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 carbonsoften used to discriminatebetween 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 NH2 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