Bhavna Biol 304 Chpt 11 - Carbohydrates & Glycoproteins

Define a carbohydrate/What are carbohydrates primarily composed of?

carbon based molecules with lots of OH groups (can have additional groups/modifications)

What is the empirical formula for carbohydrates?

(CH2O)n

What is another name for carbohydrates?

Polyhydroxy Aldehydes & Ketones (and their derivatives)

Define monosaccharides

simplest carbohydrates that are aldehydes or ketones & contain 2 or > hydroxyl groups.

How many carbons can monosaccharides contain?

3-7 carbons in length where n = 3-7 in this empirical formula: (CH2O)n

What's another name for monosachharides?

simple sugars

What is the nomenclature of a monosaccharide based on?

length of the carbon chain & identity of the most oxidized group

Ketone vs Aldehyde

Give the nomenclature for 3C, 4C, 5C, 6C, 7C, keto group & aldehyde group

3C - Triose

4C - Tetrose

5C - Pentose

6C - Hexose

7C - Heptose

Keto Group - Ketose (most oxidized group is ketone)

Aldehyde Group - Aldose (most oxidized group is aldehyde)

Define molecular formula & structural formula

Molecular Formula - # and type of atoms

Structural Formula - arrangement of atoms

Define an isomer

molecules with the same molecular formula but different structural formula

What are the two main types of isomers in carbohydrates?

Constitutional isomers and Stereoisomers

Define constitutional isomers/structural isomers

molecules with identical molecular formulas but differ in how the atoms are ordered/diff skeletons or diff functional groups

Define stereoisomers

molecules with same molecular formula & same bonding order but the atoms differ in spatial arrangement in space

Stereoisomers have which 2 configurations?

D or L, which is determined by the config of the asymmetric C atom farthest from the aldehyde or ketone group

Most vertebrae monosaccharides are in which config?

D config

Stereoisomers can be _______ or _______

Enantiomers - mirror images of e/o

Diastereoisomers - NOT mirror images of e/o, differ a chiral centers

Monosaccharides that have >3 C atoms and multiple asymmetric C's can exist as either or.

What's the formula for the # of possible stereoisomers?

2^n where n = # of asymmetric C's

Ex: For a 6C aldose with 4 asymmetric C atoms, there are 2^4 = 16 possible stereoisomers

Is an epimer a monosaccharide?

Yes

Define an epimer

Sugars that are diastereoisomers differing in configuration at a single asymmetric center

Ex: D-Glucose & D-Mannose are epimeric at C2. D-Glucose & D-Galactose are epimeric at C4.

Ketoses have 1 less _______ than aldoses with the same # of C atoms

asymmetric C

What is the predominant form of monosaccharides?

ring formation where the open chain forms of these sugars cyclize into rings

Define a hemiacetal

aldehyde + alcohol = hemiacetal

Explain the formation of intramolecular hemiacetal in open-chain form of glucose

ring that forms when C5 OH group attacks O atom of C1 aldehyde group in the open-chain form of glucose

What 2 anomeric forms can result from pyranose formation?

𝛼 & β are both in rapid equilibrium with the open-chain/linear form

This resulting cyclic hemiacetal is called ?

pyranose due to its similarity to pyran (6 membered ring)

Define a hemiketal

ketone + alcohol = hemiketal

Explain the formation of an intramolecular hemiketal

C2 ketone group in open-chain form of a ketohexose such as fructose reacts with either its own C6 OH group (forming a 6 membered cyclic hemiketal) or C5 OH group (forming a 5 membered cyclic hemiketal)

This 5 membered ring formed when C-5 OH group attacks the C2-ketone group in the open chain form of fructose is called ?

furanose due to its similarity to furan (5 membered ring)

What 2 anomeric forms can result from furanose formation?

𝛼 & β but only 𝛼 is shown

Define an anomer

diastereoisomeric form of sugars that forms when a cyclic hemiacetal is created, resulting in an additional asymmetric center

In glucose, which C atom is the anomeric C atom?

C1

What are the two ring structures formed by glucose?

C1 becomes an asymmetric center resulting in 2 ring structures: α-D-glucopyranose and β-D-glucopyranose

Define α-D-glucopyranose

OH group attached to C1 is on opposite side of ring as C6

Define β-D-glucopyranose

OH group attached to C1 is on same side of ring as C6

In D-Fructose (furanose ring form), which C is the anomeric/asymmetric C?

