MEDizzy - A woman with nonclassical galactosemia is considering

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Glycogen Metabolism

A woman with nonclassical galactosemia is considering becoming pregnant and is concerned that she will be unable to synthesize lactose in order to breast-feed her child. Her physician, who recalls her biochemistry, tells her this should not be a problem, and that she will be able to synthesize lactose at the appropriate time. This is true due to the presence of which of the following?

ExplanationFor this woman to synthesize lactose, she needs to synthesize the precursors UDP-galactose and glucose, both of which are available from glucose. Glucose is converted to glucose-6-phosphate by hexokinase in the breast, and then phosphoglucomutase will convert this to glucose-1-phosphate (G1P). The G1P will react with UTP in the glucose-1-phosphate uridyl transferase reaction, producing UDP-glucose. The C4 epimerase will then produce UDP galactose from UDP glucose. The UDP galactose then condenses with free glucose (using lactose synthase) to produce lactose and UDP. The other enzymes listed as answers are not required to produce lactose from the single precursor glucose. Fructokinase is unique for fructose metabolism. Aldolase is a glycolytic enzyme, which is deficient in hereditary fructose intolerance. Classical galactosemia (severe, type 1) is a deficit of galactose-1-phosphate uridyl transferase. Patients cannot metabolize galactose, and the accumulating galactose-1-phosphate interferes with glycogen degradation. Nonclassical galactosemia (type 2) is a deficit in galactokinase, such that galactose cannot be phosphorylated. The complications in type 1 galactosemia due to the accumulation of galactose-1-phosphate are not seen in type 2 galactosemia. In either case, the missing enzymes are not required for the synthesis of lactose.