Question

The "missing" molecule that allowed Krebs to work out the urea cycle pathway is_____________.

fumarate

carbamoyl phosphate

ornithine

citrulline

arginosuccinate

The first nitrogen in the citric acid cycle is introduced by what molecule?

glutamic acid

carbamoyl phosphate

glutamine

directly from ammonia

please answer both of them

Answer 1

The urea cycle is also known as ornithine cycle involving a sequence of biochemical reactions to produce the urea from ammonia. The ureotelic organisms like some bony fishes, amphibians and mammals produce urea by converting th toxic ammonia to urea. The urea cycle was discovered by Krebs and Kurt Henseleit in 1932, basing on the idea of A. Kossel and H. D. Dakin who had shown that arginine could be hydrolysed by the enzyme arginase to form ornithine and urea in an inorganic reaction. Krebs and Henseleit postulated that a similar reaction could occur in living cells. The ornithine and citrulline could be the intermediate reactions. Krebs started working on the synthesis of arginine. He used Warburg manometer and mixed a slice of liver with purified ornithine and citrulline. He found that citrulline acted as a catalyst in the metabolic reactions of urea from ammonia and carbon dioxide. The arginosuccinate was the missing link that stimulate the invention of urea cycle. Therefore, the option 5 is correct.

The ciric acid cycle involves the series of biochemcial reactions involved in carbohydrate metabolism .The glucose is broken down to pyruvate during glycolysis. The pyruvate combines with acetyl CoA to enter cotric acid cycle as 4 - carbon compound oxaloacetate (OAA). The OAA is converted to 6 - carbon compound citrate and then to isocitrate. The isocitrate is converted to $\alpha$ -ketoglutarate. Here, the glutamate or glutamic acid provides first nitrogen in citric acid cycle. Therefore, the option 1 is correct.