The Isoenzyme Hexosaminidase a (HEXA) Is Composed of Subunits α

question 113

Multiple Choice

The isoenzyme hexosaminidase A (HEXA) is composed of subunits α and β and breaks down molecules containing terminal N-acetyl hexosamines.Tay-Sachs disease is caused by a recessively inherited loss-of-function mutation in the gene coding for the α subunit, which is the subunit that hydrolyzes the lipid GM2 ganglioside.Accumulation of this lipid in the brain leads to progressive deterioration of the nervous system and death, usually by age 4.The most common mutation in the HEXA gene is an insertion of four base pairs in exon 11 that creates a nonfunctional truncated protein.HEXA activity is 0 to 6 percent in homozygous recessives and 7 to 35 percent in heterozygous carriers, compared with noncarriers (100 percent) .What evidence presented here strongly suggests that this truncated α subunit can still associate with the normal β subunit, thereby inhibiting its association with functional α subunit?

• A) There is a decrease in hydrolysis of GM2 ganglioside.
• B) HEXA activity in blood serum is 0 to 6 percent in homozygous recessives.
• C) The mutation only occurs in the α subunit.
• D) GM2 ganglioside accumulates in the brain.
• E) HEXA activity in heterozygous carriers is significantly less than 50 percent of that in noncarriers.

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