C. hexokinase can efficiently phosphorylate galactose to galactose 1phosphate.
D. the enzyme affected in galactosemia is activated by a hormone produced in the mammary gland.
Correct answer = B. Uridine diphosphate (UDP)-glucose is converted to UDPgalactose by UDP-hexose 4-epimerase, thereby providing the appropriate form of galactose for lactose synthesis. Isomerization of fructose to galactose does not occur in the human body. Galactose is not converted to galactose 1phosphate by hexokinase. A galactose-free diet provides no galactose. Galactosemia is the result of an enzyme (galactose 1-phosphate uridylyltransferase) deficiency.
2.2. A 5-month-old boy is brought to his physician because of vomiting, night sweats, and tremors. History revealed that these symptoms began after fruit juices were introduced to his diet as he was being weaned off breast milk. The physical examination was remarkable for hepatomegaly. Tests on the baby’s urine were positive for reducing sugar but negative for glucose. The infant most likely suffers from a deficiency of:
A. aldolase B.
B. fructokinase.
C. galactokinase.
D. β-galactosidase.
Correct answer = A. The symptoms suggest hereditary fructose intolerance, a deficiency in aldolase B. Deficiencies in fructokinase or galactokinase result in relatively benign conditions characterized by elevated levels of fructose or galactose in the blood and urine. Deficiency in β-galactosidase (lactase) results in a decreased ability to degrade lactose (milk sugar). Congenital lactase deficiency is quite rare and would have presented much earlier in this baby (and with different symptoms). Typical lactase deficiency (adult hypolactasia) presents at a later age.
2.3. Lactose synthesis is essential in the production of milk by mammary glands. In lactose synthesis:
A. galactose from galactose 1-phosphate is transferred to glucose by galactosyltransferase (protein A), generating lactose.
B. protein A is used exclusively in lactose synthesis.
C. α-lactalbumin (protein B) regulates the specificity of protein A by decreasing its affinity for glucose.
D. protein B expression is stimulated by prolactin.
Correct answer = D. α-Lactalbumin (protein B) expression is increased by the hormone prolactin. Uridine diphosphate–galactose is the form used by the galactosyltransferase (protein A). Protein A is also involved in the synthesis of the amino sugar N-acetyllactosamine. Protein B decreases the Michaelis constant (Km) and, so, increases the affinity of protein A for glucose.
2.4. A 3-month-old girl is developing cataracts. Other than not having a social smile or being able to track objects visually, all other aspects of the girl’s examination are normal. Tests on the baby’s urine are positive for reducing sugar but negative for glucose. Which enzyme is most likely deficient in this girl?
A. Aldolase B
B. Fructokinase
C. Galactokinase
D. Galactose 1-phosphate uridylyltransferase