Biochem Exam #3

What are carbohydrates?

carbon-based molecules high in hydroxyl groups

What are carbohydrates better described as?

polyhydroxy aldehydes and ketones (and their derivatives)

What is a monosacharide?

aldehydes or ketones that contain two or more hydroxyl groups.

What is the other name for monosacherides?

Simple sugars

How long are monosaccharides typically?

3-7 carbons in length

What is nomenclature based on?

• carbon-chain length

• the identity of the most oxidized group

What are constitutional isomers?

molecules with identical molecular formulas that differ in how the atoms are ordered

What are stereoisomers?

molecules that differ in spatial arrangement but not bonding order

• can be enantiomers or diastereoisomers

What are the two types of configurations stereoisomers can be?

D or L

What are enantiomers?

mirror images of each other

What are diastereoisomers?

not mirror images of each other

How do you know the number of possible stereoisomers?

2n where n is the number of asymmetric carbon atoms

What are epimers?

sugars that are diastereoisomers differing in configuration only at a single asymmetric center

What is a hemiacetal?

The product formed when an aldehyde reacts with with an alcohol

What is a hemiketal?

The produced formed when a ketone reacts with an alcohol

What is an anomer?

a diastereoisomeric form of sugars that forms when a cyclic hemiacetal is formed and an additional asymmetric center is created

In glucose, C-1 (the anomeric carbon atom) becomes an asymmetric center, forming two ring structures:

α-D-glucopyranose and β-D-glucopyranose

Describe the β-D-glucopyranose ring structure of glucose.

The hydroxyl group attached to C-1 is on the same side of the ring as C-6

Describe the α-D-glucopyranose ring structure of glucose.

hydroxyl group attached to C-1 is on the opposite side of the ring as C-6

What is C-2 in D-Fructose?

the anomeric carbon atom.

Why does the pyranose form predominate in D-Fructose?

due to reduced steric hindrances.

When does the furanose form predominate?

in fructose derivatives.

Pyranose rings are not planar because …

because of the tetrahedral geometry of its saturated carbon atoms.

What are the two conformations Pyranose rings can adopt?

boat and chair.

Describe the chair form of pyranose rings.

substituents on the carbon ring atoms can be axial or equatorial

What is axial?

Nearly perpendicular

What is equatorial?

Nearly parallel

When do Axial substituents sterically hinder each other?

When on the same side of the ring

Why does the chair form of B-D-Glucose predominate?

because all axial positions are occupied by hydrogens.

Why is the boat form of B-D-Glucose disfavored?

because it is sterically hindered.

What is blood sugar?

D-glucose circulating in the blood

Why is blood sugar so important?

• only fuel used by the brain in non-starvation conditions

• only fuel used by red blood cells

What are potential reasons WHY D-glucose is an important fuel?

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

• glucose is relatively inert

• the most stable ring structure is β-D-glucopyranose

D-Glucose is a reducing sugar that reacts _______ with hemoglobin.

Nonenzymatically

When can glucose react with oxidizing agents?

In its linear form

What is Fehling's solution?

solutions of Cu2+ that test for the presence of sugars that adopt an open structure

What are reducing sugars?

sugars that react with oxidizing agents

• all monosaccharides that can adopt linear structures in solution

What are non-reducing sugars?

sugars that do not react with oxidizing agents

What is glycation?

nonenzymatic addition of a carbohydrate to another molecule

D-glucose has a low tendency to glycate proteins unless…

concentrations of sugar and protein are very high for long periods of time.

What is advanced glycation end products (AGEs)?

products resulting from cross-linking following the primary modification

When are advanced glycation end products (AGEs) found?

in aging, arteriosclerosis, diabetes, and other pathological conditions

Provide an example of glycation.

Reducing sugars nonspecifically react with free amino groups on proteins (often Lys or Arg) to form a stable covalent bond.

What is glycated hemoglobin (hemoglobin A1c, A1C)?

The end product of D-glucose reacting with hemoglobin

• Has no effect on oxygen binding

What percentage of hemoglobin is glycated in nondiabetic individuals? In patients with uncontrolled

diabetes?

<6% for nondiabetic

10% for diabetic (uncontrolled)

When is glycated hemoglobin eliminated?

When red blood cells die off (life span is about 120 days)

Monosaccharide biochemical properties can be modified by reactions with:

• alcohols

• Amines

• Phosphates

What do modifactions to monosacherides do?

• increase biochemical versatility

  • Can serve as signal molecules

  • Can facilitate metabolism

What is O-glycosidic linkage?

covalent linkage formed between the anomeric carbon atom of a carbohydrate and the oxygen atom of an alcohol

What is N-glycosidic linkage?

covalent linkage formed between the anomeric carbon atom of a carbohydrate and the nitrogen atom of an amine

What is Phosphorylation?

a common modification of sugars in metabolic reactions

What is the purpose of phosphorylation?

