Carbohydrates Flashcards
functions of monosaccharides and disaccharides
energy stores, fuel, metabolic intermediates
structural backbones in nucleic acids (ribose, deoxyribose)
functions of polysaccharides
storage molecules, structural elements, cellular recognition functions (glycoproteins for example)
two types of sugar and their functional groups
aldoses - aldehyde functional group
ketoses - ketone functional group
what is meant by poly-alcohol?
sugars are poly-alcohols as they contain multiple -OH groups
general formula for monosaccharide
CnH2nOn
which optical isomer predominates in naturally occuring sugars?
D configuration
how to recognise D/L configuration of sugar
fischer projection
look at penultimate carbon (second to furthest from terminal aldehyde / ketone group)
-OH group on right = D
-OH group on left = L
epimers
isomers that differ at only one chiral centre
name two epimers of glucose
Galactose (differs at C4)
Mannonse (differs at C2)
what type of ring closure is seen in aldoses?
hemiacetyl ring closure
(try drawing structure)
what type of ring closure is seen in ketoses?
hemiketal ring closure
(try drawing structure)
what are anomers
stereoisomers differing in the arrangement of groups around the anomeric carbon
position of anomeric carbon on aldose
C1
position of anomeric carbon on ketose
C2
difference between a- and B- anomers of glucose
a-anomer - OH on anomeric carbon (C1) on opposite side of ring to C6
B-anomer - OH on anomeric carbon (C1) on same side of ring as C6
difference between a-anomer and B-anomer of pyranose sugar
a-anomer - OH on anomeric carbon (C2) below ring (C1 and C6 on same side of ring)
B-anomer - OH on anomeric carbon (C2) above ring (C1 and C6 on oppsite sides of ring)
which anomer of glucose is favoured at equilibrium and why?
B-anomer
because: hexose ring in chair conformation. On B-anomer, -OH on anomeric carbon is in equatorial position - less sterically hindered so more favourable. On a-anomer the -OH is in the axial position, less favourable due to 1,3 diaxial interactions
ratio of a-anomer to B-anomer at equilbibrium
1/3 : 2/3
name for six membered sugar ring
pyranose
five membered sugar ring
furanose
which type of glycosidic linkage attaches alcohol groups to modified monosaccharides?
O-linkage
which type of glycosidic linkage attaches amine groups to modified monosaccharides?
N-linkage
importance of phosphorylated monosaccharides
forms reactive intermediates that are important for metabolism
also, phosphorylation makes sugars anionic
what kind of reaction forms the glycosidic bond between two monosaccharides?
dehydration reaction, removal of H2O
which glycosidic linkage joins glucose and galactose monomers in lactose?
Gal-B-1,4-Glc
which linkage joins glucose monomers in maltose?
Glc-a-1,4-Glc
which linkage joins glucose and fructose monomers in sucrose?
a-Glc-1,2-B-Fru
Describe the structure of glycogen
branched homopolymer of glucose
a-1,4 linkages
branching via a-1,6 linkages every 8-12 units
forms open helical structure
Describe the structure of starch
contains two homopolymers of glucose:
amylose - unbranched, a-14 glycosidic linkages
amylopectin - branched via a-1,6 linkages every 24-30 units
forms open helical structure
Describe the structure of cellulose
unbranched homopolymer of glucose
B-1,4 linkages
forms long straight chains
H-bonding between parallel chains forms fibrils - high tensile strength
Describe the structure of chitin
unbranched homopolymer of GlcNAc (acetylglucosamine)
Describe the structure of peptidoglycan
heteropolysaccharide of two modified monosaccharide monomers (N-acetylmuramic acid, N-acretyl-D-glucosamine)
B-1,4 linkages
crosslinking between polysaccharide chains with oligopeptides
in which biological structure is peptidoglycan found?
bacterial cell wall
N-linked glycosidic bonds are found in which glycoproteins
oligosaccharide joined to aspariagine residue
O-linked glycosidic bonds are found in which glycoproteins?
oligosaccharides joined to serine or threonine
where does N-linked glycosylation occur
begins in lumen of endoplasmic reticulum, continues in Golgi complex
where does O-linked glycosylation occur?
entirely in the Golgo complex
