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Biochem > Ch. 4: Carbohydrate Structure and Function > Flashcards

Ch. 4: Carbohydrate Structure and Function Flashcards

1
Q

what are the 4 things that carbohydrates can be classified by?

A
  1. the number of sugar moieties that make them up
  2. the number of carbons in each sugar
  3. the functional groups present on the molecule
  4. the stereochemistry of the sugar
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2
Q

what are the basic structural units of carbohydrates?

A

monosaccharides

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3
Q

defn: trioses

A

the simplest monosaccharides

contain 3 carbon atoms

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4
Q

what are carbohydrates with 4, 5, and 6 carbons referred to?

A

4 = tetroses
5 = pentoses
6 = hexoses

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5
Q

defn: aldoses vs. ketoses

A

ALDOSE = carbohydrates that contain an aldehyde group as their most oxidized functional group

KETOSE = carbohydrates with a ketone as their most oxidized functional group

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6
Q

defn: aldohexose, ketopentose

A

aldohexose = a six-C sugar with an aldehyde group

ketopentose = a 5-C sugar with a ketone group

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7
Q

what is the simplest aldose?

A

glyceraldehyde, which is an aldotriose

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8
Q

the numbering of C atoms in a monosaccharide follows the rules described in orgo, what does this mean for monosaccharides? (2)

A
  1. the carbonyl C is the most oxidized (and will always have the lowest number)
  2. in an aldose, the aldehyde C will always be C-1
  3. in a ketose, the carbonyl C is C-2 (still most oxidized)
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9
Q

defn: glycosidic linkages

A

linkages that the aldehyde carbon in an aldose or the carbonyl C in a ketose can participate in

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10
Q

defn: glycosyl residues

A

sugars acting as substituents via glycosidic linkages

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11
Q

what is the simplest ketone sugar/ketose?

A

dihydroxyacetone (a ketotriose)

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12
Q

on every monosaccharide, what carbons carry a hydroxyl group?

A

every C other than the carbonyl C

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13
Q

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?

A
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14
Q

defn: stereoisomers/optical isomers

A

compounds that have the same chemical formula and differ from one another only in terms of the spatial arrangement of their component atoms

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15
Q

defn: enantiomer

A

a special type of isomerism between stereoisomers that are nonidentical, nonsuperimposable mirror images of each other

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16
Q

any molecule that contains chiral carbons and no internal planes of symmetry has ….

A

an enantiomer

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17
Q

diagram: enantiomers of glyceraldehyde

A
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18
Q

what determines a compound’s absolute configuration?

A

the particular 3-D arrangement of the groups attached to the chiral C

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19
Q

what determines the number of possible stereoisomers of a compound + eqn?

A

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

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20
Q

Do D and L correspond to + and - rotation or vice versa?

A

Neither, the direction of rotation must be determined experimentally and CANNOT be determined from the D or L designation for the sugar

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21
Q

how are monosaccharides assigned the D or L configuration on the MCAT?

A

based on their relationship to glyceraldehyde

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22
Q

defn + func: Fischer projection

A

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

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23
Q

using a Fischer projection, what do all D sugars have? what do all L sugars have?

A

D-sugars –> the hydroxide of their highest-numbered chiral center on the right

L-sugars –> that hydroxide is on the left

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24
Q

Are R/S and D/L interchangeable?

A

no! they are not paired one to one!

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25
Q

what is true about the chiral centers in D and L glucose because they are enantiomers?

A

EVERY chiral center in D-glucose has the opposite configuration in L-glucose

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26
Q

what 3 types of stereoisomers should you be familiar with?

A
  1. enantiomers
  2. diastereomers
  3. epimers
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27
Q

defn: enantiomers

A

the same sugars, in different optical families (D-glucose and L-glucose)

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28
Q

defn: diastereomers

A

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

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29
Q

defn: epimers

A

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)

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30
Q

look at the 4 stereoisomers of an aldotetrose to understand these compounds

A

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)

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31
Q

can a compound have multiple enantiomers? diasteroemers?

A

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)

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32
Q

why can monosaccharides undergo intramolecular reactions to form cyclic hemiacetals (from aldoses) and hemiketals (from ketoses)?

A

because they contain both a hydroxyl group (can act as a nucleophile) and a carbonyl group (the most common electrophile)

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33
Q

what are the only 2 cyclic molecules stable in solution and why?

A

6-membered PYRANOSE rings
5-membered FURANOSE rings

why: due to ring strain

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34
Q

what happens to the hydroxyl group and the oxygen when these ring structures (hemiacetals or hemiketals) form?

A
  1. the hydroxyl group acts as the nucleophile during ring formation
  2. oxygen becomes a member of the ring structure
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35
Q

defn: anomeric carbon

A

the carbonyl carbon becomes chiral during the formation of a hemiacetal or hemiketal

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36
Q

defn: anomers

A

molecules that differ at the anomeric carbon

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37
Q

what emerges from the cyclization of a sugar molecule?

how does this apply to glucose?