• makes sugars anionic to prevent crossing the lipid-bilayer membranes and interacting with transporters of the unmodified sugar

• blocks the formation of alternative ring conformation

• creates reaction intermediates that more readily undergo Metabolism

What are oligosaccharides?

sugars that contain two or more monosaccharides linked by O-glycosidic bonds

Describe the reducing end.

has a free anomeric carbon atom that can form the open-chain form

Describe the nonreducing end.

has an anomeric carbon in a glycosidic linkage that cannot covert to the open-chain form

What is the directionality of oligosaccharies defined by?

their reducing and nonreducing ends

What is the α-1,4-glycosidic linkage?

glycosidic linkage between the α-anomeric form of C-1 on one sugar and the hydroxyl oxygen atom on C-4 of the adjacent sugar

What is a disaccharide?

two sugars Joined by an O-glycosidic Linkage

Cleavage products of disaccharides can be processed to provide what?

energy in the form of ATP.

What is Sucrose?

disaccharide of sugar cane or sugar beets that consists of glucose linked to fructose

• this is not a reducing sugar

What can sucrose be cleaved by?

Sucrose (invertase)

the configuration is _____ for glucose and ____ for fructose.

1. Alpha

2. Beta

the anomeric carbon of glucose is linked to what?

the anomeric carbon of fructose

What is Lactose?

A disaccharide of milk that consists of a galactose linked to a glucose

What is lactose linked by?

linked by a β-1,4-glycosidic linkage.

What can lactose be hydrolyzed by?

lactase in human beings and by β-galactosidase in bacteria

What is maltose?

disaccharide resulting from the hydrolysis of large oligosaccharides that consists of two linked glucose molecules

After a meal, starch and glycogen are degraded by ….

α-amylase.

What leads to lactose intolerance?

A lack of lactose

What can maltose be hydrolyzed by?

Maltese (a-glucosidase)

What joins together maltose?

an α-1,4-glycosidic linkage

Oligosaccharides generated by α-amylase are further digested by what?

Maltose

What are the competitive inhibitors of maltase?

Acarbose (Precose) and miglitol (Glyset)

There are ______ different oligosaccharides have been identified in human milk

> 150

Human milk oligosaccharides may do what two things?

• serve as a fuel source for beneficial bacteria.

• prevent the attachment of microbial pathogens to the intestinal wall of the newborn.

Why are human milk oligosaccharides so important?

They play a significant role in protecting them against bacterial Infection (e.g., Streptococcus bacteria that may be transmitted during vaginal childbirth)

Free glucose cannot be stored because….

high concentrations will disturb the cell's osmotic balance.

What are polysaccharides (glycans)?

large polymeric oligosaccharides formed by the linkage of multiple monosaccharides

What do polysaccharides (glycans) do?

plays roles in energy storage and structural integrity

What is a homopolymer?

polymer in which all the monosaccharide units are the same

What is glycogen?

A large, branched polymer of glucose residues

• storage form of glucose

What is the most common homopolymer in animal cells?

Glycogen

What are most glucose units are linked by?

α-1,4-glycosidic linkages

What are the branches of glycogen formed by?

α-1,6-glycosidic linkages

What hydrolyzed glycogen?

α-amylase

Why is branching in glycogen so important?

It increases the surface area to allow better access for enzymes to rapidly breakdown glycogen.

What is starch?

A homopolymer that serves as the nutritional reservoir in plants

What are the two forms of starches?

amylose and amylopectin

What is amylose?

An unbranched type of starch composed of glucose residues in α-1,4 linkage

What is amylopectin?

The branched type of starch with ~1 α-1,6 linkage per 30 α-1,4 linkages

What is the difference between amylopectin and glycogen?

They have identical structures but amylopectin has a lower degree of branching

Amylose and amylopectin are hydrolyzed by …

α-amylase.

What is cellulose?

unbranched polymer of glucose residues joined by β-1,4 linkages

What is the role of cellulose?

Structural, not nutritional

The β configuration allows Cellulose to what?

to form long, straight chains that interact with one other through hydrogen bonds, yielding a rigid, supportive structure

The α linkages of starch and glycogen form what type of structure?

compact hollow cylinders suitable for accessible storage.

How come mammals cannot digest cellulose?

Because they lack the cellulases to

What do Insoluble fibers do?

increase the rate at which digestion products pass through the large intestine, softening stools and makes them easier to pass

What are the two examples of soluble fibers?

• pectin

• Polygalacturonic acid

What do soluble fibers do?

slow the

movement of food through the gastrointestinal tract.

– facilitating absorption of nutrients from the diet