BIOL 300 Lecture 09 Carbohydrates and Metabolism

What are fats broken down into? Polysaccharides? Proteins?

Fatty acids and glycerol. Glucose and other monosaccharides. Amino acids.

What are the breakdown products fed into metabolism as? What will happen to them?

As small molecules. They can be either further oxidized to release energy, excreted, or used to build new molecules.

What are the types of carbohydrates?

Monosaccharides, disaccharides, polysaccharides.

What are monosaccharides?

Single sugar units. They contain multiple hydroxyl groups, one carbonyl group, and have a general formula of CnH2nOn.

What are the types of monosaccharides and their differences?

Aldoses: Carbonyl group at end of carbon chain.

Ketoses: Internal carbonyl group.

How are monosaccharides classifed?

By carbon number and whether they contain aldehyde or ketone group.

What is an example of isomers in sugars?

Glucose (C6H12O6) is an aldohexose.

Fructose (C6H12O6) is a ketohexose.

What are stereoismoers?

Same molecular formula and same order of atoms, but differing in 3D arrangement of groups around certain carbons.

What is a chiral carbon?

Carbon attached to 4 different groups that allow two different 3D arrangments that are mirror images but not superimposable.

What are the chiral carbons in sugars?

Carbonyl carbon (C1) is not chiral because of its C=O double bond.

CH2 carbon (C6) is not chiral because of two identical H atoms.

C2-C5 are chiral because they are attached to four different groups.

What is the step-by-step classifcation of monosaccharides? (To be editted)

Same chemical formula? (Yes —> Isomers)

Same connectivity? (Yes —> Stereoisomers)

Mirror Images? (Yes —> Enantiomers)

Not mirror images? (Yes —> Diastereomers)

Different at only one chiral center? (Yes —> Epimers)

Different specifically at anomeric carbon? (Yes —> Anomers)

What are enantiomers?

Stereoisomers that are mirror images of each other.

What are examples of enantiomers in sugars?

D-glucose and L-glucose because they have the same molecular formula, same connectivity, opposite 3D arrangement at all chiral carbons, mirror-image relationship, and different at all chiral centers.

What are diastereomers?

Sugars that differ in configuration around more than one chiral carbon atom.

How are diastereomers different from enantiomers?

Diastereomers are not mirror images, only being different at some chiral centers.

What are epimers?

Two sugars that differ only in configuration around one carbon atom.

What are two examples of epimers through D-glucose?

D-mannose differs from D-glucose only at C2, so D-mannose is the C2 epimer of D-glucose.

D-galactose differs from D-glucose only at C4, so D-galactose is the C4 epimer of D-glucose.

What is the difference between enantiomers and epimers?

Enantiomeric forms will always be the same sugar (mirror images).

Epimeric forms will always be different sugars (not mirror images).

What is the difference between diastereomers and epimers?

Diastereomers are sugars that differ in configuration around more than one chiral carbon atom.

Epimers are sugars that differ only in the configuration around one chiral carbon atom.

How does the cyclization of glucose create a new chiral center?

Linear D-glucose contains aldehyde group at C1. In water, C5 hydroxyl and C1 carbonyl cyclizes into cyclic hemiacetal. C1 becomes new chiral carbon, called anomeric carbon.

How much chiral carbons does linear glucose contain? Cyclic?

4 chiral carbons. 5 chiral carbons.

How are alpha and beta glucose anomers?

In alpha-D-glucose, anomeric OH is down.

In beta-D-glucose, anomeric OH is up.

How do the alpha and beta glucose interconvert?

Through the open-chain form by mutarotation.

What can anomeric carbons be used for? What are their types?

Attachment of sugar to proteins, forming glycoproteins.

In O-linked glycosylation, sugar attaches to OH group of Ser or Thr.

In N-linked glycosylation, sugar attaches to amide nitrogen of Asn.

What are glycans and what are they used for?

Sugar chains formed from glycosylation with anomeric carbons. They affect protein folding, localization, signaling, and cell-cell recognition.

Why are linear forms of aldoses reducing sugars?

The free aldehyde is reactive and can be oxidized.

How can linear forms of aldoses react with proteins?

Small fraction of glucose opens into linear aldehyde form with carbonyl group that can be oxidized. They can react with the amino acids in proteins, leading to non-enzymatic glycation of proteins.

How are disaccharides formed by condensation of two cyclic monosaccharide molecules?

Glycosidic bond forms when anomeric OH of one sugar bonds with OH group of another sugar through condensation reaction (release of water).

What is the process of condensation for maltose?

