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Carbohydrates. Andy Howard Introductory Biochemistry, Fall 2008 16 September 2008. Now we’ll study sugars!. Sugars are vital as energy sources, and they also serve as building blocks for lipid-carbohydrate and protein-carbohydrate complexes. Notes about upcoming midterm Sugar Concepts

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carbohydrates

Carbohydrates

Andy HowardIntroductory Biochemistry, Fall 200816 September 2008

Biochemistry: Carbohydrates

now we ll study sugars
Now we’ll study sugars!
  • Sugars are vital as energy sources, and they also serve as building blocks for lipid-carbohydrate and protein-carbohydrate complexes

Biochemistry: Carbohydrates

what we ll discuss
Notes about upcoming midterm

Sugar Concepts

Monosaccharides

Oligosaccharides

Glycosides

Polysaccharides

Starch & glycogen

Cellulose and chitin

Glycoconjugates

Proteoglycans

Peptidoglycans

Glycoproteins

What we’ll discuss

Biochemistry: Carbohydrates

midterm is tuesday 23 sep
Midterm is Tuesday 23 Sep
  • Internet students can take it between 9am Tuesday and 5pm Wednesday
  • Find a proctor or arrange to take it in class
  • Details about how the midterm works are in the Course Introduction document

Biochemistry: Carbohydrates

what the midterm will cover
What the midterm will cover
  • Everything up through today’s lecture
  • Thursday’s lecture will be on the second midterm
  • Exam syllabus will be posted by the weekend to help you study
  • Exam help-sheet too (don’t memorize what’s on the help sheet!)
  • Yes, I curve these exams; but the grade cutoffs are determined at the end of the course, not now

Biochemistry: Carbohydrates

carbohydrates6
Carbohydrates
  • These are polyhydroxylated aldehydes and ketones, many of which can exist in cyclic forms
  • General monomeric formula (CH2O)m, 3 < m < 9
  • With one exception (dihydroxyacetone) they contain chiral centers
  • Highly soluble
  • Can be oligomerized and polymerized
  • Oligomers may or may not be soluble
  • Most abundant organic molecules on the planet

Biochemistry: Carbohydrates

how do we measure solubility for very soluble compounds
How do we measure solubility for very soluble compounds?
  • (Note: this is not a serious chemical topic: it’s an example of how statistics can be abused…)
  • The assertion is that, with highly soluble compounds like sugars, it’s difficult to use conventional approaches to compare their solubilities
  • The suggestion is that we might use the amount of time it takes to dissolve (for example) 50g of solute in 100mL of cold water: if it’s fast, the solute is more soluble than if it’s slow.

Biochemistry: Carbohydrates

solubility measured by dissolution time
Solubility measured by dissolution time

6

  • Assertion: more polar groups means shorter dissolution time for a given class of compounds

5

4

3

# of polar groups

2

1

Time required for dissolution

Biochemistry: Carbohydrates

what if we extrapolate to n 6
What if we extrapolate to n=6?

Extrapolated point

6

  • We get a negative dissolution time!
  • That is, the solid goes into solution 6 seconds before we put it in the water!
  • This causes serious psychological problems (what if I change my mind?) and philosophical problems (is this pre-ordained?)

5

4

3

# of polar groups

Observed points

2

1

Time required for dissolution

Biochemistry: Carbohydrates

whose idea is this
Whose idea is this?
  • Isaac Asimov, that’s who!
  • “The endochronic properties of resublimated thiotimolene”:Astounding Science Fiction, March1948
  • My point: extrapolations and other misuses of statistics are dangerous
  • Benjamin Disraeli (popularized by Mark Twain):There are three kinds of untruth:lies, damn lies, and statistics.
  • Okay: let’s get back to the science.

Biochemistry: Carbohydrates

aldoses ketoses
Aldoses & ketoses
  • If the carbonyl moiety is at the end carbon (conventionally counted as 1), it’s an aldose
  • If carbonyl is one carbon away (counted as 2), it’s a ketose
  • If it’s two or more carbons from the end of the chain, it’s not a sugar

Biochemistry: Carbohydrates

simplest monosaccharides
Simplest monosaccharides
  • Glyceraldehyde and dihydroxyacetone
  • Only glyceraldehyde is chiral:D-enantiomer is more plentiful in biosphere
  • All longer sugars can be regarded as being built up by adding-(CHOH)m-1 to either glyceraldehyde or dihydroxyacetone, just below C2

Biochemistry: Carbohydrates

how many aldoses are there
How many aldoses are there?
  • Every -(CHOH) in the interior offers one chiral center
  • An m-carbon aldose has (m-2) internal -(CHOH) groups
  • Therefore: 2m-2 aldoses of length m
  • For m=3, that’s 21=2; for m=6, it’s 24=16.

