Lectins, Cell Recognition, and Viral Entry

Vocabulary practice flashcards covering lectin biology, affinity versus avidity, selectin-mediated leukocyte rolling, ABO blood group antigens, and influenza hemagglutinin-sialic acid viral entry.

Lectin

A carbohydrate-binding protein that recognizes specific sugar structures on glycoproteins, glycolipids, and free polysaccharides without catalyzing reactions or modifying the sugars.

Glycosyltransferases

Enzymes that synthesize and attach carbohydrate structures ("write" the sugar code), functioning in tandem with lectins ("readers").

Afinity (in carbohydrate binding)

The binding strength of an individual lectin binding site to a single sugar molecule, typically characterized by a weak dissociation constant with a $K_d$ in the $\text{mM}$ range.

Avidity

The overall cumulative binding power achieved when a multivalent lectin forms multiple simultaneous weak contacts with multivalent glycans, increasing effective binding strength to a $K_d$ in the $\text{nM}$ to $\mu\text{M}$ range.

Selectins

Endothelial cell-surface carbohydrate-binding proteins (including P-, E-, and L-selectins) displayed during inflammation or infection to mediate leukocyte tethering and rolling.

Sialyl-Lewis X

A specific tetrasaccharide sugar epitope displayed on white blood cell glycans that is recognized and bound by endothelial selectins.

Leukocyte rolling

The physiological process in which repeated, reversible grab-and-release binding events between selectins and sialyl-Lewis X slow down white blood cells along the vascular endothelium, allowing them to sense chemokine signals.

Integrin-mediated adhesion

The firm, tight adhesion step in which activated integrins on a slowed white blood cell bind endothelial ICAM or VCAM, stopping the cell so it can crawl out into the inflamed tissue.

H-antigen

The core glycan scaffold attached to red blood cell membranes that serves as the base structure for ABO blood group antigens.

Type O blood

A blood group characterized by having only the unmodified H-antigen scaffold with no additional terminal sugar on red blood cells, with plasma containing both anti-A and anti-B antibodies.

Type A blood

A blood group defined by the addition of $N\text{-acetylgalactosamine}$ (GalNAc) as the terminal sugar on the H-antigen scaffold, with plasma containing anti-B antibodies.

Type B blood

A blood group defined by the addition of galactose (Gal) as the terminal sugar on the H-antigen scaffold, with plasma containing anti-A antibodies.

Type AB blood

A blood group displaying both GalNAc and Gal terminal sugars on red blood cell surfaces, with plasma containing neither anti-A nor anti-B antibodies (universal recipient).

Hemagglutinin (HA)

A multivalent viral surface lectin on the influenza envelope that mediates cell entry by binding to terminal sialic acid on host respiratory epithelial glycans.

Sialic acid

A negatively charged terminal carbohydrate found on host glycoproteins in the respiratory tract (nose, throat, and lungs) that serves as the receptor for influenza hemagglutinin.

Neuraminidase (NA)

An influenza viral enzyme that cleaves terminal sialic acid residues to allow newly assembled progeny virions to bud and release from the host cell surface.

$\alpha2,6$-linked sialic acid

The specific linkage conformation of sialic acid predominantly expressed in human upper airway cells, preferentially bound by human influenza hemagglutinin.

$\alpha2,3$-linked sialic acid

The specific linkage conformation of sialic acid predominantly expressed in deep avian lung tissues, preferentially bound by bird flu (avian influenza) hemagglutinin.

Concanavalin A

A well-known plant lectin isolated from jackbeans that specifically binds certain carbohydrate structures.

Ricin

A potent plant toxin and lectin that binds carbohydrates on cell surfaces.