Study Guide: Biochemical Energy, Carbohydrates, and Carbohydrate Metabolism (Ch. 20, 21, 22.1-6)

Chapter 21: The Generation of Biochemical Energy

Overview of Biochemical Energy

Biochemical energy is essential for all cellular processes. Cells obtain energy primarily through the oxidation of nutrients, which is then stored and utilized in the form of ATP (adenosine triphosphate).

• ATP: The main energy currency of the cell, produced by catabolic pathways.

• Coupled Reactions: Endergonic (energy-requiring) reactions are driven by coupling them to exergonic (energy-releasing) reactions, often involving ATP hydrolysis.

• Example: The phosphorylation of glucose is coupled to ATP hydrolysis to make the overall process energetically favorable.

Coupling Reactions Using Table 21.1

• To allow an endergonic reaction (ΔG > 0) to occur, it is coupled with an exergonic reaction (ΔG < 0), such as ATP hydrolysis.

• Equation: $\Delta G_{\text{overall}} = \Delta G_1 + \Delta G_2$

• Example: Glucose + Pi → Glucose-6-phosphate (endergonic) is coupled with ATP → ADP + Pi (exergonic).

Electron Carriers: FAD and NAD+

• FAD (Flavin Adenine Dinucleotide): Accepts two electrons and two protons to become FADH2.

• NAD+ (Nicotinamide Adenine Dinucleotide): Accepts two electrons and one proton to become NADH.

• Reduction Reactions: Involve the gain of electrons (and usually protons) by these cofactors.

• Example: $\text{NAD}^+ + 2e^- + 2H^+ \rightarrow \text{NADH} + H^+$

Citric Acid Cycle (Krebs Cycle)

• Purpose: Oxidizes acetyl-CoA to CO2 and captures high-energy electrons in NADH and FADH2.

• Key Steps:

  • Reaction 1: Oxaloacetate + Acetyl-CoA → Citrate (via citrate synthase; involves aldol condensation).

  • Reactions 2 & 3: Citrate → Isocitrate → α-Ketoglutarate (involves hydration, dehydration, and oxidation).

  • Reactions 6, 7, 8: Succinate → Fumarate → Malate → Oxaloacetate (involves oxidation, hydration, and oxidation).

• Types of Reactions: Aldol condensation, hydrolysis, hydration (Markovnikov/anti-Markovnikov), dehydration, oxidation, β-decarboxylation.

• Energy Yield: Steps that produce GTP, NADH, FADH2, and CO2 should be recognized.

Electron Transport Chain (ETC) and ATP Synthesis

• Complexes I-IV: Transfer electrons from NADH and FADH2 to O2, pumping protons across the mitochondrial membrane.

• ATP Synthase (Proton Translocating ATPase): Uses the proton gradient to synthesize ATP from ADP and Pi.

• Equation: $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

Calculating ATP Yield from the Citric Acid Cycle

• Each turn of the cycle yields:

  • 3 NADH (≈ 7.5 ATP)

  • 1 FADH2 (≈ 1.5 ATP)

  • 1 GTP (≈ 1 ATP)

• Total: ≈ 10 ATP per acetyl-CoA oxidized.

Chapter 20: Carbohydrates

Monosaccharide Structures: Fischer and Haworth Projections

• Fischer Projection: A two-dimensional representation showing the configuration of chiral centers.

• Haworth Projection: A cyclic representation showing the ring form of sugars as α or β anomers.

• Conversion: Given one form, you should be able to draw the other, specifying ring size (5- or 6-membered).

• Example: D-glucose Fischer projection to α-D-glucopyranose Haworth projection.

Reactions of Monosaccharides

• Reduction to Alditols: Converts the carbonyl group to an alcohol.

• Oxidation to Aldonic Acids: Oxidizes the aldehyde group to a carboxylic acid.

• Keto-Enol Tautomerization: Interconversion between keto and enol forms.

• Formation of Phosphate Esters: Addition of phosphate groups, important in metabolism.

• Oxidation to Uronic Acids: Oxidizes the terminal CH2OH group to a carboxylic acid.

Disaccharide Formation

• Glycosidic Bonds: Monosaccharides are joined by α or β-1,4-glycosidic bonds or 1,2-anomeric links.

