CHAPTER 23: Metabolism &
Energy Production
General, Organic, & Biological Chemistry
Janice Gorzynski Smith
CHAPTER 23: Metabolism & Energy Production
Learning Objectives:




Stages of Metabolism
ATP: structure, hydrolysis, & formation
Coupling reactions
Coenzymes
 NAD+ & NADH
 FAD & FADH2
 Coenzyme A
 Citric Acid Cycle: all 8 steps
 Electron Transport Chain
 ATP synthesis by Oxidative Phosphorylation
CH 23 Homework:
End of Chapter problems: 20, 22, 24, 26, 28, 32, 34, 36, 40, 42, 46, 48,
50, 52, 56, 60, 64, 74
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
2
Metabolism
Definition
•Metabolism is the sum of all the chemical
reactions that take place in an organism.
•Catabolism is the breakdown of large
molecules into smaller ones; energy is
generally released during catabolism.
Anabolism is the synthesis of large
molecules from smaller ones; energy is
generally absorbed during anabolism.
•Often, the process is a series of
consecutive reactions called a metabolic
pathway, which can be linear or cyclic.
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
3
Metabolism
Overview
Stage [1] – Digestion
Stage [2] – Formation of Acetyl CoA
Stage [3] – The Citric Acid Cycle
Stage [4] – Electron Transport Chain
& Oxidative Phosphorylation
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
4
Metabolism
Stage [1] – Digestion
Carbohydrates
Proteins
Triacylglycerols
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
5
Metabolism
Smith, Janice Gorzynski. General, Organic,
Stage [2] – Formation of Acetyl CoA
6
Metabolism
Stage [3] – The Citric Acid Cycle
•The citric acid cycle is based in the
mitochondria, where the acetyl CoA is
oxidized to CO2.
•The cycle also produces energy stored as
a nucleoside triphosphate and the reduced
coenzymes.
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
7
Metabolism
Stage [4] – Electron Transport Chain
& Oxidative Phosphorylation
•Within the mitochondria, the
electron transport chain and
oxidative phosphorylation
produce ATP (adenosine 5’triphosphate).
•ATP is the primary energycarrying molecule in the body
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
8
ATP
Definition
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
9
ATP
Hydrolysis: Energy Released
•Hydrolysis of ATP cleaves 1 phosphate group.
•This forms ADP and hydrogen phosphate (HPO42−),
releasing 7.3 kcal/mol of energy.
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
10
ATP
Phosphorylation: Energy Absorbed
•Phosphorylation is the reverse reaction, where
a phosphate group is added to ADP, forming ATP
requiring 7.3 kcal/mol of energy.
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
11
ATP
Coupled Reactions
•Coupled reactions are pairs of reactions that occur together.
•The energy released by one reaction is absorbed by the other reaction.
•Coupling an energetically unfavorable reaction with a favorable one that
releases more energy than the amount required is common in biological
reactions.
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
12
Coenzymes
Oxidation & Reduction
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
13
Coenzymes
Coenzyme A
•Coenzyme A (HS-CoA) is neither an oxidizing
nor a reducing agent.
•When the thioester bond is broken, 7.5 kcal/mol
of energy is released.
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
14
Citric Acid
Cycle
Overview
•The citric acid cycle produces high-energy compounds
for ATP synthesis in stage [4] of catabolism.
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
15
Citric Acid
Cycle
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
Overview
16
Citric Acid
Cycle
Steps 1 - 4
•Step [1] reacts acetyl CoA with oxaloacetate to form
citrate, and it is catalyzed by citrate synthase.
•Step [2] isomerizes the 3o alcohol in citrate to the 2o
alcohol in isocitrate; it is catalyzed by aconitase.
•Step [3] isocitrate loses CO2 in a decarboxylation reaction
catalyzed by isocitrate dehydrogenase. Also, the 2o alcohol
of isocitrate is oxidized by the oxidizing agent NAD+ to form
the ketone a-ketoglutarate and NADH
•Step [4] releases another CO2 with the oxidation of aketoglutarate by NAD+ in the presence of coenzyme A to
form succinyl CoA and NADH. Catalyzed by a-ketoglutarate
Smith, Janice Gorzynski. General, Organic,
17
dehydrogenase
& Biological Chemistry 2nd Ed.
Citric Acid
Cycle
Steps 5 - 8
•Step [5] the thioester bond of succinyl CoA is hydrolyzed to
form succinate, releasing energy that converts GDP to GTP.
•Step [6] succinate is converted to fumarate with FAD and
succinate dehydrogenase; FADH2 is formed.
•Step [7], water is added across the C=C; this transforms
fumarate into malate, which has a 2o alcohol.
•Step [8], the 2o alcohol of malate is oxidized by NAD+ to
form the ketone portion of oxaloacetate and NADH. The
product of step [8] is the starting material for step [1].
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
18
Citric Acid
Cycle
Overall Reaction
The main function of the citric acid cycle is to
produce reduced coenzymes (NADH and FADH2).
These molecules enter the electron transport chain
and ultimately produce ATP.
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
19
Electron
Transport
Definition
•The electron transport chain is a multistep
process using 4 enzyme complexes (I, II, III and IV)
located along the mitochondrial inner membrane.
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
20
Electron
Transport
Electron Transport Chain
•The reduced coenzymes (NADH and FADH2) are
reducing agents, and can donate e− when oxidized.
•NADH is oxidized to NAD+ and FADH2 is oxidized
to FAD when they enter the electron transport
chain.
•The e− donated by the coenzymes are passed
down from complex to complex in a series of
redox reactions, which produces some energy.
•These e− and H+ react with inhaled O2 to form water.
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
21
ATP Synthesis
Oxidative Phosphorylation
•The electron transport chain provides the energy
to pump H+ ions across the inner membrane
of the mitochondria.
•The concentration of H+ ions in the inter membrane
space becomes higher than that inside the matrix
creating a potential energy gradient.
•To return to the matrix, H+ ions travel through a
channel in the ATP synthase enzyme (catalyzes
phosphorylation of ADP to ATP).
•The energy released as the H+ ions return to the
matrix is the energy stored in the ATP molecule.
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
22
ATP Synthesis
Oxidative Phosphorylation
•Each NADH entering the electron transport chain
produces enough energy to make 2.5 ATPs.
•Each FADH2 entering the electron transport chain
produces enough energy to make 1.5 ATPs.
•The citric acid cycle produces overall:
3 NADH x 2.5 ATP = 7.5 ATP
1 FADH2 x 1.5 ATP = 1.5 ATP
1 GTP =
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
1 ATP
10 ATP
23
ATP Synthesis
Oxidative Phosphorylation
Smith, Janice Gorzynski. General, Organic,
& Biological Chemistry 2nd Ed.
24