Urea Cycle

Well, the urea cycle is actually the place where the urea is made from that previous figure that I've just showed. It's therefore important that we consider the reactions of the urea cycle in the slides ahead. The urea cycle occurs primarily in the liver and also in the kidney as we saw. The urea cycle consists of four different cycle reactions and one feeder reaction in the overall process as I will show you. That involves five different enzymes. The feeder reaction is shown at the top, and it involves the incorporation of one molecule of ammonia or ammonium, same thing, and one carbon dioxide. That creates a molecule that then feeds into the cycle. It's necessarily for that reaction to occur in order for the overall cycle to occur. So this process I've just described involves then the incorporation of one ammonium ion from the feeder reaction and one amine from an amino acid. In this way, both the amines and the ammonium ions get balanced. So the output of the cycle is one molecule of urea per turn of the cycle, and that means that urea contain one of its nitrogens from ammonium and one of its nitrogens from amine. The net reactions per turn of the cycle involve the incorporation of two amines, in this case NH3 coming from ammonium or NH2 that becomes NH3 coming from an amine. Carbon dioxide is also needed, three ATPs and water. That creates urea, two molecules of ADP, four phosphates, and an AMP. Let's consider the reactions of the urea cycle. You remember that the very first reaction is a feeder reaction. It creates a molecule that then feeds into the overall cycle. In that reaction, ammonium ion that's produced by the catabolic process of amino acid catabolism is...