Passage
The influenza A virus is an enveloped RNA virus with potential for zoonosis (transmission from animals to humans) . It obtains its plasma membrane from its host cell, but the composition of the viral envelope and vertebrate plasma membrane are distinct. The envelope of H6-G225D consists mostly of phosphatidylethanolamine as well as some negatively charged glycerophospholipids. The outer membrane contains hemagglutinin (HA) proteins that bind to the carbohydrate known as sialic acid (SA) on host cell receptor proteins, facilitating viral entry. It also contains neuraminidase enzymes that break the bond between SA and galactose in the host membrane glycans, allowing new virions to be released.Minute changes in the genes that code for HA proteins can alter the virulence, host, and attachment mechanism. In general, influenza A infects avian species by attaching to tracheal tissue, which displays SA groups bound to receptors through an α-2,3-glycosidic linkage to galactose (Figure 1) . Avian viruses generally cannot attack human cells, in which SA is linked to galactose through an α-2,6-glycosidic linkage instead of an α-2,3 linkage.
Figure 1 Structure of the influenza A receptor in avian cell membranesIn a recent study, however, a G225D substitution in HA, caused by a GGC to GAU codon mutation, enabled avian virions to infect human cells. The stability of G225D HA proteins was compared to that of H6-P186L, which is created by a single CCC to CUC mutation and exclusively infects avian cells, and H1N1, which targets only humans. The viral RNA of H6-P186L, H6-G225D, and H1N1 was cloned using cDNA and expressed in embryonic kidney cells. The melting curve of each purified HA protein was determined using differential scanning calorimetry, with peaks representing melting temperatures (Figure 2) .
Figure 2 Hemagglutinin melting curves for H6N1 and H1N1 influenza A strains
Adapted from De vries RP, Tzarum N, Peng W, et al. A single mutation in Taiwanese H6N1 influenza hemagglutinin switches binding to human-type receptors. EMBO Mol Med. 2017;9(9) :1314-1325.
-Which statement best describes the structure of the avian receptor of influenza A shown in Figure 1?

Question

Passage
The influenza A virus is an enveloped RNA virus with potential for zoonosis (transmission from animals to humans) . It obtains its plasma membrane from its host cell, but the composition of the viral envelope and vertebrate plasma membrane are distinct. The envelope of H6-G225D consists mostly of phosphatidylethanolamine as well as some negatively charged glycerophospholipids. The outer membrane contains hemagglutinin (HA) proteins that bind to the carbohydrate known as sialic acid (SA) on host cell receptor proteins, facilitating viral entry. It also contains neuraminidase enzymes that break the bond between SA and galactose in the host membrane glycans, allowing new virions to be released.Minute changes in the genes that code for HA proteins can alter the virulence, host, and attachment mechanism. In general, influenza A infects avian species by attaching to tracheal tissue, which displays SA groups bound to receptors through an α-2,3-glycosidic linkage to galactose (Figure 1) . Avian viruses generally cannot attack human cells, in which SA is linked to galactose through an α-2,6-glycosidic linkage instead of an α-2,3 linkage.

Figure 1 Structure of the influenza A receptor in avian cell membranesIn a recent study, however, a G225D substitution in HA, caused by a GGC to GAU codon mutation, enabled avian virions to infect human cells. The stability of G225D HA proteins was compared to that of H6-P186L, which is created by a single CCC to CUC mutation and exclusively infects avian cells, and H1N1, which targets only humans. The viral RNA of H6-P186L, H6-G225D, and H1N1 was cloned using cDNA and expressed in embryonic kidney cells. The melting curve of each purified HA protein was determined using differential scanning calorimetry, with peaks representing melting temperatures (Figure 2) .

Figure 2 Hemagglutinin melting curves for H6N1 and H1N1 influenza A strains
Adapted from De vries RP, Tzarum N, Peng W, et al. A single mutation in Taiwanese H6N1 influenza hemagglutinin switches binding to human-type receptors. EMBO Mol Med. 2017;9(9) :1314-1325.
-Which statement best describes the structure of the avian receptor of influenza A shown in Figure 1?

Quizplus · Accepted Answer

11ecba2d_0eb2_3ff3_a3bf_d178bd47319b_MD0008_00 Carbohydrates on the outer surface of a cell can act as markers for cell recognition and signaling. During post-translational modification and lipid processing, sugar chains (glycans) are attached to proteins or lipids, converting them into glycoproteins or glycolipids, respectively. These sugars can be classified as reducing if they contain a free anomeric carbon that can be oxidized, and as nonreducing when this carbon is linked to another molecule. In linear form, the free anomeric carbon is either an aldehyde or a ketone. In cyclic form, reducing sugars have hemiacetal or hemiketal configurations at their anomeric carbons whereas nonreducing sugars have acetal or ketal groups. Figure 1 shows that the influenza A receptor is a glycoprotein because the carbohydrate chain is linked to a protein through the amino acid asparagine. The terminal group of the chain consists of SA bound to two, linked sugar molecules (a disaccharide). Both sugars in the disaccharide are bound to another molecule at their anomeric carbons, so neither contains a hemiacetal or hemiketal. Therefore, the disaccharide is a nonreducing sugar, and the correct description of the receptor should state that it is a glycoprotein with SA bound to a nonreducing disaccharide. (Choices B and D) The influenza A receptor is a glycoprotein. Glycolipids have carbohydrates attached to lipids such as fatty acids, glycerol, or sphingosine backbones. The terminal group of the sugar chain contains three bonds through anomeric carbons, not just one. (Choice C) To be a reducing sugar, the disaccharide bound to the anomeric carbon of SA would need to have a free anomeric carbon. Instead the anomeric carbon is bound to an R group, making it a nonreducing sugar. Educational objective: Reducing sugars contain free anomeric carbons that provide reducing power when they are oxidized. In linear form the anomeric carbon is an aldehyde or a ketone, and in cyclic form reducing sugars have hemiacetal or hemiketal configurations. Nonreducing sugars contain acetal or ketal structures in their cyclic forms.