• Example: Maltose (α-1,4), lactose (β-1,4), sucrose (α-1,2).

Chapter 22: Carbohydrate Metabolism (Sections 1-6)

Glycolysis Pathway

Glycolysis is the metabolic pathway that converts glucose into pyruvate, generating ATP and NADH.

• Key Steps: 10 steps, each catalyzed by a specific enzyme.

• Fill-in-the-Pathway: Be able to identify or draw missing structures in the pathway.

Coupled Reactions and ΔG in Glycolysis

• Some steps are coupled to ATP hydrolysis or synthesis.

• ΔG (Gibbs Free Energy): Indicates whether a reaction is spontaneous.

• Equation: $\Delta G = \Delta H - T\Delta S$

• For steps 1, 3, 7, 10, be able to write coupled reactions and calculate total ΔG.

Intermediates and Mechanisms in Glycolysis

• Enol Intermediates: Steps 2 and 5 involve enol forms; be able to draw these.

• Reverse Aldol/Aldol Cleavage: Step 4 involves splitting fructose 1,6-bisphosphate into two 3-carbon sugars.

• Oxidation and Dehydration: Step 6 is an oxidation (NAD+ reduced to NADH); step 9 is a dehydration.

ATP and NADH Production/Consumption in Glycolysis

• ATP Consumed: Steps 1 and 3.

• ATP Produced: Steps 7 and 10.

• NADH Produced: Step 6.

Fate of Pyruvate

• Aerobic Conditions: Pyruvate is converted to acetyl-CoA, entering the citric acid cycle.

• Anaerobic Conditions (Muscle): Pyruvate is reduced to lactate.

• Yeast Cells: Pyruvate is converted to ethanol and CO2 (fermentation).

• Equations:

  • Aerobic: $\text{Pyruvate} + \text{NAD}^+ + \text{CoA} \rightarrow \text{Acetyl-CoA} + \text{NADH} + \text{CO}_2$

  • Anaerobic: $\text{Pyruvate} + \text{NADH} + H^+ \rightarrow \text{Lactate} + \text{NAD}^+$

  • Yeast: $\text{Pyruvate} \rightarrow \text{Acetaldehyde} + \text{CO}_2$; $\text{Acetaldehyde} + \text{NADH} + H^+ \rightarrow \text{Ethanol} + \text{NAD}^+$

Energy Yield from Complete Glucose Metabolism

• Glycolysis: Net 2 ATP, 2 NADH per glucose.

• Citric Acid Cycle: Each acetyl-CoA yields ≈ 10 ATP (see above).

• Total Yield: Complete oxidation of one glucose molecule yields approximately 30-32 ATP.

Summary Table: Key Steps in Glycolysis and Citric Acid Cycle

Pathway	Step	Type of Reaction	ATP/NADH/FADH2 Produced or Consumed
Glycolysis	1 (Hexokinase)	Phosphorylation (Coupled to ATP hydrolysis)	ATP Consumed
Glycolysis	3 (PFK-1)	Phosphorylation (Coupled to ATP hydrolysis)	ATP Consumed
Glycolysis	6 (GAPDH)	Oxidation/Phosphorylation	NADH Produced
Glycolysis	7 (Phosphoglycerate kinase)	Substrate-level phosphorylation	ATP Produced
Glycolysis	10 (Pyruvate kinase)	Substrate-level phosphorylation	ATP Produced
Citric Acid Cycle	3 (Isocitrate DH)	Oxidation/Decarboxylation	NADH, CO2 Produced
Citric Acid Cycle	4 (α-KG DH)	Oxidation/Decarboxylation	NADH, CO2 Produced
Citric Acid Cycle	5 (Succinyl-CoA synthetase)	Substrate-level phosphorylation	GTP Produced
Citric Acid Cycle	6 (Succinate DH)	Oxidation	FADH2 Produced
Citric Acid Cycle	8 (Malate DH)	Oxidation	NADH Produced

Additional info: This guide expands on the exam guidelines by providing definitions, examples, and equations for each topic. For detailed mechanisms and structures, refer to textbook figures and lecture handouts.