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Fumarase Deficiency

Fumarase deficiency is an extremely rare disease in humans. People with this disease can survive although they are significantly affected.

a) Given the importance of fumarase to the citric acid cycle how is it possible for people with fumarase deficiency to produce adequate ATP for survival?

b) what would be the overall consequences of this deficiency for fuel catabolism in these people?

c) What would be the expected impact of this disease on the urea cycle?

Solution Preview

When fumarase is absent (or nonfunctional), the Krebs cycle as we know it cannot function properly. The step between fumarate and malate won't take place. As a result, oxaloacetate cannot be formed. Therefore, there is no oxaloacetate present to combine with acetylCoA to form citrate.

Now, what will happen? Clearly, such an individual needs ATP production. How does he do it? With the buildup of acetylCoA, we also see the buildup of pyruvate. Make sense? Remember, pyruvate is converted to acetylCoA by the pyruvate dehydrogenase enzyme complex. With pyruvate present in higher concentrations, the excess can be shuttled into lactate. This reduction reaction is a "dead end" pathway. The lactate can go nowhere except be excreted. As a result, the individual suffers from metabolic acidosis as well. (Lactate/pyruvate/fumarate, etc. are all acids.) However, the conversion of pyruvate to lactate allows for more glucose to be converted into pyruvate. This process, as you know, yields 2 ATP (net) for every molecule of glucose oxidized to pyruvate. This "anaerobic" production of ATP keeps the fellow alive, albeit tenuously.

Clearly, such an ...

Solution Summary

Fumarase Deficiency in humans is discussed. The important for fumarase to the citric acid cycles is discussed.