Quiz 5

What is the simplest carbohydrate?

Monosaccharides

Monosaccharides

Aldehydes or ketones that contain 2 or more alcohol groups

The smallest monosaccharides are composed of how many carbons?

3 carbons

What is the general chemical formula of monosaccharides?

(CH2O)n

What is the structural difference between an aldehyde and a ketone?

Both have an O double bonded to carbon.

Aldehyde: has R group and H bonded to carbon

Ketone: had R group and R’ group bonded to carbon

Ketose

Monosaccharides that contain a keto group

Aldose

Monosaccharides that contain an aldehyde group

Is dihydroxyacetone a ketose or an aldose?

Ketose

Is glyceraldehyde a ketose or an aldose?

Aldose

Which side is the OH group on in the fischer projections of glyceraldehyde?

D-Glyceraldehyde: OH group on left side

L-Glyceraldehyde: OH group on right

What are the names of simple monosaccharides that contain 3, 4, 5, 6, or 7 carbons?

3C: trioses

4C: tetroses

5C: pentoses

6C: hexoses

7C: heptoses

In fischer projections, what do the horizontal and vertical bonds represent?

Horizontal: in front of the plane

Vertical: behind the plane

Isomers

Same molecular formula but different structures

What are the 2 types of isomers?

1. Constitutional Isomers
2. Stereoisomers

Constitutional Isomers

Differ in the order of attachment of atoms

Stereoisomers

Atoms are connected in the same order but differ in spatial arrangement

What are the 2 types of stereoisomers?

1. Enantiomers
2. Diastereoisomers

Enantiomers

Nonsuperimposable mirror images

Diastereoisomers

Isomers that are not mirror images

What are the 2 types of diastereoisomers?

1. Epimers
2. Anomers

Epimers

Differ at one of several asymmetric carbon atoms

Anomers

Isomers that differ at a new asymmetric carbon atom formed on ring closure

Most sugars found in biological systems are in what conformation?

D conformation

List 6 common monosaccharides, are they pentose or hexose sugars?

1. D-Ribose (pentose)
2. D-Deoxyribose (pentose)
3. D-Glucose (hexose; aldohexose)
4. D-Mannose (hexose; aldohexose)
5. D-Galactose (hexose; aldohexose)
6. D-Fructose (hexose; ketohexose)

Which 3 common monosaccharides are isomers of D-Glucose?

1. D-Mannose
2. D-Galactose
3. D-Fructose

How do sugars form ring structures?

An aldehyde or ketone group will react with an alcohol

Hemiacetal

When an aldehyde reacts with an alcohol

Hemiketal

When a ketone reacts with an alcohol

What is the resulting hemiacetal of glucose?

6C, called pyranose because of resemblance to pyran

What is the resulting hemiketal of ketohexose fructose?

5C, called furanose because of resemblance to furan

What are the 2 types of conformations of anomers?

1. Alpha
2. Beta

Where is the hydroxyl at C-1/C-2 (depending on the ring) for the alpha and beta conformation?

Alpha: Hydroxyl below plane of the ring

Beta: Hydroxyl above the plane of the ring

Fructose can form both the pyranose and furanose form, where are they predominantly found?

Pyranose form: when fructose is free in solution

Furanose form: commonly seen in fructose derivatives (more biologically relevant)

When glucose is in solution, how much of each configuration (alpha, beta, open-chain) will be found? Why?

Beta: \~2/3

Alpha: \~1/3

Open-chain:

How can you change the configuration from alpha to beta or vice versa?

Transition happens through the linear form, can’t just be flipped in the ring

What are the 2 types of conformations that pyranose rings can adopt?

1. Boat conformation
2. Chair conformation

What are the 2 orientations of substituents on the carbon ring?

Axial (vertical) and equatorial (slanted)

Beta-D_glycopyranose will adopt which conformation?

Chair conformation because axial positions are occupied by hydrogen atoms which reduces steric hindrance

Reducing Sugar

Sugars that react with oxidizing agents

Is glucose a reducing or nonreducing sugar?

Reducing sugar, open-chain form will react with oxidizing agents

HbA1c

Hemoglobin A1c: when glucose reacts with hemoglobin

* Determining the amount of HbA1c in the blood allows for long-term monitoring of blood glucose levels in diabetics
* A high HbA1c is bad

What usually happens when a carbohydrate reacts with a protein?

