MCAT Biology Ch14: Carbohydrates Flashcards
carbohydrates are compounds that contain
C, H, and O in form of polyhydroxylated aldehydes or ketones, Cn(H2O)n
monosaccharide
single carbohydrate (simple sugar)
disaccharide
two sugars
oligosaccharides
short carb chain
polysacs
long carb chains
monosaccharide
- simplest units
- # of carbons they possess; num. prefix and suffix -ose (ex: triose, tetrose); begins w/ C close to carbonyl (c-1)
- simplest: glyceraldehyde
- polyhydroxlyated aldehyde (aldose)
- simplest ketone sugar (ketose): dihydroyacetone; ketone will receive lowest possible number; ketone group C-2 on MCAT
- every carbon (except carbonyl) carry -OH
aldose
aldehyde sugar
ketose
simplest ketone sugar
stereochemistry of monosacs
- used glyceraldehyde –> D(+)and L(-) config
- all other monosacs assigned to D and L in relationship to glyceraldehyde
- highest # chiral center (farthest from carbonyl) right is D and left is L
- same sugars, diff. optical fam (D and L–> enantiomers
- nonidentical (nonmirror image) w/in same fam (ketose/aldoses, same # C) –> diastereomers
- diastereomers differ only at one chiral center –> epimers
glyceraldehyde
http://upload.wikimedia.org/wikipedia/commons/a/a1/D-glyceraldehyde-2D-Fischer.png
tetroses
http://www.scientificpsychic.com/fitness/erythrose.gif
trioses
http://www.fao.org/docrep/field/003/AB470E/AB470E24.gif
dihydroxyacetone
http://upload.wikimedia.org/wikipedia/commons/2/28/Dihydroxyacetone.png
D-fructose
http://web.pdx.edu/~wamserc/CH331F97/Egifs/E2_3a.gif
D-glucose
http://www.uspto.gov/web/patents/classification/uspc536/c536s1-11-2.gif
D-galactose
http://upload.wikimedia.org/wikipedia/commons/5/58/D-galactose.png
D-mannose
http://1.bp.blogspot.com/-YH0ZTmm6VX0/T5gNNDJhm2I/AAAAAAAABF4/ptwRkYrNdYw/s200/2+D-mannose.png
ring properties
- hydroxyl group (Nu) and carbonyl (electrophile) –> intramolecular rxn –> cyclic hemiacetal (from aldoses) or hemiketals (ketoses)
- ring strain –> only cyclic in sol is pyranose (6) or furanose (5)
- oxygen becomes member of ring
- c-1 becomes chiral (anomeric carbon), compound known as anomers
- when w/ water –> spon open and reform
- single bond between C1 and C2 rotate freely –> alpha or beta (mutarotation)
pyranose
- 6 membered ring formed from intramolecular Nu acyl substitution of hydroxyl group (Nu) and carbonyl (electrophile)
- chairlike config, min steric hindrance
- Fisher Projection –> Haworth (right –> down, left –> up)
- from 6 carbon aldose or seven carbon ketose
- OH is either alpha (trans) or beta (cis)
furanose
5 membered ring formed from intramolecular reaction of hydroxyl group (Nu) and carbonyl (electrophile)
-from 5 carbon aldose or 6 carbon ketose
anomer
cyclic stereoisomer that differ about new chiral carbon
anomeric carbon
- carbon that’s chiral in ring structure of sugar
- attached to two oxygen (O in ring and OH)
alpha
OH is down, trans to CH2OH substituent
beta
OH is up, cis to CH2OH substituent
mutarotation
- a or b config after ring exposed to water
- occurs more rapidly when catalyzed w/ acid or base
- mix both in eq. concentration
- process of one anomer changing into the other anomer by opening and reclosing
- alpha –> less favored due to OH axial –> steric strain
anomerization
forming one another or another from the straight-chain sugar
Monosac rxns
- ester formation, oxidation of monosacs, glycosidic rxn
- since OH, same rxns as simple alcohols
Monosac rxns: ester formation
- monosacs –> esters
- using acid anhydride and a base (CH3CO)2O
- all hydroxyls are esterified
Monosac rxns: oxdation of monosacs
- switching between anomeric config, hemiacetal rings spend short period of time in open-chain aldehyde form
- aldehydes can be oxidized to carb acids (oxidized aldoses called aldonic acids); reducing agents
aldonic acids
oxidized aldoses
reducing sugar
- hemiacetal ring
- Tollen’s (reducing Ag+ to metallic silver) and Benedict’s (red ppt of Cu2O) reagent detect presence - positive
- ketose also reducing sugar and is pos, but since can’t be oxidized to carb acid, it isomerizes to aldoses via keto-enols shifts
- more powerful oxidizing agent like dilute HNO3 w/ oxidize both aldehyde and primary alcohol to carb acid
glycosidic rxns
hemiacetals + alcohols –> acetals
- under acidic conditions
- anomeric hydroxyl group transformed into alkoxyl group –> mix of alpha and beta acetal (w/ water as LG) - glycoside
- forms glycosidic linkage (C-O)
- Sn2, sugar as nucleophile
glycoside
result of hemiacetals + alcohols –> acetals
glycosidic linkage
resulting C-O from hemiacetals + alcohols –> acetals
disacs
- two monosacs join, hydroxides act like alcohol
- most common is glycosidic linkage, 1,4 linkage (a - maltose, b - cellobiose), there’s also 1,6 and 1,2
- either alpha or beta, depending on orientation of OH group on anomeric carbon
- often cleaved in presence aq. acid
polysaccharides
- large chains of monosac linked by glycosidic bonds
- three most important: cellulose, starch, and glycogen (diff function, same monosac, D-glu)
- diff in config about anomeric carbon and position of glycosidic bonds – bio diff.
cellulose
- 1-4, B glycosidic bond
- components of plants
- not digestable by humans (fiber)
starch
- digestible by humans
- plants store energy
- 1,4 alpha glycosidic bonds, although occasional 1,6 alpha glycosidic bonds off chain
glycogen
- similar to starch, more 1,6 alpha glycosidic bonds (1 every 12 glu)
- highly branched compound (dendrimer)
