6 1 MCB 450 Lecture 6 H O Monosaccharides Glycosidic bonds Polysaccharides 6 2 Carbohydrates Most abundant biomolecules on Oxidation of carbohydrates is central energy yielding pathway in most non photosynthetic cells Roles of carbohydrate polymers structural in plants insects fungi bacteria archaea storage starch glycogen protective lubrication of joints cell cell recognition and adhesion 6 3 Classes of saccharides Monosaccharides single polyhydroxy alcohol or ketone unit cyclized e g 6 carbon glucose most abundant in nature Oligosaccharides short chain of 2 20 monosaccharides joined by glycosidic bonds e g disaccharide sucrose glucose fructose oligosaccharides 3 residues are often joined to protein or lipid in glycoconjugates Polysaccharides chains 20 to 1000s of monosaccharides in length linear e g cellulose glucose n branched e g glycogen starch glucose n depending on the sugar residues in a polysaccharide the linkages between them polysaccharides can have very different biological roles 6 4 Monosaccharides 1 Have unbranched carbon chains in which all C atoms are linked by single bonds In the linear open chain form one of the Cs is double bonded to O carbonyl each of the other Cs has an OH H2O soluble If the C O is at the end of a carbon chain i e in an aldehyde monosaccharide aldose at any other position i e in a ketone group monosaccharide ketose Simplest monosaccharides have 3 carbons 6 5 Monosaccharides 2 Common monosaccharides have 5 and 6 carbons 6 6 Monosaccharides have asymmetric centers All monosaccharides except dihydroxyacetone have one or more asymmetric carbons In glyceraldehyde middle C chiral center so molecule has 2 different optical isomers or enantiomers stereoisomers that are non superimposable mirror images of one another By convention one enantiomer the D isomer the other L Stereoisomers of glyceraldehyde 6 7 Ways of representing stereoisomers HORIZONTAL PROJECTS OUT FROM PLANE VERTICAL PROJECTS BEHIND PLANE 6 8 D and L configurations of monosaccharides Stereoisomers of monosaccharides 3 C divided into 2 groups differ in the configuration about the chiral center most distant from the carbonyl C 1 2 3 HIGHEST NUMBERED ASYMMETRIC CARBON MOST DISTANT FROM CARBONYL CARBON 4 5 OH GROUP ON RIGHT CONFIGURATION D 6 OH GROUP ON LEFT CONFIGURATION L MIRROR Most of the hexoses in living organisms are D isomers 6 9 D and L configurations of monosaccharides In general a molecule with n chiral centers can have 2n stereoisomers 1 2 3 4 5 6 MIRROR IN ALDOHEXOSES C 2 C 3 C 4 AND C 5 CHIRAL CENTERS SO 24 16 POSSIBLE ALDOHEXOSES 8 D AND 8 L HOW MANY IN ALDOPENTOSES 6 10 D or L glucose aldose fructose ketose 6 11 D or L 1 1 2 3 4 4 erythrulose ketose 5 arabinose aldose 6 12 Series of D aldoses 6 13 Epimers 2 sugars that differ only in the configuration around one carbon 6 14 Series of D ketoses Have 1 chiral center less than aldoses C 4 and C 5 ketoses designated by adding ul into the name of their corresponding aldose e g D ribulose ketopentose corresponding to D ribose 6 15 Intramolecular cyclization of pentoses hexoses 1 In aqueous solution monosaccharides with 5 C occur mainly as cyclic ring structures in which the carbonyl group has formed a covalent bond with the O of an OH group along the chain O H C CH2OH C OH C OH O OH C H C C HO C C OH C OH CH2OH OH HOH2C C C C C C OH OH O C H HO OH Rotation about C C C O bonds permits cyclization OH 6 16 Intramolecular cyclization of pentoses hexoses 2 Reaction between C 1 aldehyde group and an OH at C 5 forms intramolecular HEMIACETAL H O C H C O C ALDEHYDE at C 1 H H C O C O H C H ALCOHOL at C 5 CYCLIC HEMIACETAL and the aldehyde C becomes a new chiral center called the anomeric carbon 6 17 Intramolecular cyclization of pentoses hexoses 3 Reaction between C 2 keto group and an OH at C 5 forms intramolecular HEMIKETAL R2 O C H R2 R1 O C KETONE at C 2 R1 O C O H ALCOHOL at C 5 H C H HEMIKETAL and the keto C becomes a new chiral center called the anomeric carbon 6 18 HAWORTH PERSPECTIVE HEAVY LINE PROJECTS OUTWARD 6 19 6 20 Fructose can also form 6 membered rings Pyranose rings formed if C 2 keto reacts w C 6 OH OH OH 6 21 6 22 How to draw the pyranose form of an D aldohexose Final position in Haworth 1 CHO 2 C 3 C 4 C 5 C 6 CH2OH anomeric C OH OH OH on right down OH on left up up CHO CH2OH C OHdown C OH O OH C H C C HO C C OHdown CH2OH on top D sugar CH2OH on bottom L sugar C HO OH C OH CH2OH CH2OH D Glc C HO O CH2OH L Glc C O OH C C OH CH2OH config unspecified O OH l C 6 23 Interconversion between anomers and anomers interconvert in solution via the linear form mutarotation D glucose solution forms an equilibrium mixture of 62 38 THIS FORM HAS CHEMICAL PROPERTIES OF AN ALDEHYDE 6 24 Conformations of pyranoses Pyranose ring is not planar as suggested by Haworth perspective because of tetrahedral geometry of saturated C atoms but can adopt two classes of conformations AXIAL Substituents on the ring carbons have two orientations or EQUATORIAL APPROX or Chair form of D Glc predominates because all axial positions are occupied by H Disfavored because of steric hindrance 6 25 Definitions 1 Stereoisomers Same structural formula but different spatial configurations Enantiomers Stereoisomers about chiral center that are non superimposable mirror images of each other e g fructose 3 chiral centers D and L enantiomers D monosaccharides predominate in nature just as L amino acids predominate 6 26 Definitions 2 Diastereoisomers Stereoisomers that have opposite configurations at one or more chiral centers but which are not mirror images of each other Epimers Diastereoisomers that differ only in the stereochemistry at only one of their chiral carbons e g at C 2 for Glc and Man D Glucose D Talose diastereoisomers Anomers D Glucose D Mannose epimers Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal ring forming carbon The hemiacetal or carbonyl carbon anomeric carbon example and anomers of D glucose 6 27 H O H O C C C H C OH C OH HO C C OH HO C CH2OH C OH CH3 6 28 pKa 7 But these are also deoxy sugars which is reflected in their formal names 2 amino 2 deoxy D glucose 2 acetylamido 2 deoxy D glucose 6 29 pKa 3 OXIDATION AT C 6 OR C 1 6 30 6 31 6 32 Reducing sugars REMEMBER RING OPENING MUTAROTATION RED COLOR The anomeric carbon of Glc and other sugars can be oxidized by mild oxidizing agents such as Cu2 providing the sugar is in its open chain form with a free carbonyl carbon at C 1 Sugars