Lecture 2: Carbohydrates Flashcards
What are the most abundant organic molecules in nature?
Carbohydrates
What are some roles of Carbohydrates?
Are a significant source of dietary calories (fuels, energy stores, metabolic intermediates); form part of structural framework of DNA and RNA; form structural components of cell walls in bacteria and plants (are the principle constituents of the fibrous materials of plants); are linked to many proteins and lipids where they act as mediators of molecular recognition events particularly at the cell surface.
What does it mean for a carbohydrate to be “polyhydroxyl”?
Contain several hydroxyl groups
What two functional groups do CHO contain? Describe their structure.
Aldehyles or Ketones. Aldehydes are a carbon double bonded to an oxygen, and single bonded to a hydrogen and one substituent. Ketones are also double bonded to an oxygen but single bonded to two substituents (R groups)
Which structures are used in Fischer projections and what do horizontal and vertical lines mean?
Only linear CHO are drawn in Fischer and horizontal lines mean the bonds are coming towards you; vertical lines means the bonds are going away from you
What is an asymmetric carbon?
Also called a chiral carbon, an asymmetric carbon is linked to four different types of atoms or groups.
What are monosaccharides called when they have 3, 4, 5, or 6 carbons attached?
triose, tetrose, pentose, hexose
What are monosaccharides? How are they classified?
Simple sugars. Classified based on number of carbon atoms they contain or whether they contain an aldehyde or ketose.
What is a CHO with a ketone or an aldehyde called?
ketose; aldose
What is a stereoisomer?
isomers that have identical chemical connectivity but have different spacial arrangement of their atoms
What is an enantiomer?
It is a stereoisomer that has a mirror image. Have the some physical and chemical properties but not the same biological properties since biological macromolecules are mostly all chiral
What is a disastereomer?
Stereoisomers that are not mirror images of one another. Generally do not have identical physical and chemical properties.
If a CHO has 4 asymmetric carbons, how many possible stereoisomers can it have?
- 2^4 = 16
What are isomers that differ at only one position called?
Epimers
How do you locate the anomeric carbon in a ring structure of CHO?
It is always next to the bound oxygen
The absolute configuration of CHO is measured where?
At the asymmetric carbon furthest away from the aldehyde or ketone (hydroxyl group is on the right side of Fischer drawing)
Are most sugars in the D- configuration or L-?
D
What happens to aldehydes and ketones during cyclization?
Aldehydes and ketones are attacked by hydroxyl groups (alcohols, water) to yield hemiacetals or hemiketals. This process is reversible. If the hydroxyl group comes from the same molecule, it is called a intramolecular hemiacetal/hemiketal.
Which carbons react with each other in glucose to form rings? Fructose?
The C-1 aldehyde in open chain glucose reacts with the C-5 hydroxyl group to form a six membered pyranose ring; the C-2 keto group in the open chain fructose reacts with the C-5 hydroxyl group to form a five membered furanose ring. An additional (new) asymmetric center is formed at the anomeric carbon atom (C-1 in aldoses and C-2 in ketoses) in these cyclizations
What two possible isomers result from hemiacetal formation?
alpha and beta anomers
The equilibrium conversion of the two anomers is known as ________
mutarotation
In solution, the alpha and beta anomers exist in _______. When glucose is in this state, how much of each anomer exists?
Equilibrium; 2/3 beta, 1/3 alpha, 1% open chain. Not all sugars are in the 2:1 ratio. The ratio will depend on the specifics of the individual sugar structures
6 membered rings formed by CHO such as glucose are referred to as ________. Why?
Pyranosides, or pyranose sugars because they resemble pyran.
5 membered rings are referred to as ______. Why?
Furanosides, or furanose sugars because they resemble furan.