And as arginine accumulates, there's no reason to continue making this enzyme.

01:27 So this regulation by arginine, of this enzyme that feeds and makes arginine is important.

01:34 On the other hand, the expression of the enzyme is increased by citrulline.

01:38 And citrulline is further behind that in the cycle and it's literally pushing the thing forward.

01:44 So this kind of regulation ensures that the cell has the proper amount of the argininosuccinate synthetase enzyme.

01:51 Defects in this enzyme lead to citrullinemia which involves an accumulation of ammonia.

01:56 And in fact, most of the enzymes in the urea cycle when their deficient result in accumulation of ammonia and all the consequences that arise as a result of that.

02:06 Like other enzyme deficiencies at the urea cycle, it is treated with a low protein diet and in some cases, with supplementation of arginine which is needed in the next reaction.

02:16 The next step in the process is catalyzed by the enzyme, argininosuccinate lyase.

02:20 We see the structure of the enzyme in the far left of the slide.

02:23 In this reaction, argininosuccinic acid or argininosuccinate, as we describe it, is converted into arginine and fumaric acid.

02:32 This involves a cleaving of the bond that shown as on the screen.

02:38 In this reaction, arginine is produced.

02:40 Now arginine really has two or even three ultimate fates.

02:44 Most commonly, it can go to proteins or it can remain and continue in the urea cycle.

02:49 The fumaric acid or fumarate, as we also call it, can be released and oxidized in the citric acid cycle.

02:56 Now, remember what's happen as we added an aspartic acid in the previous reaction and we've lost fumarate in this reaction.

03:03 The net result was an additional amine got built into our molecule in this case in the form of arginine.

03:09 Now this reaction is very important for the production of arginine.

03:13 Cells need a lot of arginine, not just for the urea cycle but also for making proteins.

03:19 It's also a source of fumarate.

03:21 And the citric acid cycle is a reaction cycle where a lot of energy is produced.

03:25 So having more fumarate available allows the cell to have more energy.

03:29 A deficiency of this enzyme, like that of other urea cycle enzymes, results in excess ammonia accumulation.

03:37 In the next step of the reaction, arginine is cleaved to make urea and ornithine in a reaction catalyzed by arginase.

03:46 Now the ornithine can go ahead and go back to the mitochondria and complete the urea cycle.

03:51 The urea cycle can be excreted into the urine.

03:54 This reaction occurs by cutting the bond shown in arginine here to make the urea.

04:01 Now this enzyme is co-expressed with nitrogen oxide synthase in smooth muscle.

04:06 These two compete for the use of arginine.

04:10 The more arginase available, the more arginase that's used in the urea cycle, the less arginine is available to make nitric oxide.

04:17 So again, we have to balance how much of each of these is use for the cell to properly function.

04:22 Nitric oxide functions and signaling to relax smooth muscle and facilitates the erection of the penis.

04:29 A deficiency of arginase is the rarest of the urea cycle enzymes.

04:33 And the reason for that is because there were two forms of arginase that are present in cells.

04:37 They provide some backup when one is deficient.