Passage
The ABO blood groups are determined by the polysaccharide antigens linked to sphingolipids on red blood cell membranes. The polysaccharide known as the H-antigen (O-type blood) consists of a polysaccharide (R-group) glycosidically linked to D-galactose, which is connected to L-fucose in an α-1,2 linkage. Type A and type B blood express the A- and B-antigens, respectively (Figure 1) . These antigens are modified forms of the H-antigen, in which D-galactose (B-antigen) or a derivative of D-galactose called GalNAc (A-antigen) is added in an α-1,3 linkage to the D-galactose moiety of the H-antigen.
Figure 1 Structures of the H-, A-, and B-antigens. The R-group represents additional carbohydrates.The addition of GalNAc or galactose to the H-antigen is catalyzed by an enzyme encoded by the ABO gene. One allele of the gene encodes GalNAcyl transferase and produces the A-antigen, while another allele encodes galactosyl-transferase and yields the B-antigen (Figure 1) . The two alleles differ at seven nucleotide positions, summarized in Table 1.Table 1 Nucleotide Positions Where the Alleles Encoding GalNAcyl Transferase and Galactosyl Transferase Differ
Individuals with both alleles express both antigens and have type AB blood. Those with type O blood have a third allele that encodes an inactive form of the enzyme. A study of blood type abundance was conducted, and the number of participants with each type is shown in Table 2. Table 3 shows several fatty-acyl chains found in the sphingolipids to which the antigens were attached.Table 2 Number of Participants That Express Each Blood Type in a Study of 10,000 Individuals
Table 3 Types of Sphingolipids to Which Polysaccharide Antigens Were Bound, as Determined by Mass Spectrometry (Note: For fatty acid chains, the numbers on the left and right of the colon indicate number of carbons and number of C=C double bonds, respectively)

-Which lipid in Table 3 would make the greatest contribution to membrane fluidity?

Question

Passage
The ABO blood groups are determined by the polysaccharide antigens linked to sphingolipids on red blood cell membranes. The polysaccharide known as the H-antigen (O-type blood) consists of a polysaccharide (R-group) glycosidically linked to D-galactose, which is connected to L-fucose in an α-1,2 linkage. Type A and type B blood express the A- and B-antigens, respectively (Figure 1) . These antigens are modified forms of the H-antigen, in which D-galactose (B-antigen) or a derivative of D-galactose called GalNAc (A-antigen) is added in an α-1,3 linkage to the D-galactose moiety of the H-antigen.

Figure 1 Structures of the H-, A-, and B-antigens. The R-group represents additional carbohydrates.The addition of GalNAc or galactose to the H-antigen is catalyzed by an enzyme encoded by the ABO gene. One allele of the gene encodes GalNAcyl transferase and produces the A-antigen, while another allele encodes galactosyl-transferase and yields the B-antigen (Figure 1) . The two alleles differ at seven nucleotide positions, summarized in Table 1.Table 1 Nucleotide Positions Where the Alleles Encoding GalNAcyl Transferase and Galactosyl Transferase Differ

Individuals with both alleles express both antigens and have type AB blood. Those with type O blood have a third allele that encodes an inactive form of the enzyme. A study of blood type abundance was conducted, and the number of participants with each type is shown in Table 2. Table 3 shows several fatty-acyl chains found in the sphingolipids to which the antigens were attached.Table 2 Number of Participants That Express Each Blood Type in a Study of 10,000 Individuals

Table 3 Types of Sphingolipids to Which Polysaccharide Antigens Were Bound, as Determined by Mass Spectrometry (Note: For fatty acid chains, the numbers on the left and right of the colon indicate number of carbons and number of C=C double bonds, respectively)

-Which lipid in Table 3 would make the greatest contribution to membrane fluidity?

Quizplus · Accepted Answer

11ecba2d_0ecf_da3a_a3bf_7da6fc58d677_MD0008_00 The fluidity and rigidity of plasma membranes is partially dependent on the structure of fatty acyl tails in the lipid bilayers. Increasing hydrophobic interactions between fatty acyl tails leads to decreased motion and more rigidity. The length and number of double bonds in a fatty acyl tail influence the number of hydrophobic interactions possible. Short tails participate in fewer hydrophobic interactions than long tails and therefore result in increased fluidity. Carbon-carbon double bonds in fatty acyl tails are typically in the cis-conformation and introduce a bend in the tail, preventing adjacent tails from interacting efficiently with each other. A higher number of double bonds leads to decreased interaction and greater fluidity. The lipid in Table 3 that has the shortest fatty acyl tail length and the most double bonds will make the greatest contribution to membrane fluidity. GL6 and GL3 have the shortest fatty acyl tails (20 carbons), but GL6 has more double bonds (one instead of zero). Therefore, GL6 makes the greatest contribution to membrane fluidity. (Choice A) GL2 has the longest tail of the lipids in the table and has zero C=C double bonds. It makes the smallest contribution to membrane fluidity. (Choice B) GL3 and GL6 both have the shortest tail lengths, but GL3 has no C=C double bonds, so it makes a smaller contribution to membrane fluidity than GL6. (Choice C) GL5 has a double bond, but it has a longer tail than GL6, so GL5 makes a smaller contribution to membrane fluidity. Educational objective: The fluidity of a cell membrane is largely dependent on the lengths of the fatty acyl tails in the bilayer and on the number of double bonds in each tail. Short chains with double bonds yield the highest fluidity because they participate in the fewest hydrophobic interactions with neighboring lipids.

Passage The ABO Blood Groups Are Determined by the Polysaccharide Antigens