C2

Fructose forms which 2 types of rings?

1. Pyranose Ring - 6 membered ring

2. Furanose Ring - 5 membered ring

Fructose forms how many rings?

1. 2 furanoses (α & β forms)

2. 2 pyranoses (α & β forms)

*α & β - refer to the OH groups attached to C2, the anomeric C. Both in rapid equilibrium with the open chain form.

The pyranose form predominates in ?

fructose that is free in solution due to reduced steric hindrances

The furanose form predominates in ?

fructose derivatives

Summary of Isomeric Forms of Carbs

Pyranose rings are not planar due to

tetrahedral geometry of its saturated C atom

Instead, pyranose rings can adopt 2 types of conformations

1. Boat

2. Chair

Describe the chair form

substituents on C ring atoms have 2 orientations: axial or equatorial

Define axial vs equatorial

Axial- nearly perpendicular to avg plane of the ring

Equatorial - nearly parallel to the plane

Axial substituents can ________ e/o if on same side of ring. In contrast, equatorial substituents are less ____.

sterically hinder, crowded

The chair form is more stable bc

H atoms occupy the axial positions, resulting in less steric hindrance unless in the boat form. The bulkier groups like OH & CH2OH emerges at less hindered perihery.

Why is the boat form of glucose disfavored?

due to steric hindrances

What's the importance of D-Glucose

important fuel for most organisms

Define blood sugar

D-glucose that circulates in blood

What is the significance of D-glucose in the body?

only fuel used by brain in non-starvation conditions + by RBC

What is glycation?

The nonenzymatic addition of a carbohydrate to another molecule.

What are some reasons D-glucose is an important fuel?

1. Glucose is formed from formaldehyde under prebiotic conditions and may have been available as a fuel source for primitive biochemical systems

2. Glucose is relatively inert/unstable

3. Most stable ring structure is β-D-glucopyranose

Define reducing vs non reducing sugars

Reducing Sugars - sugars that react or open up into a form that contains a reactive aldehyde group that can react with a free amino group (NH2) on a protein

Nonreducing Sugars - sugars that do NOT react

Reducing sugars often nonspecifically react with

free AA groups (amino groups) on Lys/Arg proteins to form a (temp cov bond first then) stable covalent bond

Lys - amino group in side chain

Arg - reactive N group

Define Glycation/Glycosylation

nonenzymatic addition of a carb (such as a sugar molecule) to another molecule such as protein or lipid

Does D-glucose have a high or low tendency to glycate proteins?

Low tendency to glycate proteins since it usually stays in its cyclic (ring) form not its reactive open-form that would open up an aldehyde group

D-glucose' low tendency to glycate proteins can be overcome by

high concentrations of sugar and protein for long periods of time

Define advanced glycation end products (AGEs)

Once the initial glucose-protein bond is formed, it can undergo further complex rxns such as cross-linking. The end products after cross-linking are called AGEs.

AGEs cause _____ b/w proteins

cross-links between other proteins making them more rigid and less flexible

Why are AGEs often detrimental? What are they involved in?

They are often detrimental bc they alter normal biochemical function of the modified proteins. These modifications (AGEs) are involved in aging, arteriosclerosis, diabetes and other conditions

As a reducing sugar, D-glucose reacts with _____ to form _________ Hb

reacts with Hb/glycates it to form glycated or glycosylated Hb

What's another word for glycated Hb?

Hb A1C - measures how much Hb has been glycated

Does this have an effect on O2 binding

No

Monitoring changes in amount of ______ is a useful means of assessing effectiveness of treatments for diabetes mellitus

glycosylated Hb. Since glycosylated Hb remains in circulation, the amount of modified Hb corresponds to long-term regulation (over 6 months of glucose levels)

In nondiabetic individuals, less than ___% of Hb is glycosylated

6%

In patients with uncontrolled diabetes, almost __% of Hb is glycated

10%

What is the lifespan of red blood cells, affecting A1C levels?

