Lecture 7: Cell surface Oligosaccharides I (March 13) - Membrane Biochemistry 2024 PDF

Lecture 7 BIOC*4580 – Membrane Biochemistry Winter 2024 Cell surface oligosaccharides Cell identity and the extracellular matrix Lehninger Chapter 7 Essentials of Glycobiology, 4th ed. free online https://www.ncbi.nlm.nih.gov/books/NBK579918/ Sugars Lehninger 8th ed. Fig 7.1a Polyhydroxy aldehydes o...

Lecture 7 BIOC*4580 – Membrane Biochemistry Winter 2024 Cell surface oligosaccharides Cell identity and the extracellular matrix Lehninger Chapter 7 Essentials of Glycobiology, 4th ed. free online https://www.ncbi.nlm.nih.gov/books/NBK579918/ Sugars Lehninger 8th ed. Fig 7.1a Polyhydroxy aldehydes or polyhydroxy ketones Simple sugars have the chemical formula (CH2O)n D-aldohexoses Of these, glucose, galactose and mannose are important for mammalian cell surfaces Note: Mannose is the C-2 epimer of glucose; Galactose is the C-4 epimer of glucose Lehninger 8th ed. Fig 7.3 Fischer projections Fischer projections are used to show the chemical structure The carbon chain is written vertically, with the oxidized carbon (aldehyde or keton) nearer the top Substituents at each chiral carbon atom (H & OH) are written left or right depending on where they would appear with the carbon seen from this perspective Sugars are D- if bottom-most chiral OH is on the right, L- if it is on the left Lehninger 8th ed. Fig 7.2 Sugar cyclization- Converts sugars from linear structures (Fischer projections) to rings (Haworth perspectives) and forms either hemiacetals or hemiketals OH groups on the right in the Fischer- points down in Haworth (below the plane of the ring) OH groups on the left in the Fischer- points up in Haworth (above the plane of the ring) The terminal –CH2OH group projects upward for D-sugars; Downward for L- sugars Anomeric OH on the same side of the ring as the CH2OH – β; when it is on the opposite side from the CH2OH – α For D-sugars Lehninger 8th ed. Fig 7.6 LAB: Left Above Beta Pyranose rings are not planar but assume one of 2 “chair” conformations Conformations and configurations Conformations can be interconverted without breaking any bonds. The relative energies of each conformation is different and therefore, the 2 conformations do not readily interconvert. Another conformation, the “boat” is found only with very bulky substituents. Glucose derivatives Other hexoses have analogous variations, and are named by analogous schemas Amino sugars (e.g., D-glucosamine) are modified by replacing OH-2 with NH2-2 In N-acetyl-amine sugars (e.g., Nacetylglucosamine), this amine is acetylated, forming an amide bond Uronate sugars (e.g., glucuronate) have the terminal carbon (here C6) oxidized to carboxylate -onate sugars (e.g., gluconate) have the aldehyde oxidized to carboxylate These can cyclize to form lactones Lehninger 6th ed. Fig 7.9 Sialic acids Sialic acids are N- or O-substituted derivatives of neuraminic acid Neuraminic acid is a nine-carbon ketose (C2), with a carboxylate group at C1 Sialic acids form pyran rings, leaving a three-carbon hydroxylated “tail” e.g., Neu5Ac has an N-acetyl group at C5 Generally, only added as the last residue of saccharide chains Here their distinct properties are easily recognized Often function as a signal to not degrade a molecule or cell Hemagglutinin (HA) Neuraminidase (NA) Human Influenza virus (H#N#) Neuraminidase enzyme –cleaves neuraminic acid residues Neuraminic Acid