Biochemistry: Chapter 4

Carbohydrates

• Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis.
• Many carbohydrates have the empirical formula (CH2O)n.
• Carbohydrates are the most abundant biomolecules on Earth.

Functions of carbohydrates

• Sugars and starches are important for diet.
• The oxidation of carbohydrates is the central energy-yielding pathway in non-photosynthetic cells.
• Insoluble carbohydrate polymers (glycans) serve as structural and protective elements.
• Some carbohydrate polymers lubricate skeletal joints.
• Complex carbohydrate polymers act as signals that determine intracellular location or metabolic fate of molecules.

Classification of carbohydrates

• Monosaccharides: Single polyhydroxy aldehyde or ketone unit.
• Disaccharides: Two monosaccharides joined by an O-glycosidic bond.
• Oligosaccharides: Short chains of monosaccharide units joined by glycosidic bonds.
• Polysaccharides: Sugar polymers with more than 20 monosaccharide units.

Classification by Functional Group

• Aldoses: Contain aldehyde group.
• Ketoses: Contain keto group.

Classification by Carbon Skeleton

• Trioses: Contain 3 carbon atoms.
• Tetroses: Contain 4 carbon atoms.
• Pentoses: Contain 5 carbon atoms.
• Hexoses: Contain 6 carbon atoms.
• Heptoses: Contain 7 carbon atoms.

Monosaccharides

• Monosaccharides can be either aldoses or ketoses.
• The simplest monosaccharides are glyceraldehyde (an aldotriose) and dihydroxyacetone (a ketotriose).

Monosaccharides are Asymmetric Compounds

• All monosaccharides except dihydroxyacetone contain one or more asymmetric (chiral) carbon atoms.
• Monosaccharides exist as optically active isomers (stereoisomers or enantiomers).

Sugars Have Optical Activity

• Compounds containing chiral centers are optically active.
• They rotate the plane of polarized light.

Stereoisomers

• A molecule with n chiral centers can have 2n stereoisomers.
• Glyceraldehyde has 21 = 2.
• The aldohexoses, with four chiral centers, have 24 = 16 stereoisomers.
• Stereoisomers of monosaccharides are divided into two groups based on the configuration at the chiral center furthest from the carbonyl carbon.
• D isomers have the same configuration as D-glyceraldehyde, and L isomers have the same configuration as L-glyceraldehyde.
• When the hydroxyl group on the reference carbon is on the right in the projection formula, the sugar is the D isomer; when on the left, it is the L isomer.
• Most of the hexoses in living organisms are D isomers.

Epimers

• Two sugars that differ only in the configuration around one carbon atom are called epimers.
• Some sugars occur naturally in their L form, such as L-arabinose.

Formation of hemiacetals and hemiketals

• Aldoses and ketoses are not mainly found in straight-chain forms.
• In aqueous solution, aldotetroses and all monosaccharides with five or more carbon atoms occur predominantly as cyclic structures.
• The carbonyl group forms a covalent bond with the oxygen of a hydroxyl group along the chain, forming hemiacetals or hemiketals.
• This reaction produces an additional asymmetric carbon atom.

Monosaccharides Have Cyclic Structures

• D-glucose exists in solution as an intramolecular hemiacetal.
• This reaction renders the carbon asymmetric and produces two stereoisomers: a and b.
• Six-membered ring compounds are called pyranoses because they resemble pyran.
• Six-membered ring forms of D-glucose are a-D-glucopyranose and b-D-glucopyranose.

Mutarotation

• The optical activity of a freshly prepared solution of a-D-glucose decreases over time and reaches a final value of +52.7°.
• This is due to the conversion of a-D-glucose to b-D-glucose.
• This rotation is called mutarotation.

Mutarotation

• Mutarotation causes the formation of an equilibrium mixture consisting of 63.6% of the β anomer and 36.4% of the a anomer.
• Enzymes use only one of these forms or derivatives.
• Mutarotase enzyme catalyzes the conversion between them.

Pyranoses and furanoses

• Five-membered ring forms of monosaccharides are called furanoses.

Monosaccharide Derivatives

• Many derivatives of monosaccharides exist.
• Modifications include replacing a hydroxyl group with another substituent or oxidizing a carbon atom to a carboxyl group.

Phosphorylated Sugars

• Phosphate esters of sugars are important intermediates in metabolism.

Amino Sugars

• In glucosamine, galactosamine, and mannosamine, the hydroxyl at C-2 is replaced with an amino group.
• The amino group is nearly always condensed with acetic acid, as in N-acetylglucosamine.

Bacterial cell walls

• Bacterial cell walls contain N-acetylmuramic acid, a derivative of glucosamine.

Deoxy Sugars

• Deoxy sugars have a hydrogen substituted for the hydroxyl group at C-6.
• L-fucose and L-rhamnose are deoxy sugars found in plant polysaccharides.

Aldonic Acids and Uronic Acids

• Oxidation of the carbonyl (aldehyde) carbon of glucose to the carboxyl level produces gluconic acid.
• Oxidation of C-6 of glucose, galactose, or mannose forms the corresponding uronic acid: glucuronic, galacturonic, or mannuronic acid.
• Aldonic and uronic acids form lactones.

Sugar Acids

• Glucose forms gluconic, glucuronic, and glucaric acid.
• Galactose forms galactonic, galacturonic, and galactaric acid.

Aldonic Acids and Uronic Acids

• N-acetylneuraminic acid (sialic acid) is a nine-carbon acidic sugar derivative.