Ch. 4: Carbohydrate Structure and Function Flashcards
what are the 4 things that carbohydrates can be classified by?
- the number of sugar moieties that make them up
- the number of carbons in each sugar
- the functional groups present on the molecule
- the stereochemistry of the sugar
what are the basic structural units of carbohydrates?
monosaccharides
defn: trioses
the simplest monosaccharides
contain 3 carbon atoms
what are carbohydrates with 4, 5, and 6 carbons referred to?
4 = tetroses
5 = pentoses
6 = hexoses
defn: aldoses vs. ketoses
ALDOSE = carbohydrates that contain an aldehyde group as their most oxidized functional group
KETOSE = carbohydrates with a ketone as their most oxidized functional group
defn: aldohexose, ketopentose
aldohexose = a six-C sugar with an aldehyde group
ketopentose = a 5-C sugar with a ketone group
what is the simplest aldose?
glyceraldehyde, which is an aldotriose
the numbering of C atoms in a monosaccharide follows the rules described in orgo, what does this mean for monosaccharides? (2)
- the carbonyl C is the most oxidized (and will always have the lowest number)
- in an aldose, the aldehyde C will always be C-1
- in a ketose, the carbonyl C is C-2 (still most oxidized)
defn: glycosidic linkages
linkages that the aldehyde carbon in an aldose or the carbonyl C in a ketose can participate in
defn: glycosyl residues
sugars acting as substituents via glycosidic linkages
what is the simplest ketone sugar/ketose?
dihydroxyacetone (a ketotriose)
on every monosaccharide, what carbons carry a hydroxyl group?
every C other than the carbonyl C
what are 4 common (frequently tested) sugars/monosaccharides on the MCAT that are referred to by their common name and don’t follow naming conventions?
defn: stereoisomers/optical isomers
compounds that have the same chemical formula and differ from one another only in terms of the spatial arrangement of their component atoms
defn: enantiomer
a special type of isomerism between stereoisomers that are nonidentical, nonsuperimposable mirror images of each other
any molecule that contains chiral carbons and no internal planes of symmetry has ….
an enantiomer
diagram: enantiomers of glyceraldehyde
what determines a compound’s absolute configuration?
the particular 3-D arrangement of the groups attached to the chiral C
what determines the number of possible stereoisomers of a compound + eqn?
as the number of chiral carbons increases, so too does the number of possible stereoisomers because one compound may have many diastereomers
where n is the number of chiral C’s in the molecule
Do D and L correspond to + and - rotation or vice versa?
Neither, the direction of rotation must be determined experimentally and CANNOT be determined from the D or L designation for the sugar
how are monosaccharides assigned the D or L configuration on the MCAT?
based on their relationship to glyceraldehyde
defn + func: Fischer projection
a simple 2-D drawing of stereoisomers
horizontal lines = wedges = out of the page
vertical lines = dashes = into the page
func: allow scientists to identify different enantiomers
using a Fischer projection, what do all D sugars have? what do all L sugars have?
D-sugars –> the hydroxide of their highest-numbered chiral center on the right
L-sugars –> that hydroxide is on the left
Are R/S and D/L interchangeable?
no! they are not paired one to one!
what is true about the chiral centers in D and L glucose because they are enantiomers?
EVERY chiral center in D-glucose has the opposite configuration in L-glucose
what 3 types of stereoisomers should you be familiar with?
- enantiomers
- diastereomers
- epimers
defn: enantiomers
the same sugars, in different optical families (D-glucose and L-glucose)
defn: diastereomers
two sugars that are in the same family (both are either ketoses or aldoses, and have the same number of C’s) that are not identical and are not mirror images of each other
defn: epimers
a special subtype of diastereomers that differ in configuration at exactly one chiral center
ex: D-ribose and D-arabinose (which only differ at C2)
look at the 4 stereoisomers of an aldotetrose to understand these compounds
D-erythrose and L-erythrose are enantiomers (nonsuperimposable images of each other)
D-erythrose and D-threose are diastereomers (not mirror images of one another) (L-erythrose and D-threose are also diastereomers)
L-erythrose and D-threose are epimers (they differ in configuration at only one chiral center)
can a compound have multiple enantiomers? diasteroemers?
only one enantiomer (the left hand to its right hand)
multiple diastereomers (depends on how many and which chiral C’s are inverted between the two molecules)
why can monosaccharides undergo intramolecular reactions to form cyclic hemiacetals (from aldoses) and hemiketals (from ketoses)?
because they contain both a hydroxyl group (can act as a nucleophile) and a carbonyl group (the most common electrophile)
what are the only 2 cyclic molecules stable in solution and why?
6-membered PYRANOSE rings
5-membered FURANOSE rings
why: due to ring strain
what happens to the hydroxyl group and the oxygen when these ring structures (hemiacetals or hemiketals) form?
