1. Ground Rules of Metabolism
Chapter 4 Part A

2. Metabolic Reactions
Metabolic reactions are orderly, enzyme-mediated sequences that build, rearrange, or breakdown molecules

3. Metabolic Reactions Terms used for metabolic reactions
Reactants or Substrates: starting molecules
Intermediates: molecules formed during a reaction sequence
End Products: molecule(s) at the end of the reaction sequence
Substrate
Intermediates
Product

4. Metabolic Reactions Some metabolic pathways are linear
Runs straight from the substrate to the end product
Substrate
Intermediates
Product

5. Metabolic Reactions Some metabolic pathways are cyclic
The substrate reacts with the last intermediate of the cycle
The end product exits the cycle
The last step regenerates the last intermediate ready for the first step
Substrate
Product
Intermediates

6. Metabolic Reactions Reactions can occur in both directions
Synthesizing or breaking down
Products become the substrate
Substrates become the products

7. Energy Metabolic reactions may require or produce energy
Energy is the capacity to do work
Various forms of energy include
Kinetic: energy of movement
Potential: stored energy
Chemical: potential energy stored in chemical bonds
Heat
Light

8. Energy Energy is defined by the Laws of Thermodynamics 1st Law
Energy cannot be created or destroyed, but can be converted form one form to another

9. Energy Energy is defined by the Laws of Thermodynamics 2nd Law
Entropy tends to increase
Entropy is a measurement of how much the energy in a system has become dispersed

10. Energy Energy is defined by the Laws of Thermodynamics 2nd Law
Entropy tends to increase
Energy flows form more usable forms (low entropy) to less usable forms (high entropy)
Least usable / highest entropy is heat energy

11. Energy Energy is defined by the Laws of Thermodynamics 2nd Law
Entropy changes can cause some reactions to occur spontaneously or to require energy input

12. Energy Energy release or use in metabolic reactions
Determined by free energy of the molecules
Free energy = bond energy and entropy level of a molecule
High free energy:
Low entropy plus high bond energy
Low free energy:
High entropy plus low bond energy

13. Energy Energy release or use in metabolic reactions
Endergonic reactions
Energy is required
Reactants that have less free energy than the products need an energy input
High free energy:
Low entropy plus high bond energy
Low free energy:
High entropy plus low bond energy

14. Energy Energy release or use in metabolic reactions
Exergonic reactions
Energy is released
Reactants that have more free energy than the products will release energy
High free energy:
Low entropy plus high bond energy
Low free energy:
High entropy plus low bond energy

15. Energy Activation energy
The minimum amount of energy required to start a chemical reaction
Even exergonic reactions require activation energy

16. Energy ATP (Adenosine triphosphate)
Used by cells to store and transfer energy
Bonds between the phosphate groups hold a lot of energy

17. Energy ATP (Adenosine triphosphate)
Contributes energy to endergonic reactions
Similar to using a battery
The mechanism
ATP  ADP + phosphate group (Pi)
Releases energy when the bond holding the phosphate group is broken
Phosphate groups can be released free in the cytoplasm or attached to a substrate/intermediate in a metabolic pathway reaction requiring energy

18. Energy ATP (Adenosine triphosphate)
Receives energy from exergonic reactions
Similar to charging a battery
The mechanism
ADP + phosphate group (Pi)  ATP

19. Energy ATP (Adenosine triphosphate)
ATP and ADP + Pi cycle between endergonic and exergonic reactions driving most metabolic reactions
ATP ADP + phosphate group (Pi)

20. Enzymes
Enzymes are used for most steps of metabolic reactions

21. Enzymes
Enzymes are catalysts: molecules that make chemical reactions occur much faster than they would on their own

22. Enzymes Nearly all enzymes are proteins
The polypeptide folds into a specific shape
Creates an active site
A pocket where substrates bind and a reaction occurs
Induced fit
The substrate doesn’t start out fitting perfectly
As the enzyme and substrate interact, they change shape slightly resulting in the interactions that will promote the chemical reaction
Impacted by any factors that affect proteins
Denaturation, temperature, pH, etc.

23. Enzymes
Enzymes help reactions occur faster by lowering the activation energy
Forcing substrates together
Brings two or more substrates close together
More likely to react
Orienting substrates
Positioned so they align for a reaction
Normally the odds of two substrates colliding randomly in the correct orientation are very low

24. Enzymes
Enzymes help reactions occur faster by lowering the activation energy
Inducing a fit between the enzyme and substrate
Causes the substrate to change shape putting pressure on its bonds
Shutting out water molecules
Changes some of the chemical interactions internally in the substrate (hydrophobic/hydrophilic)

25. Enzymes
Enzymes are not consumed or altered by the reaction and can work again and again

26. Enzymes Enzymes are helped by cofactors and coenzymes Metal ions
Stabilize the structure of an enzyme
Functional role by readily losing or gaining electrons

27. Enzymes Enzymes are helped by cofactors and coenzymes Coenzymes
Small organic molecules
Many vitamins are cofactors (or precursors for them)
Coenzymes are often modified during a reaction, but they are regenerated elsewhere

28. Enzymes Enzymes are helped by cofactors and coenzymes Coenzymes
NAD+ is a coenzyme
Derived from niacin
Accepts electrons and hydrogen ions becoming NADH
Then donates the electrons and hydrogen ions forming NAD+ again
NAD+ + electrons + H+ NADH NAD+ + electrons + H+

29. Control of Metabolic Pathways
Cells control their metabolic pathways to control amounts of various reactants, intermediates, and products

30. Control of Metabolic Pathways
Control enzyme activity
Temperature
Enzymes have optimal ranges of temperature
Too low slows enzymes down
Too high results in denaturation
Adapted to environment
Arctic verses hot springs

31. Control of Metabolic Pathways
Control enzyme activity
Denaturation
Destroys the three-dimensional shape of the active site
Environmental causes
High temperature pH
Salinity
Enzymes have optimal ranges

32. Control of Metabolic Pathways
Control enzyme activity
Regulatory molecules
Binding of an allosteric regulator alters the shape of the enzyme
Can either inhibit or enhance the enzyme’s function

34. Control of Metabolic Pathways
Feed-back inhibition

35. Control of Metabolic Pathways
Feed-back inhibition
The pathway produces a lot of the end product
Excess product binds to the first enzyme inhibiting its action
The cell uses the remaining product
Product levels decrease and release the first enzyme
The pathway is active again

36. Summary Metabolic Reactions Energy Enzymes Control of reactions
Terminology
Linear and cyclic
Energy
ATP
Enzymes
Control of reactions