Where does the NADH for oxidative phosphorylation come from? Cellular oxidation-reduction reactions such as dehydrogenations We will be spending quite a bit of time before the next exam covering numerous aspects of carbohydrate metabolism

Classes of carbohydrates Monosaccharide – single polyhydroxy aldehyde or ketone unit Oligosaccharide – short chain of monosaccharides joined by glycosidic bonds –Disaccharides, I.e. sucrose Polysaccharides – 20 or more monosaccharides –Starch, glycogen, etc.

Monosaccharides are either aldoses or ketoses If the carbonyl group is at the end of the carbon chain in an aldehyde group, it’s an aldose If the carbonyl is at any other position in a ketone group it is a ketose

Examples of aldoses and ketoses Note that also # of carbons often used to discriminate between sugars

You will be responsible for knowing the structures of sugars with boxed names in Fig 7-3 a and b

Epimers are sugars that differ only in the configuration around one carbon atom

Note the number convention The carbons of a sugar are numbered beginning at the end of the chain nearest the carbonyl group. Also, most biologically significant hexoses are D isomers, you do not have to be able to recognize nor memorize D and L configurations, just know that two exist and L is more rare

Monosaccharides have cyclic structures For convenience, sugars are often represented in straight chain forms, but in solution aldotetroses and all mono- saccharides >5 carbons are predominantly cyclic in structure The carbonyl group forms a covalent bond with the oxygen of a hydroxyl group along the chain to form hemiacetals or hemiketals

When sugars cyclize they form isomers Isomers of monosaccharides differ only in their configuration about the hemiacetal or hemiketal carbon atom and are called anomers. The hemiacetyal or carbonyl carbon atom is called the anomeric carbon.

Alpha and Beta anomers in  -sugars (  -anomers), the -OH group on carbon no. 1 is below the ring in  -sugars (  -anomers), the -OH group on carbon no. 1 is above the ring

Five-membered or six-membered rings can be formed by aldohexoses Six-membered rings are called pyranoses –On the previous slide, you may note the molecule is called glucopyranose –Only aldoses of five or more carbon atoms can form pyranose rings Five-membered rings are called furanoses –However, the six-membered aldopyranose ring is much more stable than aldofuransoe ring and predominates

Ketohexoses also occur in alpha and beta anomeric forms In these compounds, the hydroxyl group on C-5 or C-6 reacts with the keto group at C-2 forming a furanose or pyranose ring containing a hemiketal linkage. Fructose predominantly forms the furanose ring

Mutarotation – spontaneous sugar isomerization For both fructose and glucose,  is the predominant anomer In aqueous solutions,  and  forms interconvert in a process known as mutarotation

Haworth projections

Various derivatives of hexoses are present in biology Several types of these, such as replacement of the OH group at C-2 of glucose with NH 2 results in a compound known as glucosamine, which is involved in many structural polymers We will cover these only in specific instances in subsequent lectures. At this time, you do NOT need to memorize Figure 7-9

Sugar reactivity Monosaccharides are reducing agents (and are called reducing sugars) donating electrons to Fe +3 or Cu +2 –Oxidation of sugars produces various acidic derivatives – Cyclization of acidic sugars forms cyclic esters known as lactones – Ascorbic acid (vitamin C) is a lactone derivative of D- glucuronic acid Monosaccharides also polymerize

Disaccharides contain a glycosidic bond When a hydroxyl group of one sugar reacts with the anomeric carbon of the other, an O-glycosidic bond is formed

Outcome of glycosidic bonds When an anomeric carbon is in a glycosidic bond, it is no longer oxidizable. the disaccharide cannot exist in linear form Glycosidic bonds are hydrolyzed by acid, but resistant to base The end of the sugar chain with a free anomeric carbon is called the reducing end

A naming convention for polysaccharides Describe the compound with its non-reducing end at the left, following these rules: –1. Configuration (  /  ) at anomeric carbon joining the left monosaccharide unit to the second is given –2. The nonreducing residue is named –3. The two carbon atoms joined by the glycosidic bond are indicated in parentheses with an arrow connecting the numbers –4. The second residue is named –5. Reiterate sequentially for additional residues, if necessary

For example,

Most carbohydrates found in nature are polysaccharides

Starch: Storage polysaccharide produced by plants Composed of glucose polymers: amylose & amylopectin Amylose is an unbranched polymer of  (1,4)- linked glucose molecules Amylopectin is a branched polymer of  (1,4)- linked glucose molecules Branches are formed by  (1,6) glycosidic bonds, (see figure 7-14)

Glycogen: Storage polysaccharide produced by vertebrates Highly-branched polymer of  (1,4)-linked glucose molecules branches may occur as frequently as every fourth glucose in the core of the molecule branches are again  (1,6)-linkages (as in the case of amylopectin) Has as many nonreducing ends as it has branches, but only one reducing end

Other polysaccharides of interest Cellulose: structural polysaccharide produced by plants; primary component in plant cell walls –Polymer of  (1,4)-linked glucose molecules Cellulose molecules are held together by intermolecular hydrogen bonds to form microfibrils, which are rigid strips and rods Chitin: Structural polysaccharide produced by arthropods and mollusks Polymer of  (1,4)-linked N-acetylglucosamine molecules Chitin forms a rigid frame-work that is stabilized and hardened by mineral deposits

Many biomolecules are “decorated” with sugar Obviously, key component of DNA, RNA Proteoglycans Glycoproteins Glycolipids

Proteoglycans Extracellular or integral membrane proteins with glycosaminoglycan chains attached Point of attachment is a serine residue Play key role in a cell’s interactions with it’s environment –Can provide an anchor –Bind extracellular macromolecules –Convey information via signal transduction

Families defined on hydrophobic anchor

Glycoproteins O-linked (Serine or Threonine) or N-linked (Asparagine) Carbohydrate typically constitutes less than 50% of molecule mass in contrast to proteoglycans Can affect protein folding, function, targeting to a specific locale, etc.

Lectins are proteins that bind carbohydrates H. pylori adheres to stomach by interaction between bacterial membrane lectin and specific oligosaccharides found on specific glycoproteins of gastric epithelial cells