Question

Imagine that you are studying the metabolism of a baby who is feeding on breast milk. You are going to compare the catabolism of two molecules of galactose from lactose in milk with the catabolism of two molecules of leucine from casein in milk. 

1. Detail the catabolic path for the molecules until their final oxidation in CO2 + urea.
2. Include all the reactions that take place and the enzymes that catalyze it.
3. Include the molecular structure and names of all intermediaries.
4. All of them must be intertwined. Use different colors for the reactions that are (a) only from galactose catabolism (b) only from leucine catabolism and (3) common to both.

Answer 1

Follow reaction below. Galactose will produce pyruvic acid via glycolysis. Pyruvic acid will enter in the urea cycle for complete break down. Leucine after catabolysis will generate succinyl CoA, then succinyl CoA will enter in the TCA cycle to produce oxaloacetate. Then, oxaloacetate enter in the urea cycle for complete degradation. Urea cycle will be common in degradation of galactose and leucine.

galactose undergoes degradation to form pyruvic acid

Now pyruvic acid will enter in the urea cycle to for Urea. Therefore complete degradation of galactose will take place.

Now let us discuss catabolism of Leucine

Now Succinyl CoA (Degradation product of leucine) will enter in the TCA cycle to generate oxaloacetate.In the figure given below kindly consider reactions from conversion of succinyl CoA to oxaloacetate. Please avoid other reactions.

The oxaloacetate formed will be enter in the urea cycle for complete catabolysis.