Nitrogen, Nitrogen Cycle and Excretion of Nitrogen

00:01 In the lectures on amino acid metabolism, I've talked about how amines are moved from one molecule to another, and how ammonia is a toxic byproduct of their metabolism.

00:11 With this lecture, I'll talk about how those amines and how that ammonia are combined and prepared for excretion.

00:18 Nitrogen in the body, as we have seen, comes in various forms.

00:22 Balance is absolutely critical.

00:24 Nitrogen is an essential nutrient, but it’s also one that two high levels can be toxic.

00:30 The body must be able to handle the nitrogen arising from amino acid metabolism.

00:35 But nitrogen is also needed for the synthesis of other molecules.

00:38 This includes the nucleotides, ATP, GTP, CTP, UTP, dATP, dCTP, dGTP and dTTP.

00:47 Nitrogen is also needed for making of non-protein amino acids.

00:51 These include ornithine, citrulline and sarcosine as shown in the screen.

00:56 And there are yet other nitrogen containing compounds that are necessary for synthesis in the body.

01:01 This includes choline, various vitamins and carnitine.

01:07 Now, the nitrogen cycle is a very important cycle in our environment.

01:10 Nitrogen is a -- the nitrogen in our atmosphere must be reduced.

01:14 And there are nitrifying organisms at the very deepest parts of our soil that grab that nitrogen and convert it into the amines that are useful for higher organisms.

01:23 That over all process is shown on the slide.

01:28 Now, nitrogen must be excreted if the balance is to be properly maintained.

01:33 As I've noted, amino acids through transamination make nitrogen mobile.

01:38 It's very easy to move nitrogen from one amino acid to a keto acid and vice versa.

01:45 The toxicity of ammonia means that the nitrogen that's produced as a byproduct of amino acid metabolism must be handled properly.

01:54 In the excretion process, there are various strategies that are employed by different organisms.

02:00 Organisms for example that are ammonotelic - excrete ammonia.

02:04 This includes fish and this is why you have to periodically clean your aquarium.

02:08 Uricotelic organisms excrete uric acid and these include birds.

02:14 Ureotelic organisms include human beings.

02:16 These excrete urea and it also includes most vertebrates and a few invertebrates.

02:22 Now, when we look at the structure of these molecules, we see of course that ammonia only contains one nitrogen but has the advantage that it's water soluble.

02:31 It has the disadvantage of course that it's toxic.

02:34 Uric acid is a very good way to get rid of a lot of nitrogen.

02:37 It has four nitrogens within it but suffers from the disadvantage that it is not very water soluble.

02:44 Urea is the sort of the perfect compromise of the two.

02:47 It contains two nitrogens for excretion and it's very water soluble.

02:52 The ammonia that's produced by amino acid catabolism is used in the urea cycle.

02:58 And the uric acid that's produced by purine catabolism is a byproduct of the breakdown of nucleotides.

03:05 And finally, urea is produced by the urea cycle.