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carbs Flashcards

1
Q

what does the term ‘carbohydrate’ derive from?

A

derives from the first isolated carbohydrate - glucose

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2
Q

what was glucose originally thought to be?

A

a hydrate of the C6(H2O)6 carbon

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3
Q

what are simple carbohydrates?

A

they are monosaccharides and cannot be hydrolysed to form smaller carbs

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4
Q

what are complex carbohydrates?

A

these are more than 2 monosaccharides joined covalently

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5
Q

what is an oligosaccharide?

A

a polysaccharide that hydrolyses to give 2-10 monosaccharides

often associated with proteins(glycoproteins) and lipids(glycolipids)

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6
Q

what do disaccharides contain?

A

an O-glycosidic (acetal) bond between C-1 on one sugar and any hydroxy group on the second sugar

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7
Q

what does enzyme hydrolysis of starch produce?

A

maltose

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8
Q

what does partial chemical hydrolysis of cellulose produce?

A

cellobiose

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9
Q

what does lactose contain?

A

d-glucose and d-galactose.

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10
Q

what does lactase do?

A

catalyses the hydrolysis of the 1-4’ O-glycosidic bond in lactose

lactose intolerance occurs when there is low levels of this enzyme and ingested lactose moves to the colon and bacterial fermentation produces CO2, H2 and organic acids

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11
Q

what is sucrose?

A

table sugar and it is the most common disaccharide

is not a reducing sugar and does not undergo mutarotation as it has no hemiacetals

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12
Q

what does hydrolysis of sucrose produce?

A

gives a mixture of D-glucose and D-fructose called ‘invert sugar’

sucrose [a]D = + 66.6
invert sugar [a]D = - 22

catalysed by the enzyme invertase or b-D-fructofuranoside

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13
Q

how is invert sugar produced in cooking?

A

heating sucrose with lemon juice or cream of tartar

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14
Q

what are heteropolysaccharides?

A

heteropolysaccharides are composed of more than one type of monosaccharide unit

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15
Q

what are homopolysaccharides?

A

homopolysaccharides are composed of just one type of monosaccharide unit

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16
Q

what are galactans?

A

galactose homopolysaccharides are called galactans

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17
Q

what are glucans

A

glucose homopolysaccharides are called glucans

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18
Q

what can polysaccharides form unlike nucleic acids and proteins?

A

branched and linear polymers - the glycosidic linkages can involve any of the hydroxy (OH) groups of a monosaccharide

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19
Q

primary structure

A

monomer builiding blocks

sequence of monomers

nature of the glycosidic linkages

basic geometry of the polysaccharide

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20
Q

cellulose

A

cellulose, a linear polysaccharide, is the most abundant organic molecule in the biosphere. It is the major structural component of: leaves (10 - 20 %), wood/bark (50 %) and cotton (90 %).

composed of D-glucose units joined by 1-4’-b-glycosidic linkages.

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21
Q

cellulase

A

common in micro-organisms

micro-organisms are found in the digestive tracts of herbivores, allowing these animals to use cellulose, indirectly, as an energy source

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22
Q

starch

A

second most abundant polysaccharide; these are found in plants and animals.

can be separated into two fractions

one insoluble in cold water - this is amylose (20 % of starch)

one soluble in cold water - this is amylopectin (80 % of starch)

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23
Q

what starch links can humans digest?

A

starch with a-glycosidic links

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24
Q

d-glucose storage

A

stored as a polymer, rather than as a large number of monomer units.

major reason is to avoid large osmotic pressures.

osmotic pressure from 1000 glucose monomers in solution would be 103 times that of 1 amylose molecule with 1000 glucose units linked together.

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25
Q

distance between branch points in amylopectin?

A

24-30 glucose units.

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26
Q

glycogen

A

an energy storage polysaccharide found in animals

like amylopectin (plants), glycogen is a polysaccharide of glucose with 1,4’-a- and 1,6’-a-glycosidic links.

glycogen is more heavily branched with 1,6’-a-glycosidic links every 6 - 8 and 3 glucose units.

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27
Q

breakdown of glycogen

A

catalysed by glycogen phosphorylase to give glucose-1-phosphate

heavily branched with many non-reducing termini at which the glycogen phosphorylase can cleave glucose units, so as to provide sufficient glucose at high energy requirement conditions

cannot cleave glycosidic linkages that are closer than four glucose units from a branch point.

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28
Q

secondary structure

A

details the way in which the backbone is folded to give the three dimensional shape

structure results from local conformational variety, which is due to rotations about the single bonds involved in the glycosidic linkages

nature of the glycosidic bond is important as certain angle combinations are less hindered, so are more stable (lower energy).