A

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)

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38
Q

diagram: cyclic sugar formation via intramolecular nucleophilic addition

A

glucose forms a 6-membered ring with two anomeric forms (alpha, left and beta, right)

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39
Q

use + char: Haworth projection

A

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

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40
Q

how does is reality different than how the Haworth projection depicts cyclic sugars?

A

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

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41
Q

how do we convert a monosaccharide from a straight-chain Fischer projection to a Haworth projection?

A

any group on the right in the Fischer projection will point down

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42
Q

what happens when you expose hemiacetal rings to water?

A

it will cause them to spontaneously cycle between the open and closed form

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43
Q

is the alpha or beta anomer formed when you expose hemiacetal rings to water? why?

A

either anomer can be formed because the substituents on the single bond between C1 and C2 can rotate freely

44
Q

defn + diagram: mutarotation

A

the spontaneous change of configuration between alpha and beta anomers about C1 (via ring opening and closing)

45
Q

what makes mutarotation occur more rapidly?

A

when the reaction is catalyzed with an acid or base

46
Q

what is the result of mutarotation?

A

a mixture that contains both alpha and beta anomers at equilibrium concentrations

47
Q

in solution, which anomeric configuration is less favored? why?

A

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

48
Q

can the anomeric configuration prefernces of solution be changed in its solid state?

A

yes, it can be mitigated by the anomeric effect, which helps stabilize the alpha anomer

49
Q

what do monosaccharides contain? (2)

A
  1. alcohols
  2. aldehydes or ketones
50
Q

what 3 types of reactions do monosaccharides undergo?

A
  1. oxidation and reduction
  2. esterification
  3. nucleophilic attack (creating glycosides)
51
Q

why is the oxidation of carbohydrates so important?

A

one of the most important biochemical reactions in the human body in order to yield energy

52
Q

defn + how do they form: aldonic acids (3)

A
  1. as monosaccharides switch between anomeric configurations, the hemiacetal rings spend a short period of time in the open-chain aldehyde form
  2. just like other aldehydes, they can be oxidized to carboxylic acids
  3. these oxidized aldoses are aldonic acids
53
Q

defn: reducing sugar

A

any monosaccharide with a hemiacetal ring

54
Q

why are aldoses considered reducing agents?

A

because they can be oxidized

55
Q

where does a lactone come from + defn + diagram

A

forms when the aldose that is oxidized is in ring form

defn: a cyclic ester with a carbonyl group persisting on the anomeric carbon

56
Q

what are Tollens’ reagent and Benedict’s reagent used for

A

to detect the presence of reducing sugars

57
Q

how is Tollens’ reagent made (3) + what happens to it when aldehydes are present

A
  1. must be freshly prepared
  2. start with silver nitrate (AgNO3) mixed with NaOH to produce silver oxide (Ag2O)
  3. 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

58
Q

what happens with Benedict’s reagent?

A

the aldehyde group of an aldose is readily oxidized, indicated by a red precipitate of Cu2O

59
Q

how does one test specifically for the presence of glucose?

A

utilize the enzyme glucose oxidase, which does not react with other reducing sugars

60
Q

how do ketose sugars interact with Tollens’ and Benedict’s test? (3)

A
  1. ketose sugars are also reducing sugars and give positive Tollens’ and Benedict’s tests
  2. although ketones cannot be oxidized directly to carboxylic acids, they can tautomerize to form aldoses under basic conditions, via keto-enol shifts
  3. while in the aldose form, they can react with Tollens’ or Benedict’s reagents to form the carboxylic acid
61
Q

defn: tautomerization

A

the rearrangement of bonds in a compound, usually by moving a hydrogen and forming a double bond

62
Q

explain how the tautomerization of ketoses to aldoses works

A

the ketone group picks up a H while the double bond is moved between 2 adjacent C’s, resulting in an enol

63
Q

defn: enol

A

a compound with a double bond and an alcohol group

64
Q

defn: alditol

A

when the aldehyde group of an aldose is reduced to an alcohol

65
Q

defn: deoxy sugar

A

contains a H that replaces a hydroxyl group on the sugar

66
Q

why can carbohydrates form esters?

A

because they have hydroxyl groups and so can participate in reactions with carboxylic acids and carboxylic acid derivatives

67
Q

what is formed from esterification in the body?

A

a phosphate ester

68
Q

diagram: esterification of glucose

A
69
Q

explain the phosphorylation of glucose (3 + diagram)

A
  1. an extremely important metabolic reaction of glycolysis
  2. a phosphate group is transferred from ATP to glucose
  3. this phosphorylates glucose while forming ADP
70
Q

what do hemiacetals form when they react with alcohols?