Biochemistry: Carbohydrates

how many ketoses are there
How many ketoses are there?
  • Every -(CHOH) in the interior offers one chiral center
  • An m-carbon ketose has (m-3) internal-(CHOH) groups
  • Therefore: 2m-3 ketoses of length m
  • For m=3, that’s 20 = 1; for m=6, that’s 23=8.

Biochemistry: Carbohydrates

review stereochemical nomenclature
Review: stereochemical nomenclature
  • Stereoisomers: compounds with identical covalent bonding apart from chiral connectivity
  • Enantiomers: compounds for which the opposite chirality applies at all chiral centers
  • Epimers: compounds that differ in chirality at exactly one chiral center
  • One chiral center: enantiomers are epimers.
  • > 1 chiral center: enantiomers are not epimers.

Biochemistry: Carbohydrates

example 2 chiral centers
Example: 2 chiral centers
  • Chiral centers u,v; compounds A,B,C,D

Biochemistry: Carbohydrates

properties
Properties
  • Enantiomers have identical physical properties (MP,BP, solubility, surface tension…) except when they interact with other chiral molecules
  • (Note!: water isn’t chiral!)
  • Stereoisomers that aren’t enantiomers can have different properties; therefore, they’re often given different names

Biochemistry: Carbohydrates

sugar nomenclature
Sugar nomenclature
  • All sugars with m ≤ 7 have specific names apart from their enantiomeric(L or D) designation,e.g. D-glucose, L-ribose.
  • The only 7-carbon sugar that routinely gets involved in metabolism is sedoheptulose, so we won’t try to articulate the names of the others

Biochemistry: Carbohydrates

fischer projections
Fischer projections

Emil Fischer

  • Convention for drawing open-chain monosaccharides
  • If the hydroxyl comes off counterclockwise relative to the previous carbon, we draw it to the left;
  • Clockwise to the right.

Biochemistry: Carbohydrates

cyclic sugars
Cyclic sugars
  • Sugars with at least four carbons can readily interconvert between the open-chain forms we have drawn and five-membered(furanose) or six-membered (pyranose) ring forms in which the carbonyl oxygen becomes part of the ring
  • There are no C=O bonds in the ring forms

Biochemistry: Carbohydrates

furanoses

1

Furanoses

5

2

4

3

furan

  • Formally derived from structure of furan
  • Hydroxyls hang off of the ring; stereochemistry preserved there
  • Extra carbons come off at 2 and 5 positions

Biochemistry: Carbohydrates

pyranoses

1

Pyranoses

6

2

3

5

  • Formally derived from structure of pyran
  • Hydroxyls hang off of the ring; stereochemistry preserved there
  • Extra carbons come off at 2 and 6 positions

4

pyran

Biochemistry: Carbohydrates

how do we cyclize a sugar
How do we cyclize a sugar?
  • Formation of an internal hemiacetal or hemiketal (see a few slides from here) by conversion of the carbonyl oxygen to a ring oxygen
  • Not a net oxidation or reduction;in fact it’s a true isomerization.
  • The molecular formula for the cyclized form is the same as the open chain form

Biochemistry: Carbohydrates

family tree of aldoses
Family tree of aldoses
  • Simplest: D-, L- glyceraldehyde (C3)
  • Add —CHOH: D,L-threose, erythrose (C4)
  • Add —CHOH:D,L- lyxose, xylose, arabinose, ribose (C5)
  • Add —CHOH:D,L-talose, galactose, idose, gulose,mannose, glucose, altrose, allose (C6)

Biochemistry: Carbohydrates

family tree of ketoses
Family tree of ketoses
  • Simplest: dihydroxyacetone (C3)
  • Add —CHOH: D,L-erythrulose (C4)
  • Add —CHOH:D,L- ribulose, xylulose(C5)
  • Add —CHOH:D,L-sorbose, tagatose, fructose, psicose (C6)

Biochemistry: Carbohydrates

haworth projections
Haworth projections
  • …provide a way of keeping track the chiral centers in a cyclic sugar, as the Fischer projections enable for straight-chain sugars

Sir Walter Haworth

Biochemistry: Carbohydrates

the anomeric carbon

O

The anomeric carbon

C

O

  • In any cyclic sugar (monosaccharide, or single unit of an oligosaccharide, or polysaccharide) there is one carbon that has covalent bonds to two different oxygen atoms
  • We describe this carbon as the anomeric carbon

Biochemistry: Carbohydrates

iclicker quiz question 1
iClicker quiz, question 1
  • Which of these is a furanose sugar?