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29
Q

1,4’ glycosides

A

1,4’-glycosides have two torsional angles to consider, psi and phi

The favourable psi and phi for the 1,4’-a-link results in a gentle turn that gives rise to a helix when extended further.

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30
Q

1,6’-glycosides

A

three torsional angles to consider, psi, phi and omega

much wider range of conformations available

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31
Q

tertiary structure

A

concerns the way the entire polysaccharide backbone is arranged in three dimensional space

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32
Q

quaternary structure

A

concerns the way polysaccharide chains aggregate with other polysaccharide chains

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33
Q

chain folding and packing

A

chain folding and packing is dictated by non-covalent , long-range interactions between the functional groups present in the monosaccharide units (-OH, -SO42-, -NH2, -PO42-, -CO2H, …).

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34
Q

ribbons

A

linear arrangement of b-linked glucose units (cellulose) or N-acetylglucosamine units (chitin) can be described as b-linked chains, or ribbons

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35
Q

cellulose ribbons

A

the resulting ‘ribbon’ is stabilised by intramolecular hydrogen bonds between OH groups on adjacent glucose units

b-ribbons associate laterally to form sheets, which are stabilised by intermolecular hydrogen bonds

further association of cellulose sheets results from inter-sheet hydrogen bonding

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36
Q

chitin

A

principal structural component of the exoskeletons of invertebrates

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37
Q

flexible helices

A

a-glycosidic bond results in chains with wide hollow helix that is stabilised by hydrogen bonding

amylose and amylopectin from starch have a-glycosidic bonds and, therefore, have a helical structure

enzymes can hydrolyse the a-glycosidic bond in these loosely packed helices

a-linked carbohydrates are found in food storage polysaccharides (eg. starch and glycogen).

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38
Q

iodine-starch test

A

molecules can be accommodated into the central cavities of polysaccharide helices to give inclusion complexes

aqueous solution of I2 and I- forms an inclusion complex with starch to give the characteristic blue-violet colour of the starch-iodine test

colour arises from charge-transfer interactions between the rows of triiodide anions, [I3]-, arranged end-to-end in the amylose cavity.

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39
Q

buckled ribbons

A

1,4’-a-linked polysaccharide chains can pack together leaving cavities - these are called ‘buckled ribbons’.

cavities can be filled with H2O or metal ions to give extra stability to the polysaccharide

pectic acid forms a buckled ribbon. It is
responsible for the formation of jellies
in fruits and berries

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40
Q

cyclic structures

A

cyclodextrins are naturally occurring macrocycles composed of sugar units.

obtained by the enzymatic degradation of starch

hydrophobic interiors and hydrophilic exteriors

hydrophobic interior is ideal for apolar guests (eg. drug molecules) and the hydrophilic exterior means that any complexes formed by cyclodextrin will be soluble in water

41
Q

more cyclic structures

A

cyclodextrins are able to bind apolar drug molecules within their cavities and simultaneously dissolve in water.

this can enable transport of the drug molecules to their receptors.

febreze is a cyclodextrin solution.

the cyclodextrins bind apolar molecules that have an odour.

the resulting complex dissolves in water and can be washed away.

42
Q

rocuronium

A

rocuronium is a muscle relaxant used as part of anaesthesia during surgery

sugammadex binds rocuronium within its hydrophobic interior.

the quaternary nitrogen (N+) of rocuronium sits outside of the sugammadex core and interacts with the pendant carboxylate groups.

43
Q

hylauronates

A

important for the vitreous humor of eyes, synovial fluid and cartilage

forms an extended rigid structure which is highly hydrated

solutions have a viscosity which is shear dependent:

at low shear rates, the molecules form tangled masses that impede flow

at high shear rates there’s a lower viscosity

these properties make the solutions excellent shock absorbers and lubricants

44
Q

Chondroitans and collagen

A

found in tendons, cartilage, and connective tissue

45
Q

Dermatan sulfate

A

component of the extracellular skin matrix

46
Q

glycoproteins

A

proteins with a covalent attachment to a carbohydrate

many natural proteins are glycoproteins: structural proteins, enzymes, membrane receptors, transport proteins, immunoglobulins, and others

vital part of recognition (blood antigenic determinants) and cell adhesion (eg. the binding of sperm to eggs).

