Chapter 7: Carbohydrates and Glycobiology

Question 1

glycoconjugates

Answer 1

• Complex carbohydrate polymers covalently attached to proteins or lipids
• act as signals that determine the intracellular destination or metabolic fate of these hybrid molecules

Question 2

Carbohydrates

Answer 2

• polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis
• Many, but not all, carbohydrates have the empirical formula (CH2O)_n
• some contain nitrogen, phosphorus, or sulfur

Question 3

Monosaccharides

Answer 3

• simple sugars
• single polyhydroxy aldehyde or ketone unit
• most monosaccharide is the six-carbon sugar D-glucose (dextrose)
• those with four or more carbons tend to have cyclic structures.

Question 4

Oligosaccharides

Answer 4

• short chains of monosaccharide units
• joined by glycosidic bonds
• most abundant are the disaccharides
  
  • two monosaccharide units
• Typical is sucrose (cane sugar), which consists of the six-carbon sugars D-glucose and D-fructose
• suffix “-ose.”
• In cells
  
  • consist of three or more units
  • don’t occur as free entities
  • joined to nonsugar molecules (lipids or proteins) in glycoconjugates

Question 5

polysaccharides

Answer 5

• containing more than 20 or so monosaccharide units
• Some are linear chains (cellulose); others are branched (glycogen)
  
  • both of these consist of recurring units of D-glucose
  • they differ in the type of glycosidic linkage

Question 6

• Many of the carbon atoms to which hydroxyl groups are attached are ______ _____, which give rise to the many sugar stereoisomers found in nature.
• Stereoisomerism is significant because the enzymes that act on sugars are ______ , typically preferring one stereoisomer to another, as reflected in ______ values or binding constants

Answer 6

• chiral centers
• stereospecific, K_m

Question 7

• One chemical reactions of the carbonyl groups of monosaccharides is the addition of a ______ group from within the same molecule, generates _____ forms having four or more backbone carbons (the forms that predominate in aqueous solution).
• This ring closure creates a new _____ _____

Answer 7

• hydroxyl, cyclic
• chiral center

Question 8

Monosaccharides

structure

Answer 8

• colorless, crystalline solids
• freely soluble in water but insoluble in nonpolar solvents
• Most have a sweet taste
• backbones of common monosaccharides
  
  • unbranched carbon chains
  • carbon atoms are linked by single bonds
  • Aldose: If carbonyl group is in an aldehyde
  • Ketose: If the carbonyl group is in a ketone
• Monosaccharides with four, five, six, and seven carbon atoms in their backbones are called, respectively, tetroses, pentoses, hexoses, and heptoses

Question 9

a molecule with n chiral centers can
have ______ stereoisomers

Answer 9

2ⁿ

Question 10

How to assign D (dextro) or L (levo)

Answer 10

• in a projection formula, when the carbonyl carbon is at the top
• look at the placement of the chiral carbon most distant from the carbonyl carbon
  
  • D isomer: hydroxyl group is on the right (dextro)
  • L isomer: hydroxyl group is on the left (levo)

Question 11

Most of the hexoses of living organisms are ____ isomers

Answer 11

D

Question 12

epimers

Answer 12

• Two sugars that differ only in the configuration around one carbon atom
• Example: D-Glucose and two of its epimers are shown as projection formulas. Each epimer differs from D-glucose in the configuration at one chiral center (shaded light red or blue).

Question 13

hemiacetals or hemiketals

Answer 13

• cyclic (ring) structures created when carbonyl group forms a covalent bond with the oxygen of a hydroxyl group along the chain
• Two molecules of an alcohol add to a carbonyl carbon
• hemiacetal: Addition to an aldose
• hemiketal: addition to a ketose
• full acetal or ketal
  
  • Addition of the second molecule of alcohol
  • bond formed is a glycosidic linkage
  • disaccharide: if both molecules that react are monosaccharides
• When the second alcohol is part of another sugar molecule, the bond produced is a glycosidic bond
• Cuz OH can attack the front or back of the carbonyl carbon, it can produce two stereoisomeric configurations, α or β

Question 14

Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom are called _____, and the carbonyl carbon atom is called the ______ ______

Answer 14

• anomers
• anomeric carbon

Question 15

pyranoses

Answer 15

Six-membered ring compounds

Question 16

Converting Fischer projection to a Haworth perspective

Answer 16

• draw the six-membered ring
• five carbons and one oxygen, at the upper right
• number the carbons clockwise, beginning with the anomeric carbon
• place the hydroxyl groups
  
  • right on Fischer projection = OH facing down
  • left on Fischer projection = OH facing up
• terminal —CH₂OH group points up for D and down for L
• OH on anomeric carbon can point up or down.
  