Biochemistry: Carbohydrates

iclicker quiz question 2
iClicker quiz, question 2
  • Which carbon is the anomeric carbon in this sugar?
  • (a) 1
  • (b) 2
  • (c) 5
  • (d) 6
  • (e) none of these.

Biochemistry: Carbohydrates

iclicker question 3
iClicker, question 3
  • How many 7-carbon D-ketoses are there?
  • (a) none.
  • (b) 4
  • (c) 8
  • (d) 16
  • (e) 32

Biochemistry: Carbohydrates

a d glucopyranose
a-D-glucopyranose
  • One of 2 possible pyranose forms of D-glucose
  • There are two because the anomeric carbon itself becomes chiral when we cyclize

Biochemistry: Carbohydrates

b d glucopyranose
b-D-glucopyranose
  • Differs from a-D-gluco-pyranose only at anomeric carbon

Biochemistry: Carbohydrates

count carefully
Count carefully!
  • It’s tempting to think that hexoses are pyranoses and pentoses are furanoses;
  • But that’s not always true
  • The ring always contains an oxygen, so even a pentose can form a pyranose
  • In solution: pyranose, furanose, open-chain forms are all present
  • Percentages depend on the sugar

Biochemistry: Carbohydrates

substituted monosaccharides
Substituted monosaccharides
  • Substitutions on the various positions retain some sugar-like character
  • Some substituted monosaccharides are building blocks of polysaccharides
  • Amination, acetylamination, carboxylation common

O

O-

OH

HO

O

HO

O

HO

HO

OH

OH

D-glucuronic acid(GlcUA)

GlcNAc

HNCOCH3

HO

Biochemistry: Carbohydrates

sugar acids fig 7 10

6

5

Sugar acids (fig. 7.10)

4

1

D--gluconolactone

2

3

  • Gluconic acid:
    • glucose carboxylated @ 1 position
    • In equilibrium with lactone form
  • Glucuronic acid:glucose carboxylated @ 6 position
  • Glucaric acid:glucose carboxylated @ 1 and 6 positions
  • Iduronic acid: idose carboxylated @ 6

Biochemistry: Carbohydrates

sugar alcohols fig 7 11
Sugar alcohols (fig.7.11)
  • Mild reduction of sugars convert aldehyde moiety to alcohol
  • Generates an additional asymmetric center in ketoses
  • These remain in open-chain forms
  • Smallest: glycerol
  • Sorbitol, myo-inositol, ribitol are important

Biochemistry: Carbohydrates

sugar esters fig 7 13
Sugar esters (fig. 7.13)

Glucose 6-phosphate

  • Phosphate esters of sugars are significant metabolic intermediates
  • 5’ position on ribose is phosphorylated in nucleotides

Biochemistry: Carbohydrates

amino sugars

OH

Amino sugars

HO

O

HO

OH

GlcNAc

HNCOCH3

  • Hydroxyl at 2- position of hexoses is replaced with an amine group
  • Amine is often acetylated (CH3C=O)
  • These aminated sugars are found in many polysaccharides and glycoproteins

Biochemistry: Carbohydrates

acetals and ketals
Acetals and ketals
  • Hemiacetals and hemiketals are compounds that have an –OH and an –OR group on the same carbon
  • Cyclic monosaccharides are hemiacetals & hemiketals
  • Acetals and ketals have two —OR groups on a single carbon
  • Acetals and ketals are found in glycosidic bonds

Biochemistry: Carbohydrates

oligosaccharides and other glycosides
Oligosaccharides and other glycosides
  • A glycoside is any compound in which the hydroxyl group of the anomeric carbon is replaced via condensation with an alcohol, an amine, or a thiol
  • All oligosaccharides are glycosides, but so are a lot of monomeric sugar derivatives, like nucleosides

Biochemistry: Carbohydrates

sucrose a glycoside
Sucrose: a glycoside
  • A disaccharide
  • Linkage is between anomeric carbons of contributing monosaccharides, which are glucose and fructose