- the hydroxyl group acts as the nucleophile during ring formation
- oxygen becomes a member of the ring structure
defn: anomeric carbon
the carbonyl carbon becomes chiral during the formation of a hemiacetal or hemiketal
defn: anomers
molecules that differ at the anomeric carbon
what emerges from the cyclization of a sugar molecule?
how does this apply to glucose?
one of two ring forms can emerge (alpha or beta)
in glucose, the alpha-anomer has the -OH group of C-1 trans to the CH2OH substituent (axial and down), whereas the beta-anomer has the -OH group of C-1 cis to the -CH2OH substituent (equatorial and up)
diagram: cyclic sugar formation via intramolecular nucleophilic addition
glucose forms a 6-membered ring with two anomeric forms (alpha, left and beta, right)
use + char: Haworth projection
use: a method for describing the 3-D conformations of cyclic structures
char: depict cyclic sugars as planar 5 or 6 membered rings with the top and bottom faces of the ring nearly perpendicular to the page
how does is reality different than how the Haworth projection depicts cyclic sugars?
reality: the 5 membered rings are close to planar, but the pyranose rings adopt a chair-like configuration, and the substituents assume axial or equatorial positions to minimize steric hindrance
how do we convert a monosaccharide from a straight-chain Fischer projection to a Haworth projection?
any group on the right in the Fischer projection will point down
what happens when you expose hemiacetal rings to water?
it will cause them to spontaneously cycle between the open and closed form
is the alpha or beta anomer formed when you expose hemiacetal rings to water? why?
either anomer can be formed because the substituents on the single bond between C1 and C2 can rotate freely
defn + diagram: mutarotation
the spontaneous change of configuration between alpha and beta anomers about C1 (via ring opening and closing)
what makes mutarotation occur more rapidly?
when the reaction is catalyzed with an acid or base
what is the result of mutarotation?
a mixture that contains both alpha and beta anomers at equilibrium concentrations
in solution, which anomeric configuration is less favored? why?
the alpha-anomeric configuration is less favored
because the hydroxyl group of the anomeric C is axial, adding to the steric strain of the molecule
can the anomeric configuration prefernces of solution be changed in its solid state?
yes, it can be mitigated by the anomeric effect, which helps stabilize the alpha anomer
what do monosaccharides contain? (2)
- alcohols
- aldehydes or ketones
what 3 types of reactions do monosaccharides undergo?
- oxidation and reduction
- esterification
- nucleophilic attack (creating glycosides)
why is the oxidation of carbohydrates so important?
one of the most important biochemical reactions in the human body in order to yield energy
defn + how do they form: aldonic acids (3)
- as monosaccharides switch between anomeric configurations, the hemiacetal rings spend a short period of time in the open-chain aldehyde form
- just like other aldehydes, they can be oxidized to carboxylic acids
- these oxidized aldoses are aldonic acids
defn: reducing sugar
any monosaccharide with a hemiacetal ring
why are aldoses considered reducing agents?
because they can be oxidized
where does a lactone come from + defn + diagram
forms when the aldose that is oxidized is in ring form
defn: a cyclic ester with a carbonyl group persisting on the anomeric carbon
what are Tollens’ reagent and Benedict’s reagent used for
to detect the presence of reducing sugars
how is Tollens’ reagent made (3) + what happens to it when aldehydes are present
- must be freshly prepared
- start with silver nitrate (AgNO3) mixed with NaOH to produce silver oxide (Ag2O)
- silver oxide is dissolved in ammonia to produce [Ag(NH3)2]+ –> this is the actual Tollens’ reagent
it is reduced to produce a silvery mirror when aldehydes are present
what happens with Benedict’s reagent?
the aldehyde group of an aldose is readily oxidized, indicated by a red precipitate of Cu2O
how does one test specifically for the presence of glucose?
utilize the enzyme glucose oxidase, which does not react with other reducing sugars
how do ketose sugars interact with Tollens’ and Benedict’s test? (3)
- ketose sugars are also reducing sugars and give positive Tollens’ and Benedict’s tests
- although ketones cannot be oxidized directly to carboxylic acids, they can tautomerize to form aldoses under basic conditions, via keto-enol shifts
- while in the aldose form, they can react with Tollens’ or Benedict’s reagents to form the carboxylic acid
defn: tautomerization
the rearrangement of bonds in a compound, usually by moving a hydrogen and forming a double bond
explain how the tautomerization of ketoses to aldoses works
the ketone group picks up a H while the double bond is moved between 2 adjacent C’s, resulting in an enol
defn: enol
a compound with a double bond and an alcohol group
defn: alditol
when the aldehyde group of an aldose is reduced to an alcohol
defn: deoxy sugar
contains a H that replaces a hydroxyl group on the sugar
why can carbohydrates form esters?
because they have hydroxyl groups and so can participate in reactions with carboxylic acids and carboxylic acid derivatives
what is formed from esterification in the body?
a phosphate ester
diagram: esterification of glucose
explain the phosphorylation of glucose (3 + diagram)
- an extremely important metabolic reaction of glycolysis
- a phosphate group is transferred from ATP to glucose
- this phosphorylates glucose while forming ADP
what do hemiacetals form when they react with alcohols?