47
Q

recognition

A

carbohydrate portion of glycoproteins are vital in infection of the cell

viruses use the carbohydrate part of a glycoprotein to bind to a cell

48
Q

stereochemistry

A

carbohydrates contain stereocentres (carbon atoms with four different groups attached and no plane of symmetry)

49
Q

stereoisomers

A

same molecular formula and sequence of bonds (configuration) but differ in the relative position of atoms in space (conformation)

50
Q

enantiomers

A

non-superimposable mirror images are called enantiomers

identical physical and chemical properties except how they interact with plane polarised light

thalidomde

optically active because they rotate plane polarised light in equal and opposite directions

51
Q

thalidomide

A

r form - treats morning sickness

s form - causes birth defects

52
Q

chirality

A

cannot be superimposed on its mirror image

53
Q

specific rotation

A

dextrorotatory molecules have the prefix (+)

levorotatory molecules have the prefix (-)

54
Q

diasterisomers

A

not mirror image related

different physical properties and similar, but not identical, chemical properties

they may exhibit different rates of chemical reaction

diastereoisomers result from the presence of two or more stereocentres within molecules

55
Q

fischer projections

A

possess 1-5 stereocentres

horizontal bonds signify bonds that are pointing to the viewer

vertical lines are pointing away from the viewer

56
Q

d labels

A

all natural sugars

57
Q

l labels

A

all standard amino acids from proteins

58
Q

aldoses

A

most common are hexose

d-glucose is the only naturally occuring as a monosaccharide

d-glyceraldehyde, d-ribose, d-mannose and d-galactose

59
Q

epimers

A

pair of sugars with one carbon atom difference

60
Q

ketoses

A

simplest ketose is dihydroxyacetone, it is achiral and not regarded as a sugar

much less common in nature than aldoses. The most abundant is D-fructose
(a ketohexose, functional group isomer of glucose)

61
Q

evidence for non linerar forms

A

no absorption at ~1725 cm-1 (C=O stretch) in the IR spectrum

no signal at ~ 200 ppm in the 13C NMR spectrum

no dimethyl acetal formation with MeOH and HCl

62
Q

hemiacetal

A

r1, r2/hydrogen, OH, OR’

63
Q

cyclisation and glucose anomers

A

cyclisation creates a new stereocentre at C-1, so there are two cyclic forms

63
Q

cylic hemiacetals

A

cyclic hemiacetal means that there is no free aldehyde group - this explains the unexpected properties of glucose

64
Q

glucose reactions

A

reaction of glucose with MeOH and HCl gives two, distinct monomethyl acetals with different physical properties:

m.p. 165 ˚C and [a]D = +157

m.p. 104 ˚C and [a]D = -34

65
Q

acetal

A

r1, r2/hydrogen, OR’, OR’

66
Q

pyranoses

A

six membered rings are called pyranoses

67
Q

equilibrium

A

in solution, monosaccharides exist as an equilibrium mixture of the various structural forms

68
Q

preferred structures

A

aldopentoses give mainly the pyranose forms

aldohexoses give mainly the pyranose forms

ketahexaoses give mixtures of the pyranose and the furanose forms

69
Q

furanoses

A

five membered cyclic hemiacetals

70
Q

why are five and six carbon ring sizes the most stable?

A

other ring sizes lead to weaker bonding (internal angles lead to poorer orbital overlap)

71
Q

mutarotation

A

change in [a]D = to an equilibrium value is called mutarotation.

accelerated by acids or bases and is caused by the reversible ring opening of the cyclic hemiacetal to the open chain form, an aldehyde or ketone , followed by re-closure

72
Q

puckered forms

A

pyranose rings take a similar structural form to cyclohexane, which can adopt ‘boat’ and ‘chair’ forms

73
Q

boat and chair forms

A

Cyclohexane can adopt either but the lowest energy arrangement is the ‘chair’ form

relative positions of the hydrogen atoms determines the stability of each conformation

29 kJ mol-1 higher in energy than the chair form, as a result of hydrogen-hydrogen repulsions

74
Q

ring flipping

A

cyclohexane rings are not rigid and can interconvert between two chair forms.

all axial positions become equatorial and all equatorial positions become axial

75
Q

stability of chair forms

A

energy barrier to ring-flipping is ~ 43 kJ mol-1, so the process occurs quite frequently (~ 105 times per second at 25 degrees)

76
Q

predicting the most stable form

A

chair form with the fewest bulky axial substituents is predicted to be more stable

77
Q

CH2OH

A

aldohexoses have the -CH2OH equatorial in the most stable conformation.

in some cases, the two conformations are nearly equal in energy, so both conformations (-CH2OH axial and -CH2OH equatorial) may be present at equilibrium.

