Cell-surface carbohydrates mediate cell-recognition Human blood groups:A Band AB. 之a2 ified sugars may contain nitrogen. more of t oups have been g 。 24 Important Concepts 24 Important Concepts 1. po的ymes. 5-mombered hain hex-etc e right. The a anomer points down,the B anomer points up. 1010 Cell-surface carbohydrates mediate cell-recognition processes. Interactions between one cell and another and between one cell and specific chemical species are called cell-recognition processes. These processes usually involve non-covalent binding (often hydrogen bonds) with molecules on the exterior surface of the cell. These recognition processes are based on carbohydrates, present on cell surfaces, which are linked to either lipids or proteins embedded in the cell membrane. The key features of this class of carbohydrates are illustrated as a molecule of glucosyl cerebroside with a polar, hydrophilic head and two hydrophobic tails. Other cell surface functions mediated by carbohydrates include: The glycolipid, GM1 pentasaccharide, which binds the cholera toxin. Human blood groups: O, A, B and AB. Modified sugars may contain nitrogen. In some sugars, one or more of the hydroxyl groups have been replaced by amino groups. These sugars are called glycosylamines when the nitrogen is attached to the anomeric carbon and amino deoxy sugars when the nitrogen is attached elsewhere. When a sugar is attached by its anomeric carbon to the hydroxy group of another complex residue, it is called a glycosyl group. The remainder of the molecule is called the aglycon. 24 Important Concepts 1. Carbohydrates – naturally occurring polyhydroxycarbonyl compounds found as monomers, dimers, oligiomers and polymers. 2. Monosaccharides – • Aldoses – aldehydes • Ketoses – ketones • Chain Length Prefix – tri-, tetr-, pent-, hex-, etc. 3. Stereochemistry – most natural carbohydrates belong to the D family: The stereocenter farthest from the carbonyl group has the same configuration as (R)- (+)-2,3-dihydroxy-propanal (D-(+)-glyceraldehyde). 24 Important Concepts 4. Cyclic Hemiacetals – keto forms of carbohydrates exist in equilibrium with the corresponding cyclic hemiacetals. • 5-membered furanoses • 6-membered pyranoses • The new asymmetric center formed is called the anomeric carbon (α and β). 5. Haworth Projections – a planar pentagon or hexagon representation of a cyclical D hemiacetal • The anomeric carbon is placed on the right. • The ether oxygen is placed at the top. • Substituents located above or below the ring are attached to vertical lines. • The α anomer points down, the β anomer points up