  • β: it’s on the same side as C-6 (top/top or bottom/bottom)
  • α: it’s on the opposite side from C-6 (top/bottom)
• *

Question 17

mutarotation

Answer 17

• α and β anomers of D-glucose interconvert in aqueous solution
• one ring form (α anomer) opens briefly into the linear form, then closes again to produce the β anomer

Question 18

• two ______ of a molecule are interconvertible without the breakage of covalent bonds, whereas two configurations can be interconverted only by breaking a covalent bond
• To interconvert α and β ______, the bond involving the ring oxygen atom would have to be broken

Answer 18

• conformations
• configurations

Question 19

• there are a number of sugar derivatives in which a ______ group in the parent compound is replaced with another substituent, or a carbon atom is oxidized to a ______ group
• In amino sugars, an ______ group replaces one of the —OH groups in the parent hexose. Substitution of —H for —OH produces a ______ ______

Answer 19

• hydroxyl, carboxyl
• —NH2
• deoxy sugar

Question 20

• During synthesis & metabolism of carbohydrates, intermediates are _____ _____. Sugar phosphates are relatively stable at neutral pH and bear a negative charge
• One effect of sugar phosphorylation within cells is to trap
• Phosphorylation also activates sugars for

Answer 20

• phosphorylated derivatives
• the sugar inside the cell; most cells do not have plasma membrane transporters for phosphorylated sugars.
• subsequent chemical transformation.

Question 21

O-glycosidic bond

Answer 21

• formed when a hydroxyl group of one sugar molecule, typically cyclic, reacts with the anomeric carbon of the other
• represents the formation of an acetal from a hemiacetal and an OH
• resulting compound is called a glycoside
• Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base
  
  • Thus disaccharides can be hydrolyzed to yield their free monosaccharide components by boiling with dilute acid

Question 22

N-glycosyl bonds

Answer 22

join the anomeric carbon of a sugar to a nitrogen atom in glycoproteins and nucleotides

Question 23

• oxidation of a sugar by _____ _____ (the reaction that defines a reducing sugar) occurs only with the ______ form, which exists in equilibrium with the ______ form(s).
• the ______ ______ can be oxidized only when the sugar is in its linear form, formation of a glycosidic bond renders a sugar _____.
• In disaccharides or polysaccharides, the end of a chain with a free anomeric carbon (one not involved in a glycosidic bond) is commonly called the ______ _____
• wavy lines, sometimes u shaped are used to indicate that the structure may be

Answer 23

• cupric ion, linear, cyclic
• carbonyl carbon, nonreducing
• reducing end
• either α or β

Question 24

reducing disaccharides

naming convention

Answer 24

• name has nonreducing end to the left
• Choose α or β, for the anomeric carbon joining the first monosaccharide unit
• Name the nonreducing residue (Glucose, Fructose, etc.)
• Use “furano” or “pyrano” for five & six-membered ring structures
• In parens show the two C joined by the glycosidic bond connected by an arrow:
  
  • (1→4) = C-1 of the first residue is joined to C-4 of the second
• Name the second residue
  
  • If there’s a third, use the same conventions for the second glycosidic bond
• Use 3-letter abbreviation of monosaccharides for complex polysaccharides
• Since most sugars in this book are D pyranose forms, a shortened version used is: Glc(α1→4)Glc

Question 25

non-reducing disaccharides

naming convention

Answer 25

• sucrose contains no free anomeric carbon atom
• the anomeric carbons of both monosaccharide units are involved in the glycosidic bond
• a nonreducing sugar is formed
• it’s stable toward oxidation
• a double-headed arrow connects the symbols specifying anomeric carbons and their configurations
  
  • Glc(α1⇔2β)Fru or Fru(β2⇔1α)Glc

Question 26

Homopolysaccharides

Answer 26

• contain only a single monomeric species
• Some serve as storage forms of monosaccharides that are used as fuels; starch and glycogen
• others (cellulose and chitin) serve as structural elements

Question 27

heteropolysaccharides

Answer 27

• contain two or more different kinds
• provide extracellular support for organisms (peptidoglycan), intracellular matrix (holds cells together and provides protection, shape, and support to cells, tissues, and organs)

Question 28

polysaccharides generally do not have defining molecular weights. Explain.

Answer 28

• proteins are synthesized on a template (messenger RNA) of defined sequence and length, by enzymes that follow the template exactly
• no template or polysaccharide synthesis t
• program for polysaccharide synthesis is intrinsic to the enzymes that catalyze the polymerization of the monomeric units
• no specific stopping point in the synthetic process; the products thus vary in length

Question 29

In addition to their important roles as stored fuels (starch, glycogen, dextran) and as structural materials (cellulose, chitin, peptidoglycans), polysaccharides and oligosaccharides are information carriers. Explain.

Answer 29

• Some provide communication between cells and their extracellular surroundings
• others label proteins for transport to and localization in specific organelles, or for destruction when the protein is malformed or superfluous
• others serve as recognition sites for extracellular signal molecules
• specific oligosaccharide chains attached to plasma membrane form a carbohydrate layer that serves as an information-rich surface shown to it surrounding
• players in cell-cell recognition and adhesion, cell migration during development, blood clotting, the immune response, wound healing, and other cellular processes

Question 30

glycoconjugate

Answer 30

carbohydrate is covalently joined to a protein or a lipid