1. Chapter Outline 15.1 Metabolic Pathways, Energy, and Coupled Reactions
15.2 Overview of Metabolism
15.3 Digestion
15.4 Glycolysis
15.7 Citric Acid Cycle
15.8 Electron Transport Chain and Oxidative Phosphorylation
15.9 Lipid Metabolism
15.10 Amino Acid Metabolism
15.5 Gluconeogenesis
15.6 Glycogen Metabolism

2. To discuss how living things manufacture or break down carbohydrates, lipids, or members of any other biochemical class of compounds it is necessary to talk in terms of groups of reactions called metabolic pathways.
The metabolic pathways can be:
1) linear – a continuous series of reactions in which the product of one reaction is the reactant in the next.
2) circular – a series of reactions where the final product is an initial reactant.
3) spiral – a series of repeated reactions is used to break down or build up a molecule.

3. 15.1: Metabolic Pathways, Energy, and Coupled Reactions

4. Coupled Rxn:
In a coupled reaction, a spontaneous reaction provides the energy needed to a nonspontaneous reaction.
Fructose 6-phosphate + Pi → fructose 1,6-biphosphate + H20
(∆G = 3.9 kcal/mol)
2. ATP + H2O → ADP + Pi (∆G = -7.3 kcal/mol)
Overall:
Fructose 6-phosphate + ATP → fructose 1,6-biphosphate + ADP
(∆G = -3.4 kcal/mol)
Is the coupled reaction spontaneous or nonspontaneous?

5. 15.2: Metabolism, the sum of all reactions that take place in a living thing, can be divided into two parts:
Catabolism. During catabolism, compounds are broken down into smaller ones in processes that, usually, release energy.
Anabolism. Anabolism involves the biosynthesis of larger compounds from smaller ones in processes that, usually, require energy.

6. -ADP and ATP are key players in metabolism.
-Energy released during catabolism is used to drive the formation of these two compounds.
-Energy obtained by hydrolyzing ATP can, in turn, be used for anabolism or other energy requiring processes, such as muscle contraction.

9. Catabolism
(break down of large molecules to small ones.)

10. Catabolism
Not all catabolic pathways take place in the same part of a cell.
Cytoplasm
glycolysis
Mitochondria
Fatty acid oxidation
Amino acid catabolism
Citric acid cycle
Electron transport chain
Oxidative phosphorylation

12. Anabolism
During anabolism, small molecules such as pyruvate, acetyl-CoA, and intermediates in the citric acid cycle are used to make fatty acids, monosaccharides, and amino acids for incorporation into lipids, polysaccharides, and proteins.

13. 15.3: Digestion

14. Example of polysaccharide break down to monosaccharide.

15. Example of triglyceride and polypeptide break down.

16. 15.4: Glycolysis

18. Glucose + 2NAD+ + 2ADP + 2Pi → 2 pyruvate + 2NADH + 2ATP + energy
15.4: Glycolysis
The net reaction for glycolysis is:
Glucose + 2NAD+ + 2ADP + 2Pi →
2 pyruvate + 2NADH + 2ATP + energy
Energy and Glycolysis:
glucose + 2ADP + 2Pi → 2lactate + 2ATP
∆G = kcal/mol
spontaneous

19. Metabolism can take pyruvate into a number of different directions.
In yeast, pyruvate undergoes alcoholic fermentation.
In this process pyruvate is split into acetaldehyde plus CO2, and acetaldehyde is reduced to ethanol.
These reactions serve to recycle NADH back into NAD+, allowing glycolysis to continue.

21. In humans, pyruvate is reduced to lactate when conditions are anaerobic (O2 deficient)
This reaction converts NADH back into NAD+, allowing glycolysis to continue.
Once produced, lactate is sent in blood to the liver, where it can be used to make glucose.
Under aerobic conditions (O2 is in sufficient supply), pyruvate is converted into acetyl-CoA, the reactant for the first step in the citric acid cycle.
Glycolysis takes place in a cell’s cyctoplasm, but the formation of acetyl-CoA and the citric acid cycle take place inside the mitochondria.

22. 15.7: Citric Acid Cycle

23. Overall Rxn: (occurs twice)
15.7: Citric Acid Cycle
Overall Rxn: (occurs twice)
Acetyl-CoA + 3NAD+ + FAD + GDP + Pi → 2CO2 + CoA + 3NADH + FADH2 + GTP
∆G = -11 kcal/mol

24. 15.8: Electron Transport Chain and Oxidative Phosphorylation
The electron transport chain is a group of proteins and other molecules embedded in the inner mitochondrial membrane.
The electron transport chain and oxidative phosphorylation use the potential energy present in NADH and FADH2 to make ATP.

26. Summary of Glucose catabolism:

27. 15.9: Lipid Metabolism

28. Four reactions in the b-oxidation of a fatty acid

29. b-oxidation Spiral of a fatty acid

30. Fatty Acid Catabolism
One common fate of fatty acids is their use as reactants in the formation of triglycerides, sphingolipids, and other lipids that contain fatty acid residues.
Their other important use is as a source of energy.
Fatty acid catabolism involves a spiral metabolic pathway, called the  oxidation spiral, where the same series of reactions is repeated on increasingly shorter reactants.

31. 15.10: Amino Acid Metabolism
Removal of the amino group is an important part of amino acid catabolism.
The two reactions most often used to do this are:
1) Transamination is the transfer of an amino group from an amino acid to an -keto acid. These reactions are catalyzed by transaminase enzymes.
2) In oxidative deamination an amino group is replaced by a carbonyl (C=O) group.

35. Amino Acid Anabolism
Of the 20 amino acids used to synthesize proteins, humans can make only half.
The others, called essential amino acids, must be obtained in the diet.

37. 15.5: Gluconeogenesis
Gluconeogenesis, the pathway involved in making glucose from noncarbohydrate sources, such as amino acids, glycerol, and lactate, takes place mostly in the liver.
One important role of this process is the conversion of lactate produced during anaerobic catabolism back into glucose, which is either transformed into glycogen or goes into the blood and is transported to other cells.

38. In addition to recycling lactate, gluconeogenesis is a provider of glucose during fasting or in the early stages of starvation, in which glucose and glycogen (a source of glucose) have been depleted.
The supply of glucose is especially important to brain cells, which use only glucose to fuel metabolism, unlike other cells in the body which can also use lipids and proteins.

39. 15.5: Gluconeogenesis

40. 15.6: Glycogen Metabolism
Glycogen, a highly branched homopolysaccharide, is found mainly in liver and muscle cells.
This carbohydrate is a glucose storage molecule that, when necessary, can be quickly broken down to release glucose.
Glycogen is synthesized from or is broken down into glucose 6-phosphate.

41. 15.6: Glycogen Metabolism