A

acetals

71
Q

how do hemiacetals turn into acetals?/hemiketals to ketals? (3 + diagram)

A
  1. the anomeric hydroxyl group is transformed into an alkoxy group, yielding a mixture of alpha and beta acetals with water as a leaving group
  2. the resulting carbon-oxygen (C-O) bonds are glycosidic bonds and the acetals formed are glycosides
  3. this occurs under acidic conditions
72
Q

what results as a result of glycosidic bonds between monosaccharides?

A

disaccharides and polysaccharides

73
Q

defn: furanosides and pyranosides

A

FURANOSIDE = glycosides derived from furanose rings

PYRANOSIDE = glycosides derived from pyranose rings

74
Q

why does breaking a glycosidic bond require hydrolysis?

A

glycoside formation is a dehydration reaction

75
Q

defn + 3 ex: complex carbohydrates

A

all carbohydrates with at least two sugar molecules linked together

disaccharides
oligosaccharides
polysaccharides

76
Q

how does a disaccharide form?

A

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

77
Q

what does it mean that an alpha or beta glycosidic linkage is nonspecific?

A

the anomeric C of a cyclic sugar can react with any hydroxyl group on any other sugar molecule

78
Q

what are glycosidic linkages named for? + example

A

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)

79
Q

how would we name a glycosidic bond formed between two anomeric carbons?

A

example: a bond formed between the anomeric C’s of two alpha-D-glucose molecules would be termed an alpha,alpha-1,1 linkage

80
Q

defn: maltose

A

2 glucose molecules linked by an alpha-1,4 glycosidic bond

81
Q

defn: cellobiose

A

2 glucose molecules joined by a beta-1,4 linkage

82
Q

what are the 3 most important disaccharides in a real-world context? how are these cells commonly produced in the cell?

A
  1. sucrose
  2. lactose
  3. maltose

produced in the cell by enzymatic activity

83
Q

what are the technical names of sucrose
lactose
maltose

+ diagrams

A

sucrose = glucose-alpha-1,2-fructose

lactose = galactose-beta-1,4-glucose

maltose = glucose-alpha-1,4-glucose

84
Q

defn: polysaccharides

A

long chains of monosaccharides linked together by glycosidic bonds

85
Q

defn: homopolysaccharide

A

a polysaccharide composed entirely or glucose or any other monosaccharide

86
Q

defn: heteropolysaccharide

A

a polymer made up of more than one type of monosaccharide

87
Q

what are the 3 most important biological polysaccharides?

A
  1. cellulose
  2. starch
  3. glycogen
88
Q

what are cellulose, starch, and glycogen all composed of?

A

D-glucose

89
Q

how do cellulose, starch, and glycogen differ?

A

in configuration about the anomeric C and the position of the glycosidic bonds, which results in notable biological differences

90
Q

why can polymer formation be linear or branched?

A

because glycosidic bonding can occur at multiple hydroxyl groups in a monosaccharide

91
Q

when do branches form in polymers?

A

when an internal monosaccharide in a polymer chain forms at least 2 glycosidic bonds

92
Q

defn + structure: cellulose

A

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

93
Q

why are humans not able to digest cellulose?

A

we lack the cellulase enzyme responsible for hydrolyzing cellulose to glucose monomers

94
Q

defn: starches

A

polysaccharides that are more digestible by humans because they are linked alpha-D-glucose monomers

95
Q

how is starch stored by plants?

A

predominantly as amylose (a linear glucose polymer linked via alpha-1,4 glycosidic bonds)

96
Q

structure: amylopectin

A

another type of starch

starts off with the same type of linkage that amylose exhibits, but also contains alpha-1,6 glycosidic bonds

97
Q

how does iodine react with starches?

A

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

98
Q

what are starches broken down by in the body and what are they used for?

A

broken down by enzymes

used as a source of energy

99
Q

what 2 enzymes is amylose degraded by?

A

alpha-amylase

beta-amylase

100
Q

func: beta-amylase

A

cleaves amylose at the nonreducing end of the polymer (the end with the acetal) to yield maltose

101
Q

func: alpha-amylase

A

cleaves amylose randomly along the chain to yield shorter polysaccharide chains, maltose, and glucose

102
Q

what helps degrade amylopectin? why?

A

debranching enzymes

because amylopectin is highly branched

103
Q

defn + structure: glycogen

A

a carbohydrate storage unit in animals

similar to starch, except has more alpha-1,6 glycosidic bonds, making it highly branched

104
Q

what benefit does branching have for glycogen? (4)

A
  1. optimizes the energy efficiency of glycogen –> 2. making it more soluble in solution –> 3. allowing more glucose to be stored in the body
  2. allows enzymes that cleave glucose from glycogen (glycogen phosphorylase) to work on many sites within the molecule simultaneously
105
Q

how does glycogen phosphorylase work?

A

by cleaving glucose from the nonreducing end of a glycogen branch and phosphorylating it, thus producing glucose 1-phosphate which is important for metabolism

Biochem (12 decks)

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