acetals
how do hemiacetals turn into acetals?/hemiketals to ketals? (3 + diagram)
- the anomeric hydroxyl group is transformed into an alkoxy group, yielding a mixture of alpha and beta acetals with water as a leaving group
- the resulting carbon-oxygen (C-O) bonds are glycosidic bonds and the acetals formed are glycosides
- this occurs under acidic conditions
what results as a result of glycosidic bonds between monosaccharides?
disaccharides and polysaccharides
defn: furanosides and pyranosides
FURANOSIDE = glycosides derived from furanose rings
PYRANOSIDE = glycosides derived from pyranose rings
why does breaking a glycosidic bond require hydrolysis?
glycoside formation is a dehydration reaction
defn + 3 ex: complex carbohydrates
all carbohydrates with at least two sugar molecules linked together
disaccharides
oligosaccharides
polysaccharides
how does a disaccharide form?
results from glycosidic bonds formed between hydroxyl groups of two monosaccharides
the hydroxyl group on the anomeric C reacts with the hydroxyl group of another sugar to form an acetal or ketal with a 1,2; 1,4; or 1,6 glycosidic linkage
what does it mean that an alpha or beta glycosidic linkage is nonspecific?
the anomeric C of a cyclic sugar can react with any hydroxyl group on any other sugar molecule
what are glycosidic linkages named for? + example
the configuration of the anomeric C and the numbers of the hydroxyl-containing C’s involved in the linkage
example: in an alpha-1,6 glycosidic bond formation between 2 D-glucose molecules, the alpha-anomeric C of the first glucose C-1 attaches to the C-6 of the second glucose (note that the second glucose could be either the alpha or beta anomer)
how would we name a glycosidic bond formed between two anomeric carbons?
example: a bond formed between the anomeric C’s of two alpha-D-glucose molecules would be termed an alpha,alpha-1,1 linkage
defn: maltose
2 glucose molecules linked by an alpha-1,4 glycosidic bond
defn: cellobiose
2 glucose molecules joined by a beta-1,4 linkage
what are the 3 most important disaccharides in a real-world context? how are these cells commonly produced in the cell?
- sucrose
- lactose
- maltose
produced in the cell by enzymatic activity
what are the technical names of sucrose
lactose
maltose
+ diagrams
sucrose = glucose-alpha-1,2-fructose
lactose = galactose-beta-1,4-glucose
maltose = glucose-alpha-1,4-glucose
defn: polysaccharides
long chains of monosaccharides linked together by glycosidic bonds
defn: homopolysaccharide
a polysaccharide composed entirely or glucose or any other monosaccharide
defn: heteropolysaccharide
a polymer made up of more than one type of monosaccharide
what are the 3 most important biological polysaccharides?
- cellulose
- starch
- glycogen
what are cellulose, starch, and glycogen all composed of?
D-glucose
how do cellulose, starch, and glycogen differ?
in configuration about the anomeric C and the position of the glycosidic bonds, which results in notable biological differences
why can polymer formation be linear or branched?
because glycosidic bonding can occur at multiple hydroxyl groups in a monosaccharide
when do branches form in polymers?
when an internal monosaccharide in a polymer chain forms at least 2 glycosidic bonds
defn + structure: cellulose
the main structural component of plants
a chain of beta-D-glucose molecules linked by beta-1,4 glycosidic bonds with hydrogen bonds holding the actual polymer chains together for support
why are humans not able to digest cellulose?
we lack the cellulase enzyme responsible for hydrolyzing cellulose to glucose monomers
defn: starches
polysaccharides that are more digestible by humans because they are linked alpha-D-glucose monomers
how is starch stored by plants?
predominantly as amylose (a linear glucose polymer linked via alpha-1,4 glycosidic bonds)
structure: amylopectin
another type of starch
starts off with the same type of linkage that amylose exhibits, but also contains alpha-1,6 glycosidic bonds
how does iodine react with starches?
iodine is a well-known reagent that tests for the presence of starch
does so by fitting inside the helix conformation amylose typically makes, forming a starch-iodine complex
what are starches broken down by in the body and what are they used for?
broken down by enzymes
used as a source of energy
what 2 enzymes is amylose degraded by?
alpha-amylase
beta-amylase
func: beta-amylase
cleaves amylose at the nonreducing end of the polymer (the end with the acetal) to yield maltose
func: alpha-amylase
cleaves amylose randomly along the chain to yield shorter polysaccharide chains, maltose, and glucose
what helps degrade amylopectin? why?
debranching enzymes
because amylopectin is highly branched
defn + structure: glycogen
a carbohydrate storage unit in animals
similar to starch, except has more alpha-1,6 glycosidic bonds, making it highly branched
what benefit does branching have for glycogen? (4)
- optimizes the energy efficiency of glycogen –> 2. making it more soluble in solution –> 3. allowing more glucose to be stored in the body
- allows enzymes that cleave glucose from glycogen (glycogen phosphorylase) to work on many sites within the molecule simultaneously
how does glycogen phosphorylase work?
by cleaving glucose from the nonreducing end of a glycogen branch and phosphorylating it, thus producing glucose 1-phosphate which is important for metabolism