78
Q

anomeric effect

A

a-anomer (with an axial -OH at C-1) is known as the anomeric effect

anomeric effect often results in electronegative substituents (halogen, OR, NR2) being axial

anomeric effect is a stabilizing electronic effect when an oxygen lone pair is trans coplanar to a polar C-X bond (X = halogen, OR, NR2)

79
Q

furanose rings

A

not planar - they are puckered, with an ‘envelope’ conformation

four atoms are coplanar, while the fifth is ~ 0.5 Å out of the ring plane

80
Q

ribose rings in nucleic acids

A

the ribose ring has all five atoms in the same plane, resulting in high ring-strain and steric repulsions

ring-strain is relieved by ring puckering

81
Q

heparin

A

2-3 sulfate groups per disaccharide unit

one of the most negatively charged bio-polymers

occurs in the lining of arterial walls

inhibits blood clotting and is used in post surgical patients

82
Q

keratan sulfate

A

component of cartilage, bone, hair, nails, horn and cornea

sulfate content is variable and contains several different monosaccharide units

83
Q

cartilage

A

consists of a mesh of collagen fibers, combined with glycoproteins which contain keratan sulfate and chondroitan sulfate

charged sulfate groups mean that cartilage is highly hydrated

mechanical compression displaces water molecules until the negatively charged sulfate groups repel significantly and resist further compression

when pressure is released, water returns

84
Q

glycosaminoglycans in bacteria

A

n-acetylmuramic acid (NAM)

n-acetylglucosamine (NAG)

85
Q

bacterial cell walls

A

both gram positive and gram negative bacteria have cell walls that consist of covalently linked polysaccharide and polypeptide chains

polysaccharide component consists of linear chains of 1,4’ beta linked NAG or NAM

86
Q

amide bonds in bacteria

A

the linkage between the polysaccharide and polypeptide chains occur through an amide functional group

formed by the carboxylic group of NAM and an amine functional group from an Ala residue

87
Q

penicillin

A

kills bacteria by inhibiting cell wall biosynthesis

binds and inactivates the enzymes responsible for cross linking the peptidoglycan strands of the bacterial cell walls

88
Q

glycoproteins

A

polypeptide part is synthesized by translation(encoded by genes)

the carb component is enzymatically generated

89
Q

glycoprotein types

A

structural protein components

in mucous secretions

immunoglobins

interferons

blood plasma proteins

90
Q

glycoproteins in membranes

A

important constituents of plasma membrane

location of carb parts determined by electron microscopy

mostly on external surfaces

intercellular communication

91
Q

O-linked glycoproteins

A

carbohydrates are often linked to the polypeptide chains via the OH of serine, threonine or hydroxylysine

92
Q

protective functions of O-linked glycoproteins

A

tend to be clustered in heavily glycosylated segments, rather than being spread along the polypeptide chain

mucus contains O-linked glycoproteins with - charged sulfate groups. These negatively charged groups repel one another and prevent any secondary structure from forming

mucins form inter-tangled networks that act as viscoelastic gels which protect and lubricate mucous membranes

93
Q

antigenic determinants

A

structure of the carb on the cell surface determines the blood group

presence of these means that blood can only be transferred between people if the donor and acceptor are compatible

otherwise blood is considered foreign and leads to clumping of erythrocytes and fatal blood vessel blockage

universal donor - O

universal acceptor - AB

94
Q

sialic acid

A

white blood cells patrol the circulatory system ready for immobilization at a trauma site

frontline scouts are cell surface carbohydrate called sialyl lewis acids and a contain a sialic acid group

95
Q

injury

A

selectins signal the site of the injury

bind sialyl lewis acid, this binding causes adhesion of leukocytes to the infected area

leukocyte recruitment is central to the inflammatory cascade

some diseases result from over enthusiastic leukocyte recruitment

96
Q

sialic acid function

A

combatting an overactive inflammatory cascade can be done by blocking selectin binding

synthetic mimetic sialyl lewis acids have helped to identify the key functional groups for recognition and binding

by preparing sialyl lewis acid analogues it is hoped that tighter binding derivatives can result, so providing new therapy for chronic inflammatory diseases

97
Q

sialic acid and glycoprotein aging

A

new serum proteins have N-linked oligosaccharifes with chains terminated with 3 strands that have sialic acid end caps covering a galactose

as the glycoproteins circulate, sialic acid groups are slowly cleaved from the glycoprotein by enzymes in the blood vessel walls and expose the galactose units

the asialoglycoprotein receptor in the liver binds to the galactose units

as more binding occurs the receptor-asia complex is absorbed and degraded by the cell

98
Q

sialic acid and viral infections

A

virus binds to the carbs on the surface and cleaves them from the glycoprotein and infect the cell

inhibitors of the enzymes (neuramidases) that cleave the sialic acid glycoside bond are used as anti flu treatments

inhibitors bind more tightly

ch112 (4 decks)

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