Biochemistry Exam 2 Study Guide

Topics: Glucose, Glycogen, Starch Pages: 13 (2519 words) Published: April 25, 2014
Chapter 7: Carbohydrates

I. Carbohydrates
A. Most abundant organic molecule in nature
a) 3 major classes:
Monosaccharides – simple sugar – polyhydroxy aldehyde or ketone; represent individual sugar Oligosaccharides – consist of short chains of monosaccharide units joined together by covalent bonds; 2-10 strings of sugar Polysaccharides – long chains having hundreds or thousands of monosaccharide units b) Functions:

Provide energy thru oxidation – glucose oxidized for energy Supply carbon for synthesis of cell components
Energy storage to form a part of structural elements of some cells and tissues; glucose stored as glycogen c) Classification
General formula (CH2O)n where n is any integer from 3-7
Monosaccharides are either aldehydes or ketones
Aldoses – monosaccharides that contain aldehydes
Ketoses – monosaccharides that contain ketones

d) Also classified by number of carbons:
3 carbons – trioses, 4 carbons – tetroses, 5 C’s – pentoses, 6 C’s – hexoses, 7 C’s – heptoses Each of these exist in 2 series: aldotrioses and ketotrioses, aldotetroses and ketotetroses, aldopentoses and ketopentoses, aldohexoses and ketohexoses, aldoheptoses and ketoheptoses

B. Stereochemistry
a) Stereoisomers: all atoms are bonded together with exactly the same bonding pattern but differ only in the arrangements of their atoms in space b) Enantiomers:
Non-superimposable mirror-image pairs of stereoisomers (left and right hand) Observed only when a carbon atom has 4 diff groups attached to it (e.g. CHXYZ also called chiral molecules) One of the enantiomeric pair is a member of the D-family and other is L-family c) Properties of Enantiomers

Have identical physical and chemical properties
They rotate the plane of the plane-polarized light by equal degrees but in opposite direction Those that rotate plane-polarized light clockwise are called dextrorotatory and are designated and plus (+). Those that rotate plane-polarized light counterclockwise are called levorotatory and are designated by a sign (-). When both enantiomers are present the net rotation of plane-polarized light is zero (i.e. optically inactive) and mixture is referred to as racemic mixtur

d) D- Or L- configuration
Monosaccharides contain chiral centers so they can exist as enantiomers known as D or L The most oxidized C or most oxidized end is drawn at the top The position of hydroxyl group on the penultimate C; chiral C furthest from carbonyl group (oxidized end) determines configuration: If –OH group is on right, the molecule has D-configuration If –OH group is on left, molecule has L-configuration

e) Stereochemistry:
Almost all monosaccharides contain chiral centers
Can rotate plane of polarized light
Exist as pair of enantiomers
Also exist as diastereomers – isomers that are not enantiomers. Diasteriomers that differ at only a single carbon other than the reference carbon are Epimers. E.g. glucose & galactose – differ at C4

C. Cyclic structure
a) Haworth Projection
Sugars that contain 4 or more C’s exist in cyclic forms
Sugars that may cyclize to 5-membered ring (furanose) or 6-membered ring (pyranose) A new chiral center is generated when ring cyclize and resulting pairs diastereisomers are called anomers. These are designated alpha or beta. In Haworth Projection of any cyclic form of a D-sugar the –CH2OH group is always “up” with the following 2 configurations. When the –OH group at the anomeric carbon is “up” or cis to –CH2OH group, sugar is B-D-sugar (B-anomer) When –OH group at anomeric carbon is “down” or trans to –CH2OH group, sugar is a-D-sugar (a-anomer)

II. Monosaccharides
A. Reactions of Monosaccharides
a) Mutarotation – interconversion between cyclic forms; The cyclization rxn is an equilibrium rxn and the various forms interconvert thru central aldehyde or ketone form (mutarotation) b) Oxidation-reduction rxns

The aldehyde group may be oxidized by metal ions...
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