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

-What is the stereochemical configuration of the galactose component of GC?

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

-What is the stereochemical configuration of the galactose component of GC?

Quizplus · Accepted Answer

11ecba2d_0e63_5c29_a3bf_afed9da19f0e_MD0008_00 Molecules may have one or more stereocenters (an atom bonded to four unique substituents), each of which can adopt one of two stereochemical configurations with distinct properties. Every stereocenter within a molecule is given an R or S designation based on the relative positions of the substituents and their priority ranking. Priority is determined by the atomic mass of each atom to which the stereocenter is bonded, with higher mass given higher priority. If two or more of the bonds are to atoms of the same atomic mass, then the atoms to which they are bonded are assessed until a difference is found. With the exception of dihydroxyacetone, all carbohydrates have at least one stereocenter. Most monosaccharides (single sugar units) contain multiple stereocenters and cannot be designated as R or S because R and S configurations refer only to individual stereocenters. Instead, each carbon is given a number starting at one end of the chain. The anomeric carbon (two bonds to oxygen) is given the lowest number possible, and the carbohydrate is then given an L or D designation based on the configuration of the stereocenter with the highest number. An R configuration at this position corresponds to a D-sugar, and an S configuration corresponds to an L-sugar. Biological structures such as the myelin sheath contain L-amino acids and D-sugars almost exclusively. The galactose component of GC shown in Figure 1 has an R configuration at carbon 5, making it a D-sugar. (Choice A) L-Galactose is not used in biological structures. Figure 1 shows D-galactose. (Choices C and D) R and S designations are used to classify individual stereocenters within a molecule. D-Galactose has both R and S stereocenters, but the molecule as a whole cannot be classified as either R or S. Educational objective: Stereocenters are designated as having R or S configurations based on the arrangement of their substituents and the priority ranking of each. Carbohydrates with multiple stereocenters are given an L or D designation based on the configuration of the highest-numbered stereocenter. Almost all carbohydrates found in nature are in the D configuration.

Passage Nerve Cells Must Maintain the Composition of Their Myelin Sheaths