Form advanced glycation end products:

* often impairs protein function
* have been implicated in a number of pathological conditions

What type of bonds join monosaccharides to alcohols and amines?

Glycosidic bonds

What are the 2 types of glycosidic bonds? What are the names of the end products?

1. O-glycosidic bond
2. N-glycosidic bond

O-Glycosidic Bond

Bond between the anomeric carbon atom of glucose and a hydroxyl group of another molecule; product is called a glycoside

N-Glycosidic Bond

Bond between the anomeric carbon atom of glucose and an amine

What type of bond joins carbohydrates and phosphates?

Ester bonds/linkages

Glucosinolates

A special class of glycosides that protect some plants from herbivory

* When hydrolyzed, isothiocyanate is produced, generates a sharp taste that discourages further eating
* Certain glucosinolates may help prevent cancer in humans; induce rare enzymes in cells that are related to detoxification

Oligosaccharides

2 or more monosaccharides linked by O-glycosidic bonds

What are 3 common disaccharides? What monosaccharides make them? What enzymes cleave them?

1. Sucrose (glucose-fructose); sucrase
2. Lactose (galactose-glucose); lactase
3. Maltose (glucose-glucose); maltase

* Enzymes are found on the epithelium of the digestive system

What is the only sugar that can be used my humans?

Glucose; monosaccharides and disaccharides have to be broken down and go through chemical processes to be converted to glucose so they can be used

Glycosyltransfereases

A large class of enzymes that catalyze the formation of glycosidic bonds

What activates the substrates for glycosyltransferases?

Monosaccharide substrates are activated by attachment of uridine diphosphate (UDP)

Polysaccharides

Large polymeric oligosaccharides

Homopolymer

A polysaccharide in which all the monosaccharides are the same

Glycogen

The homopolymer that is the storage form of glucose in animal cells

How are the glucose monosaccharides attached in glycogen?

* Units in glycogen are linked by alpha-1,4-glycosidic bonds
* Branches formed by alpha-1,6-glycosidic bonds every 12 glucose units

How is glucose stored in plants?

Stored as starch

What are the 2 types of starch?

1. Amylose
2. Amylopectin

Amylose

Linear polymer, linked by alpha-1,4-glycosidic bonds

Amylopectin

Branched polymer, linked by alpha-1,6-glycosidic bond for every 30 alpha-1,4-glycosidic bond

What carbohydrate is used as a structural component of plants?

Cellulose

Cellulose

Homopolymer of glucose units linked by Beta-1,4-glycosidic bonds

How does type of linkage (alpha or beta) effect the overall structure of the polymers?

Alpha (starch and glycogen): Form compact hollow cylinders suitable for accessible storage

Beta (cellulose): a straight chain capable of interacting with other cellulose molecules to form strong fibrils

Can mammals digest cellulose and other plant fibers?

No they can not digest it but soluble fibers (ex. polygalacturonic acid) aid in digestion

What is the most abundant carbohydrate in the world?

Cellulose

What is the 2nd most abundant carbohydrate in the world?

Chitin

Chitin

A glycosaminoglycan, found in the exoskeleton of insects and cell walls of fungi

Glycoproteins

Proteins with carbohydrates attatched

What percent of the proteome are glycoproteins?

50%

What are the 3 main classes of glycoproteins?

1. Glycoproteins (mostly protein by mass)
2. Prroteoglycans (mostly carbohydrate by mass)
3. Mucins/Mucoproteins (mostly carbohydrate by mass)

What are the 2 ways carbohydrates can be linked to proteins?

1. N-linkage: carbohydrate attached to nitrogen atom in side chain of asparagine
2. O-linkage: carbohydrate attached to oxygen atom in side chain of serine or threonine

What 2 components do N-linked oligosaccharides contain?

1. A common pentasaccharide core consisting of 3 mannoses
2. 2 N-acetylgalactosamine units

* Y-shaped

What are the 2 types of N-linked oligosaccharides? Describe them.

1. Mannose type: mannose is the only sugar present
2. Complex type: other monosaccharides are attached to mannose (the core)

What is the role of the subclass of glycoproteins?