120 days

The biochemical properties of a monosaccharide can be modified by reactions with 3 types of molecules

1. Alcohols

2. Amines (NH3)

3. Phosphates

These modifications of monosaccharides increase _______ ________

biochemical versatility, enabling them to now serve as signaling molecules or facilitate metabolism

Define a glycosidic linkage/bond

covalent linkage b/w anomeric/asymmetric C of a carb + O atom of an alcohol or N atom of an amine

Glycosidic bonds are prominent when

carbs are linked together to form long polymers & when a carb is attached to proteins

Since sugars contain OH grous, _______ bonds can join 1 monosaccharide to another

glycosidic

Define an O-glycosidic linkage

covalent bond formed b/w anomeric C atom of a carb such as glucose & O atom of an alcohol/OH group

Ex: O-glycosidic bond linking glucose to a methyl group

Define an N-glycosidic linkage?

covalent linkage b/w the anomeric/asymmetric C atom of a carb + N atom of an amine

Ex: Nitrogenous bases attached to Ribose units to form nucleosides via N-glycosidic linkages

Carbs can also be modified by attachment of _______ groups to Cs other than OH groups at the anomeric/asymmetric C

functional

Define phosphorylation

common modification of sugars in metabolic reactions via attachment of phosphoryl groups

What's the charge on sugars after phosphorylation?

anionic = negative charge

What's the role of phosphorylated sugars?

1. prevents sugars from crossing the lipid-bilayer membranes spontaneously and interacting with transporters of the unmodified sugar

2. blocks the formation of alternative ring conformation

3. creates rxn intermediates that more readily undergo metabolism

The first step in breakdown of glucose to obtain energy is

glucose' conversion into glucose-6-phosphate. Other intermediates in this path include G-6PP, DHAP and GAP.

Define oligosaccharides

two or more monosaccharides/sugars linked by O-glycosidic bonds (

What defines the directionality of oligosaccharides?

Their reducing and nonreducing ends NOT reducing sugars

Define reducing vs nonreducing end

Reducing End - free anomeric C atom that can form the open-chain form

Nonreducing End - anomeric C in glyosidic linkage that cannot convert to the open-chain form

Define an α-1,4-glycosidic linkage

glycosidic linkage between the α-anomeric form of C-1 on one sugar and the O atom of OH group on C-4 of the adjacent sugar

What does α vs β refer to?

α - OH on the anomeric C pointing down

β - OH on anomeric C pointing up

What is maltose?

A disaccharide composed of glucose + glucose molecules linked by an α-1,4-glycosidic linkage

Glucose molecule on RT: can assume open-chain form which can act as a reducing agent/oxidize other molecules since it has a free anomeric C. It's still called reducing even when it's bound to another molecule such a a protein and may not longer have reducing properties.

Glucose molecule on LT: CANNOT assume open-chain form since the C1 is bound to another molecule

Maltose is a _____ sugar

reducing sugar since it has a free anomeric carbon on one of its two glucose units, which can open to form an aldehyde group

The fact that monosaccharides have multiple OH groups means

that many diff glycosidic linkages can be formed since these bonds join monosaccharides together via OH groups

What are the common disaccharides?

Sucrose, lactose, and maltose.

Define a disaccharide

2 sugars joined by O-glycosidic bond

Cleavage products of disaccharides can be processed to provide energy in the form of

ATP

Define sucrose & its components

disaccharide of sugar cane or sugar beets, consisting of glucose (6 membered ring) + fructose (5 membered ring)

What type of linkage connects fructose to glucose in sucrose?

anomeric carbon of glucose is linked to the anomeric carbon of fructose

What type of sugar is sucrose classified as?

Sucrose is not a reducing sugar

Why is sucrose not a reducing sugar?

neither sugar can access the open chain form since both anomeric C's are involved in the glycosidic linkage

Doese sucrose have anomeric ends?

No

What's the configuration of glucose and fructose in sucrose?

α for glucose and β for fructose

How is sucrose hydrolyzed?

Sucrose can be cleaved by the enzyme sucrase (invertase).

Define lactose and its components

a disaccharide of milk that consists of galactose + glucose

What type of linkage connects galactose to glucose in lactose?

by a β-1,4-glycosidic linkage - 1st sugar has a β-anomeric carbon at C1 (β - OH pointing up) that bonds to the C4-OH group on the 2nd sugar