Lecture Notes - Carbohydrates and Energy

What We Learned Last Lecture

• Energy is required by all cells to maintain cell viability and to build new biological macromolecules for reproduction.

• Cells bring energy from the external environment and convert it to a usable form; adenosine triphosphate (ATP) is a major molecule for energy storage.

• Heterotrophs obtain energy from imported organic molecules.

• Catabolism breaks down molecules to regenerate ATP and provide materials for anabolism (building new macromolecules).

• Cells are thermodynamically open systems.

• Enthalpy (H) is the total energy in the cell and its surroundings.

• Gibb’s free energy (G) is the usable energy from catabolic reactions.

• Free energy change ($\Delta G$) can be released (exergonic, $-\Delta G$) or required (endergonic, $+\Delta G$).

• Entropy (S) is unusable energy; a positive $\Delta S$ means less ordered products and is increased by temperature (-T$\Delta S$).

• The overall free energy equation is: $\Delta G = \Delta H - T\Delta S$

• Most ATP regeneration occurs during oxidative phosphorylation.

• Catabolic oxidation of inorganic (lithotrophic) or organic (heterotrophic) molecules provides electrons to power oxidative phosphorylation.

• In eukaryotic cells, 24 electrons are harvested from one glucose molecule via glycolysis, pyruvate oxidation, and the citric acid cycle.

• These electrons are carried by NAD+ (20 electrons) and FAD (4 electrons), with each molecule carrying 2 electrons.

• Electrons from NADH and FADH2 feed into electron transport proteins in the plasma membrane (prokaryotes) or inner mitochondrial membrane (eukaryotes).

• The free energy released ($\Delta G$) as electrons move through these proteins drives ions (protons) across the membrane against their gradient.

• In aerobic cells, two electrons are placed onto oxygen atoms to generate $O_2$, which combines with protons to form water.

• The removal of electrons allows more electrons to enter the chain, reforming the ion gradient.

• The proton gradient's free energy drives the ATP synthase complex rotation.

• This rotation directly provides the energy to form a covalent bond between adenosine diphosphate (ADP) and inorganic phosphate (Pi), forming ATP.

• Oxidative phosphorylation generates 34 of the 38 ATP molecules from complete glucose oxidation.

• The four macromolecule groups in cells are proteins, lipids, carbohydrates, and nucleic acids.

• We have studied the structures and functions of lipids and proteins.

• We have not yet studied carbohydrates and nucleic acids.

• Understanding the structure and function of carbohydrates, especially glucose, is important.

• Understanding catabolic pathways for glucose oxidation and ATP regeneration is important.

• Enzymes are essential proteins that increase the rate of most reactions in cells.

Carbohydrates: Monosaccharide Structure

• Carbohydrates contain carbon, hydrogen, and oxygen.

• Monosaccharides (simple sugars) are the monomeric units of all carbohydrates.

• Most monosaccharides have a carbon:hydrogen:oxygen ratio of 1:2:1 (CnH{2n}On)).

• Disaccharides form by covalently bonding two monosaccharides.

• Polysaccharides are chains of covalently attached monosaccharides.

Basic Structure of Monosaccharides

• Monosaccharides must have at least 3 carbon atoms.

• All monosaccharides have a single carbonyl and two or more hydroxyl functional groups on different carbon atoms.

• Sugars are defined by:

  • Number of carbons: trioses (3), pentoses (5), hexoses (6).

  • Position of the carbonyl group: terminal (aldose) or internal (ketose).

  • Orientation of the hydroxyl group furthest from the carbonyl group.

Nomenclature

• Defined by:

  • Number of carbon atoms [trioses (3), pentoses (5) and hexoses (6)]

  • Carbonyl group position: terminal (aldose) or internal (ketose).

  • Carbon #1 is the terminal carbon with a carbonyl or the closest to the carbonyl.

  • In D-sugars, the hydroxyl on the chiral carbon furthest from the carbonyl is oriented as in D-glyceraldehyde.

• Examples include:

  • D-glyceraldehyde: Intermediate product of the glycolysis and photosynthesis

  • D-ribose-Required for ATP synthesis, DNA, and RNA.

  • D-glucose: Main circulating energy source in mammals, stored as polymers in muscle and liver; important intermediate in glycolysis and photosynthesis.