Biochemistry: Carbohydrates

other disaccharides
Other disaccharides
  • Maltose
    • glc-glc with -glycosidic bond from left-hand glc
    • Produced in brewing, malted milk, etc.
  • Cellobiose
    • -glc-glc
    • Breakdown product from cellulose
  • Lactose: -gal-glc
    • Milk sugar
    • Lactose intolerance caused by absence of enzyme capable of hydrolyzing this glycoside

Biochemistry: Carbohydrates

reducing sugars
Reducing sugars
  • Sugars that can undergo ring-opening to form the open-chain aldehyde compounds that can be oxidized to carboxylic acids
  • We describe those as reducing sugars because they can reduce metal ions or amino acids in the presence of base
  • Benedict’s test:2Cu2+ + RCH=O + 5OH-Cu2O + RCOO- + 3H2O
  • Cuprous oxide is red and insoluble

Biochemistry: Carbohydrates

ketoses are reducing sugars
Ketoses are reducing sugars
  • In presence of base a ketose can spontaneously rearrange to an aldose via an enediol intermediate, and then the aldose can be oxidized.

Biochemistry: Carbohydrates

sucrose not a reducing sugar
Sucrose: not a reducing sugar
  • Both anomeric carbons are involved in the glycosidic bond, so they can’t rearrange or open up, so it can’t be oxidized
  • Bottom line: only sugars in which the anomeric carbon is free are reducing sugars

Biochemistry: Carbohydrates

reducing nonreducing ends
Reducing & nonreducing ends
  • Typically, oligo and polysaccharides have a reducing end and a nonreducing end
  • Non-reducing end is the sugar moiety whose anomeric carbon is involved in the glycosidic bond
  • Reducing end is sugar whose anomeric carbon is free to open up and oxidize
  • Enzymatic lengthening and degradation of polysaccharides occurs at nonreducing end or ends

Biochemistry: Carbohydrates

nucleosides
Nucleosides
  • Anomeric carbon of ribose (or deoxyribose) is linked to nitrogen of RNA (or DNA) base (A,C,G,T,U)
  • Generally ribose is in furanose form
  • This is an example of an N-glycoside

Diagram courtesy of World of Molecules

Biochemistry: Carbohydrates

polysaccharides
Polysaccharides
  • Homoglycans: all building blocks same
  • Heteroglycans: more than one kind of building block
  • No equivalent of genetic code for carbohydrates, so long ones will be heterogeneous in length and branching, and maybe even in monomer identity

Biochemistry: Carbohydrates

categories of polysaccharides
Categories of polysaccharides
  • Storage homoglycans (all Glc)
    • Starch: amylose ((14)Glc) , amylopectin
    • Glycogen
  • Structural homoglycans
    • Cellulose ((14)Glc)
    • Chitin ((14)GlcNac)
  • Heteroglycans
    • Glycosaminoglycans (disacch.units)
    • Hyaluronic acid (GlcUA,GlcNAc)((1  3,4))

Biochemistry: Carbohydrates

storage polysaccharides
Storage polysaccharides
  • Available sources of glucose for energy and carbon
  • Long-chain polymers of glucose
    • Starch (amylose and amylopectin):in plants, it’s stored in 3-100 µm granules
    • Glycogen
    • Branches found in all but amylose

Biochemistry: Carbohydrates

amylose
Amylose
  • Unbranched, a-14 linkages
  • Typically 100-1000 residues
  • Not soluble but can form hydrated micelles and may be helical
  • Amylases hydrolyze a-14 linkages

Diagram courtesyLangara College

Biochemistry: Carbohydrates

amylopectin
Amylopectin
  • Mostly a-14 linkages; 4% a-16
  • Each sidechain has 15-25 glucose moieties
  • a-16 linkages broken down by debranching enzymes
  • 300-6000 total glucose units per amylopectin molecule
  • One reducing end, many nonreducing ends

Biochemistry: Carbohydrates

glycogen
Glycogen
  • Principal storage form of glucose in human liver; some in muscle
  • Branched (a-14 + a few a-16)
  • More branches (~10%)
  • Larger than starch: 50000 glucose
  • One reducing end, many nonreducing ends
  • Broken down to G-1-P units
  • Built up fromG-6-P  G-1-P  UDP-Glucose units

Biochemistry: Carbohydrates

glycogen structure
Glycogen structure

Biochemistry: Carbohydrates