Passage
Nerve cells must maintain the composition of their myelin sheaths to function properly. Galactocerebroside (GC) , shown in Figure 1, is a hydrolyzable lipid and an important component of the myelin sheath.
Figure 1 Structure of the myelin sheath component galactocerebrosideHomeostasis of GC is maintained by its dynamic synthesis and degradation, as shown in Figure 2. GC is synthesized by the enzyme ceramide galactosyltransferase (CGT) using UDP-galactose (UDP-Gal) and ceramide (Cer) as substrates. It is degraded by the enzyme galactocerebrosidase (GalC) , which hydrolyzes the glycosidic bond to release galactose (Gal) and ceramide. A deficiency in GalC activity leads to a buildup of GC, and prior research has connected this buildup with neurodegeneration.
Figure 2 Schematic of GCT and GalC activity in healthy and GalC deficient cellsOver 100 inactivating GalC mutations have been identified. Five of the most common mutations are listed in Table 1. The changes to both DNA and amino acid sequence are shown.Table 1 Five Common Galactocerebrosidase Mutations

-Based on the information in the passage, which bond is cleaved by GalC?

Question

Passage
Nerve cells must maintain the composition of their myelin sheaths to function properly. Galactocerebroside (GC) , shown in Figure 1, is a hydrolyzable lipid and an important component of the myelin sheath.

Figure 1 Structure of the myelin sheath component galactocerebrosideHomeostasis of GC is maintained by its dynamic synthesis and degradation, as shown in Figure 2. GC is synthesized by the enzyme ceramide galactosyltransferase (CGT) using UDP-galactose (UDP-Gal) and ceramide (Cer) as substrates. It is degraded by the enzyme galactocerebrosidase (GalC) , which hydrolyzes the glycosidic bond to release galactose (Gal) and ceramide. A deficiency in GalC activity leads to a buildup of GC, and prior research has connected this buildup with neurodegeneration.

Figure 2 Schematic of GCT and GalC activity in healthy and GalC deficient cellsOver 100 inactivating GalC mutations have been identified. Five of the most common mutations are listed in Table 1. The changes to both DNA and amino acid sequence are shown.Table 1 Five Common Galactocerebrosidase Mutations

-Based on the information in the passage, which bond is cleaved by GalC?

Quizplus · Accepted Answer

11ecba2d_0e65_30eb_a3bf_8b7af12f320f_MD0008_00 All monosaccharides have one carbon atom that has two bonds to oxygen, known as the anomeric carbon. The chain-terminating carbon that is closest to the anomeric carbon is designated as position 1, and all other carbons are numbered sequentially. In their linear form, aldoses such as galactose contain an aldehyde group. Because aldehydes are always at the end of a carbon chain (chain-terminating groups), the anomeric carbon of an aldose is always carbon 1. Ketoses contain ketone groups, which are not chain-terminating. Although a ketose could theoretically have a carbonyl at any non-chain-terminating carbon, biologically important ketoses (eg, fructose) have their anomeric carbon at carbon 2. A glycosidic bond links the anomeric carbon of a carbohydrate to another molecule such as a lipid, a protein, a nitrogen base, or another carbohydrate. According to the passage, GalC hydrolyzes a glycosidic bond in GC to release the carbohydrate galactose and the lipid ceramide. Therefore, GalC cleaves the bond between carbon 1 of galactose and an oxygen atom of ceramide. (Choice B) The bond between carbon 1 of galactose and the oxygen atom in the galactose ring links galactose to itself, not to another molecule. It is not the glycosidic bond. (Choice C) The oxygen atom in the ring of galactose does not link galactose to another molecule. In addition, carbon 4 is not bonded to the ring oxygen. (Choice D) Carbon 4 is not the anomeric carbon and does not participate in the glycosidic bond. Educational objective: Glycosidic bonds link the anomeric carbon of a carbohydrate to another molecule, such as a lipid, a protein, a nitrogen base, or another carbohydrate. The anomeric carbon has two bonds to oxygen, and for aldoses it is carbon 1. For biologically important ketoses, the anomeric carbon is carbon 2.

Passage Nerve Cells Must Maintain the Composition of Their Myelin Sheaths