Download Principles of Biochemistry 4/e

Carbohydrates
• What are they?
– Sugars, starches & much more
– Most abundant molecules on Earth
– End products of photosynthesis
Carbohydrates
• Functional Roles
– Carbohydrate polymers act as energy
storage molecules
– Cell walls and protective coatings
– Cell recognition and interaction
– Part of DNA, RNA, coenzymes
Classifications
• Monosaccharides
– Smallest unit of carbohydrate structure
– Hydrates of carbon (CH2O)n n = 3-9
Most common n = 5-6
• Oligosaccharides
– Polymers of 2-20 monosaccharides
– Most common – disaccharides (2
monosaccharides
• Polysaccharides
– Polymers of many (usually >20)
monosaccharides
Other Definitions
• Homoglycan – a polymer of identical
monosaccharides
• Heteroglycans – a polymer of
different monosaccharides
• (note: glycan is a term for carbohydrate
polymers)
• Glycoconjugates – carbohydrate
derivatives
– One or more carbohydrate chains
covalently linked to peptide, proteins or
lipids
More Definitions
• Stereoisomers – compounds that have
the same molecular formula but
different arrangements of their
atoms in space
• Enantiomers – 2 stereoisomers that
are mirror images of one another
• Diastereomers – 2 stereoisomers that
are not mirror images of one another
• Chiral Carbon- a carbon atom with 4
different groups bonded to it
• Epimers – molecules that differ in
configuration at only one of several
chiral centers
Monosaccharides
Water-soluble, white, crystalline solids
with a sweet taste
Are polyhydroxy alcohols with at least
3 carbons
• Two Types
– Aldoses
• Contain an aldehyde on C 1
– Ketoses
• Contain a ketone, usually on C2
• Smallest monosaccharides - trioses
Glyceraldehyde has 2 Forms
• It contains one chiral carbon – is
optically active
• D-form rotates polarized light to the
right
• L-form rotates polarized light to the
left
• In solution, monosaccharides exist in cyclic
conformations
• Carbonyl carbons react with a hydroxyl
group to produce:
– Pyranose – a 6 membered ring
– Furanose – a 5 membered ring
• The carbon attached to 2 O’s is called an
anomeric carbon – can be alpha or beta
Haworth Projections
• A ring structure
• The ring is projected as lying
perpendicular to the page
• Anomeric Carbon is on right or top
• OH’s on right are drawn down
• OH’s on left are drawn up
• On anomeric carbon
– Alpha configuration – OH is down
– Beta configuration – OH is up
Other Furanose Conformations
• Envelope – one of the five ring atoms
is out-of-plane & others are coplanar
• Twist – two of the five ring atoms are
out-of-plane, one on either side of
the plane formed by the other 3
atoms
Other Pyranose Conformations
• Chair –
– Most stable – minimize steric hindrance
– Substituents may be:
• Axial (above or below plane of ring)
• Equatorial (in the plane of the ring)
• Boat –
Derivatives of Monosaccharides
• Sugar Phosphates
– Phosphate ester of one of the –OH
groups
– Often on the terminal carbon
– Important in glucose metabolism (e.g.,
glucose 6-phosphate, glucose 1phosphate)
Derivatives of Monosaccharides
• DeoxySugars
– A hydrogen atom replaces a hydroxyl
group
– Very important in DNA synthesis (e.g.,
deoxyribose)
– L-Fucose (6-deoxy-L-galactose) is
common in polysaccharides
Derivatives of Monosaccharides
• Amino Sugars
– One of the hydroxyl groups in a
monosaccharide is replaced by:
• Amino group
• Acetylated amino group
• Important in glycoconjugates
N-Acetylneuraminic acid (NeuNAc)
• Starts with N-acetylmannosamine and
pyruvate
• Cyclizes to a pyranose in a unusual
way – carbonyl group at C-2 (from
pyruvate) reacts with –OH at C-6
• NeuNAc & its derivatives = sialic
acids
• Important in glycoproteins
Derivatives of Monosaccharides
Sugar Alcohols
A carbonyl oxygen is reduced to an
-OH
Derivatives of Monosaccharides
Sugar Acids
Derived from aldoses by either:
oxidation of the aldehyde carbon
oxidation of the highest numbered
carbon