Variety of roles; membrane proteins and involved in cell adhesion

Describe erythropoietin (a glycoprotein)

* Secreted by the kidney, stimulates red blood cell production
* N-glycosylated at 3 asp residues and O-glycosylates at 1 serine residue


* Glycosylation of erythropoietin enhances the stability of the protein in the blood
* Used as a medication to treat anemia, especially in cancer patients

GlcNAc

* ClcNAc transferase attached N-acetylglucosamine (GlcNAc) to serine or threonine residues of protein
* Concentration of GlcNAc reflects active metabolism of carbohydrates, amino acids, and fats, indicating nutrients are abundant
* More than 1000 proteins, including transcription factors and components of signaling pathways, are modified this way
* GlcNAcylation is reversible, GlcNAcase catalyzes the removal of the carbohydrate
* Dysregulation of GlcNAc transferase has been linked to insulin resistance, diabetes, cancer, and neurological pathologies

What is the role of the subclass proteoglycans?

Structural roles or acting as lubricant, often found in the extracellular matrix, attached to glycosaminoglycan (a type of polysaccharide)

Glycosaminoglycan (GAGs)

* Makes up \~95% of the mass of a proteoglycan
* Polysaccharide composing of repeating units of a disaccharide
* One glucosamine (amino sugar) and one glucuronic acid (uronic sugar): carries negative charge as carboxylate or sulfate
* Will attract and retain water
* Exception: Keratan has galactose instead of uronic sugar

List 5 important glycosaminoglycans

1. Chondroitin 6-sulfate
2. Keratan sulfate
3. Heparin
4. Dermatan sulfate
5. Hyaluronate

What is cartilage composed of?

* Part of it is composed of aggrecan (proteoglycan) and collagen
* Glycosaminoglycan component of aggrecan cushions joints by releasing water on impact and then rebinding water

Mucopolysaccharidoses

Pathological conditions that result from the inability to degrade proteoglycans

Hurler Disease

Mucopolysaccharidosis type I: glycosaminoglycans can’t be degraded and the excess is stored in the soft tissue of the facial regions

* Symptoms: wide nostrils, depressed nasal bridge, thick lips and earlobes, and irregular teeth

What is the role of the subclass mucins/mucoproteins?

Often lubricants

How are mucins/mucoproteins often attached to the carbohydrate?

N-acetylgalactosamine

Describe the structure of mucins/mucoproteins.

* Regio of protein backbone: rich in serines and threonines called variable number of tandem repeats (VNTR) and is the site of glycosylation
* Protein component: extensively O-glycosylated to serine and threonine residues beginning with an N-acetylgalactosamine sugar residue
* Often look like long chains

Describe how blood groups are based on protein glycosylation patterns.

Human ABO blood groups reflect the specificity of glycosyltransferases:

* All of the blood groups share oligosaccharide foundation called O
* A: N-acetylgalactosamine is added to O by specific glycosyltransferase
* B: galactose is added by another transferase
* O: produces no active glycosyltransferase

Lipids

Molecules that are not soluble in water but are soluble in organic solvents

What are the 5 classes of lipids?

1. Free fatty acids
2. Triacylglycerols
3. Phospholipids
4. Glycolipids
5. Steroids

Free Fatty Acids

Chains of hydrogen-bearing carbon atoms that have a carboxylic acid at one end and a methyl group at the other end; may be saturated or unsaturated with hydrogens

* A common fuel

How are fatty acid carbon atoms usually numbered?

Usually begins with carboxyl terminal carbon atom; carbon 2 and 3 are referred to as alpha and Beta

* Can also be numbered from the methyl carbon atom (omega carbon)

Can there be 2 double bonds next to each other in polyunsaturated fatty acids?

No, has to be separated by at least 1 methylene group

How are the positions of double bonds indicated?

With delta symbol with the first atom of the double bond indicated by a superscript number

* Ex. delta^9 indicates double bond between atoms 9 and 10

Are fatty acids usually referred to as their carboxylate form or as unionized acid?

Carboxylate form since fatty acids are ionized at physiological pH

Fatty acids in biological systems usually contain how many carbon atoms?

Usually contain an even number, between 16-18 carbons (optimal length for membrane)

When double bonds are present, which configuration do they commonly have?

Cis

The properties of fatty acids depend on what 2 things?

1. Chain length
2. Degree of unsaturation

What chain length and bond conformation (degree of unsaturation) results in increased fluidity of fatty acids?

Short chain length and cis double bonds (unsaturated); important for overall membrane fluidity

How does length effect melting temperature of a fatty acid?

Longer length increases melting temperature

Why are cis polyunsaturated fatty acids essential to human diets?

Because humans cannot synthesize them, they are precursors to a variety of hormones, and offers some protection from coronary heart disease