Chapter 7 Flashcards
(33 cards)
What is the function of carbohydrates?
Needed for energy, protein modification, DNA/RNA, structure, cell-cell communication, etc.
High structural diversity
Describe the structure of carbohydrates
Built from monosaccharides containing three to nine carbons
Variation in size
Variation in stereochemistry
Can be linked together in linear or branched structures
What are monosaccharides?
Aldehydes or ketones that have two or more hydroxyl groups
Smallest are dihydroxyacetone and glyceraldehyde
Both are 3 carbon chains (trioses)
describe enantiomers
stereoisomers that are non-superimposable mirror images
In sugars that contain many chiral centers, only the one that is most distant from the carbonyl carbon is designated as D or L
D and L isomers of a sugar are enantiomers
Most hexoses in living organisms are D stereoisomers
Some simple sugars occur in the L-form, such as L-arabinose
describe epimers
Monosaccharides with same number of carbons that differ in configuration at only one carbon are epimers:
D-glucose and D-mannose are epimeric at C-2
describe anomers
same number of carbons, ring configuration, different only at C1 carbon (a and b forms)
Why do pentoses and hexoses cyclize?
Pentoses and hexoses tend to be cyclic rather than linear
Rings are energetically more stable
How are hemiacetals ad hemiketals formed?
Aldehyde and ketone carbons are electrophilic
Alcohol oxygen atom is a nucleophilic
When aldehydes are attacked by alcohols, hemiacetals form
When ketones are attacked by alcohols, hemiketals form
How are acetals and ketals formed?
hemiacetal and heiketals react with an HO-R
what is a pyranose ring
Pyranoses are six membered rings
Aldoses with six carbons tend to form pyranose rings
Oxygen is one atom in ring
what is a furanose ring
Furanose are five membered rings
Ketoses with six carbons can form either furanose or pyranose
Oxygen is one atom in ring
How does glucose form glucopyranose?
- C-1 aldehyde of open-chain glucose reacts with C-5 hydroxyl
- C-1 carbon is now another asymmetric center in ring
- Can have a configuration or b configuration, determined by whether C-1 hydroxyl group is on same side or opposite side of ring from the C-6 group
Describe the hayworth projection of glucopyranose
Ring structure is shown
C’s not indicated
Heavy line indicates projection toward you
Alpha form drawn with –OH below the ring, beta with –OH above the ring
What are the sugars of nucleic acids?
- they are furanoses
- 5 carbon chains can also cyclize to form furanose rings
- ribose has 3 OH groups attached to the carbons of the ring while deoxyribose has 2
What kind of configuration do 6 membered rings have?
- Six membered rings have “chair” and “boat configurations
- Chair has axial and equatorial groups
- Axial means parallel to the axis, equatorial is perpendicular to the axis
- The chair conformation is favored over the boat (which is less stable)
What kind of configuration do 5 membered rings have?
- Tend to have four carbons in plane, one “puckered”
- Envelope form
- Named by which carbon is out of plane
- if the carbon is puckered up it is endo if it is puckered down it is exo
What are O and N glycosidic linkages?
- Modification can happen through reaction with amines and alcohols
- Bond formed between carbon atom of sugar and oxygen (alcohol) or nitrogen (amine) atom
- Once modified, no longer easily converted to open-chain form
Reducing vs non-reducing sugars
- If unmodified, can interconvert with open-chain form that has free aldehyde or ketone
- Free aldehyde and ketone can react with oxidizing agents and other molecules (aldehyde/ketone becomes oxidized)
- If aldehyde/ketone is free to act as a reducing agent, we call this a “reducing sugar
How do the test strips work that test for glucose in the urine?
- Free glucose can easily bind to many compounds
- Diabetes causes high blood glucose
- Test strip contains glucose oxidase and a dye that reacts with H2O2
- glucose gets oxidized by glucose oxidase and O2 gets reduced to H2O2
- Readout is optical and correlates with concentration of blood glucose
describe the glycosidic bond
- Two sugar molecules can be joined via a glycosidic bond between an anomeric carbon and a hydroxyl carbon
- The glycosidic bond (an acetal) between monomers is less reactive than the hemiacetal at the second monomer
- The disaccharide formed upon condensation of two glucose molecules via 1 4 bond is called maltose
- The free anomeric carbon of the hemiacetal categorizes this sugar as a “reducing sugar.”
Describe non reducing disaccharides
- Two sugar molecules can be also joined via a glycosidic bond between two anomeric carbons
- no hemiacetals
- There are no reducing ends, this is a nonreducing disaccharide
What are common disaccharides and what are the enzymes that break them down into monosaccharides?
- Sucrose, lactose and maltose are most common
- Sucrose broken down by sucrase into glucose plus fructose
- Lactose is sugar found in milk. Broken down by lactase (people) or b-galactosidase (bacteria)
- Maltose made by hydrolysis of starch. Broken down to monosaccharides of glucose by maltase
describe glycogen
- Glycogen is a branched homopolysaccharide of glucose
- Glucose monomers form (alpha 1 to 4) linked chains
- Branch-points with (alpha 1 to 6) linkers every 8-12 residues
- Molecular weight reaches several millions
- Functions as the main storage polysaccharide in animals
Describe starch
-Starch is a mixture of two homopolysaccharides of glucose
-Amylose is unbranched polymer of (alpha 1 to 4) linked residues
-Amylopectin is branched like glycogen but the branch-points with
(alpha 1 to 6) linkers occur every 24-30 residues
-Molecular weight of amylopectin is up to 200 million
-Starch is the main storage homopolysaccharide in plants