  • D-glyceraldehyde: triose

  • D-ribose: pentose

  • D-glucose: hexose

  • D-glyceraldehyde: triose aldose (C3H6O3)

  • dihydroxyacetone: triose ketose (C3H6O3)

  • D-glucose: hexose aldose (C6H{12}O6)

  • D-fructose: hexose ketose (

  • C6H{12}O6)

• Numbering carbons:

  • In D-glyceraldehyde and D-glucose, the terminal carbonyl is carbon #1; subsequent carbons are numbered sequentially.

  • Phosphate bonded to the hydroxyl oxygen on the carbon atom furthest from the carbonyl group in glucose would be labelled as glucose 6-phosphate.

  • In D-fructose, carbon #1 is the terminal carbon closest to the carbonyl since it is a ketose sugar.

  • Dihydroxyacetone carbons are unnumbered because the carbonyl group is equidistant from both terminal carbons.

• Ketoses: Dihydroxyacetone, Ribulose.

• Aldoses: Fructose, Glyceraldehyde, Ribose, Glucose, Galactose.

  • Trioses (C3H6O3)

  • Pentoses (C5H{10}O5)

  • Hexoses (C6H{12}O6)

  • Structral isonomers are compounds with the same molecular formula but different structural arrangements.

  • Optical isomers where the structures are non-superimposable mirror images of each other .

  • Glycoysis: Glucose—- glucose 6-phosphate—-frutose 6-phosphate— frutose 1-6bisphonphate

Chirality

• Most simple sugars have one or more chiral carbons.

• A chiral carbon is bonded to four different groups.

• A carbonyl carbon is not chiral because it is attached to only 3 groups.

• The carbon in $R-CH_2OH$ is not chiral because it is attached to 2 separate hydrogen atoms.

• Sugars with group position differences at a chiral carbon have stereoisomers (enantiomers or optical isomers).

• D- vs. L- designates enantiomers at a specific carbon position in the simple sugar.

Chiral Carbons

• Chiral carbons have four different covalently bonded groups.

• Glyceraldehyde is the simplest example of a monosaccharide with an asymmetric carbon.

• If the hydroxyl is to the right, it is D-; if to the left, it is L-.

• Most biologically active sugars are in the D-conformation.

Designations

• If the hydroxyl is to the left in the Fischer projection, the sugar is L-.

• If the hydroxyl is to the right in the Fischer projection, the sugar is D-.

Representations of Glucose

• Linear structures are Fischer projections (D- or L-glucose).

• Sugars with 5+ carbons cyclize in aqueous solution to form ring structures (Haworth projections, D-glucopyranose and L-glucopyranose).

• Ring structures can also be represented as conformational projections.

Drawing Haworth Structure of Glucose

• Draw a six-member ring with 5 carbon atoms and 1 oxygen atom.

• Label carbons 1’ through 5’ clockwise, starting to the right of oxygen.

• For D-glucopyranose, place the $CH_2OH$ group of carbon 6’ in the up position from carbon 5’.

• Place the hydroxyl on 4’ down, 3’ up, and 2’ down; carbon 5 has no hydroxyl group in the ring.

• Atoms on carbons without 4 bonds are hydrogen.

• For $\alpha$-D-glucopyranose, the hydroxyl on carbon 1 points below the ring; for $\beta$-D-glucopyranose, it points above.

From Haworth to Fischer Structure

• Draw six covalently connected carbon atoms in a linear arrangement.

• Since glucose is an aldose, place a carbonyl and hydrogen atom on carbon 1 and $CH_2OH$ on carbon 6.

• If the hydroxyl is down in the Haworth projection, place it to the right in the Fischer projection; if it is up, place it to the left.

• Since this is D-glucose, the hydroxyl on carbon 5 is to the right.

• The remaining positions on the carbons are hydrogen atoms.

Ring Formation

• The most stable conformation of simple sugars with 5+ carbons in aqueous solution is as closed rings.

• In glucose, the carbonyl carbon forms a bond with the oxygen on the hydroxyl group of carbon 5 to form a hemiacetal; the oxygen at carbon 5 incorporates into the ring.

• In glucose, when the ring closes, the carbonyl oxygen at carbon 1 converts to a hydroxyl group.

  • In the alpha ($\alpha$) position, the hydroxyl is below the ring in the Haworth projection.

  • In the beta ($\beta$) position, it is above the ring in the Haworth projection.