Glucose units are joined by alpha(1,4) glycosidic bond.

Left glucose anomeric carbon is locked as acetal.

Right glucose anomeric carbon still has free hemiacetal/anomeric carbon, so maltose will be reducing agent.

How can a sugar open to its linear form? What are some situations?

If anomeric carbon is locked in glycosidic bond, sugar cannot open.

If disaccharide still has one free anomeric carbon, it is reducing.

If both anomeric carbons are used in glycosidic bond, so no free anomeric carbon, it is nonreducing.

How is HbA1c formed?

When the aldehyde at C1 in open-chain glucose reacts with the N-terminal valine amino group of the hemoglobin beta-chain.

What are some common disaccharides?

Sucrose: Glucose + Fructose.

Lactose: Galactose + Glucose.

Maltose: Glucose + Glucose.

What is the nonreducing/reducing property of sucrose? Explain.

Both anomeric carbons are used in glycosidic bond. Glucose C1 and fructose C2 are both locked. No free anomeric carbon remain.

Sucrose cannot open to linear carbonyl form, so nonreducing.

What is the nonreducing/reducing property of lactose? Explain.

Galactose C1 is used in glycosidic bond. Glucose C1 is still free.

Lactose can open to linear form, so reducing.

What is the nonreducing/reducing property of maltose? Explain.

One glucose C1 is used in glycosidic bond. Other glucose C1 is still free.

Maltose can open to linear form, so reducing.

How are carbohydrates used as metabolic fuel?

Glucose is a central metabolic hub.

• Oxidized by glycolysis to produce pyruvate and capture energy.

• Stored as glycogen or starch.

• Enter pentose phosphate pathway to make ribose 5-phosphate (for DNA, RNA, ATP).

• Used to build structural polysaccharides.

What is molecular equation of the aerobic catabolism for glucose?

C6H12O6 + 6O2 —> 6CO2 + 6H2O + 30-32 ATP molecules.

What is the aerobic catabolism of glucose?

Complete oxidation of glucose to CO2 and H2O releases large amount of free energy.

Why is the aerobic catabolism of glucose done in steps?

If glucose is burned directly, energy is released all at once as mostly heat and light. Cells cannot efficiently use this outburst.

Stepwise oxidation allows energy to be captured in useful forms, especially ATP and reduced electron carriers like NADH.

What occurs in respiration?

Stepwise oxidation of sugar in cells, where small activation energies are overcome by body temperature.

What occurs in combustion?

Direct burning of sugar, where large activation energy is overcome by a fire and all free energy is released as heat. None is stored.

Why does chirality matter in biology?

Many biological sugar-binding sites are 3D and asymmetric, so they can distinguish D-sugars from L-sugars.

What chiral carbon glucose do glucose transporters prefer?

D-glucose over the L-glucose.

What is the thermodynamic rule of ΔG?

Gibbs free energy for a reaction means change in usable energy as reactants become products. This tells us whether a reaction releases or requires usable energy.

What is the formula for ΔG? What are the interpretations of ΔG?

ΔG = Free energy of products - Free energy of reactants.

Negative ΔG: Exergonic reactions that release free energy.

Positive ΔG: Endergonic reactions that require free energy.

What is the relationship between ΔG and ATP coupling?

ATP hydrolysis has large negative ΔG, so cell couple the reaction with an unfavorable reaction to make it an overall favorable reaction.

Why does ATP hydrolysis release free energy?

Products of ADP + Pi are more stable than ATP because:

• Electrostatic repulsion of close negatively charged phosphate groups in ATP.

• Hydrolysis relieves repulsion.

• Inorganic phosphate (Pi) is stabilized by resonance.

• ADP and Pi are better stabilized in water than ATP.

What are the reactions in glucose phosphorylation?

Endergonic: Glucose + Pi —> Glucose 6-phosphate.

Exergonic: ATP —> ADP + Pi.

Exergonic: Glucose + ATP —> Glucose 6-phosphate + ATP.

Does every energy coupling require ATP?

High-energy compounds (like PEP) can either regenerate ATP or directly drive unfavorable reactions through enzyme-coupled group transfer.

What is the polysaccharide forms that consist of glucose?

Linear amylose is made up of D-glucose in alpha(1-4) linkages.

Branched amylopectin is made up of D-glucose in alpha(1,4) main linkages and alpha(1,6) branch point linkages.

Why does branching in amylopectin result in faster glucose release?

The multiple exposed nonreducing end of amylopectin allows enzymes to rapidly remove glucose units from starch, which can be metabolized to produce energy.