Lecture 4

• Circularization of glucose: alpha-glucose vs. beta-glucose

  • carbonyl carbon circles around and interacts with the next to last carbon

  • alpha-glucose: H above plane

  • beta-glucose: H below plane

• some monosaccharides have identical formulas but different structures (called isomers)

  • aldose (aldehyde sugar) vs ketose (ketone sugar)

    • aldose — carbonyl group is the first carbon

      • ex: glucose, galactose (beta configuration)

      • max number of carbons in ring (ex: 6 carbon rings for glucose and galactose)

    • ketose — carbonyl group is within the carbon chain

      • ex: fructose has carbonyl at carbon 2

      • makes the ring size the max number of carbons minus one (ex: fructose has a five carbon ring)

• other monosaccharides have similar (but not identical) formulas, similar structures, and related functions:

  • deoxyribose vs oxyribose (deoxyribose has one less oxygen)

• monosaccharides are typically found with 3, 5, or 6 carbons

  • pentoses (C5H10O5)

    • aldose: ribose

    • ketose: ribulose

• two monosaccharides can be brought together to form a very simply polysaccharide called a disaccharide via a covalent bond called a glycosidic linkage

  • alpha-glucose + alpha glucose = maltose + H2O

    • the glucose molecule contributing its C1 is an alpha glucose, making the resulting glycosidic linkage an alpha-1,4 glycosidic linkage

  • cellobiose is a disaccharide of beta-glucose and another glucose connected via a beta-1,4 glycosidic linkage

• not all disaccharides have to be the same monomers:

  • lactose is a disaccharide of glucose and galactose with a beta-1,4 glycosidic linkage

    • galactose is has its 4-carbon upside down from glucose, so only beta-glucose can form a bond

  • sucrose (table sugar) is an disaccharide of glucose and fructose

    • alpha-glucose + fructose = sucrose + H2O

    • alpha-1,4 glycosidic linkage

• the chemical formula for a disaccharide of hexose sugars is C12H22O11

  • subtract one H2O for each bond you form

• carbohydrates can be modified:

  • linkage of oligosaccharides to other molecules

    • when covalently linked to membrane proteins (glycoproteins) or lipids (glycolipids), carbohydrates act as identification and recognition molecules (chemical markers), as in blood typing

  • addition of chemical groups

    • phosphorylation (PO4)

      • ex: fructose-1,6-biphosphate

    • amino (R-NH2)

      • glucose → glucosamine (2-carbon OH → NH2)

      • galactose → galactosamine (2-carbon OH → NH2)

    • carboxyl (R-COOH)

    • carbonyl (R-COH or C=O)

    • hydroxyl (-OH)

    • sulfhydryl (R-SH)

Cellulose

• the most abundant carbon-containing (i.e., organic) compound on Earth

• used for structural support in cell walls of plants and many algae

• linear, unbranched polymer of glucose

  • monomers covalently linked by β-1,4-glycosidic linkages

    • very hard to hydrolyze; perfect for structure

  • linear polymers held together by hydrogen bonding with neighboring strands

• bonds together in alternating directions, where every other glucose is flipped upside down

  • creates straight strands without branching

Starches

• found chiefly in seeds, fruits, tubers, roots, and stems of plants

• used for energy storage in plant cells (ex: potatoes)

• helical, unbranched or loosely branched polymers of glucose

  • monomers within chains covalently linked by alpha-1,4 glycosidic linkages

    • easy to hydrolyze

  • chains branch by connecting with other chains by alpha-1,6 glycosidic linkages

• don’t have alternating directions

• amylose - unbranched helix

• amylopectin - branched helices (alpha-1,6 glycosidic linkages)

Glycogen

• found in muscle and liver cells of animals; energy storage

• helical, highly branched polymers of glucose

  • much more branched than starch

  • monomers within chains covalently linked by alpha-1,4 glycosidic linkages

  • chains branch by connecting with other chains by alpha-1,6 glycosidic linkages