1. An Introduction to Metabolism
Chapter 8

2. Metabolism Chemical reactions are occurring continuously in our bodies
The totality of these reactions is called metabolism
There are two broad categories of metabolic reactions:
Anabolic pathways
Catabolic pathways

3. Metabolic Reactions Anabolic Catabolic
Link simple molecules to form more complex molecules
Require the input of energy and capture it in the chemical bonds that are formed
Ex) photosynthesis making glucose or the synthesis of a protein from amino acids
The breakdown of complex molecules into simpler ones
Energy is released from the chemical bonds
Ex) cellular respiration breaking down glucose

4. Catabolic and anabolic reactions are often linked
Catabolic and anabolic reactions are often linked. The energy released in catabolic reactions is often used to drive anabolic reactions.
Ex) the energy released by the breakdown of glucose (catabolism) is used to drive anabolic reactions such as the synthesis of nucleic acids and proteins.

5. Energy
Energy comes in many forms: chemical, heat, electrical, light, and mechanical
All forms of energy can be categorized into to basic types:
Potential
Kinetic

6. Types of Energy Potential Kinetic
The energy of state or position; stored energy
Can be stored in many forms – in chemical bonds, as a concentration gradient, or even as an electric charge imbalance
The energy of movement
Energy that does work and makes things change

7. Example of an Energy Conversion
A cat leaping requires the potential energy stored in the covalent bonds of its muscles (chemical energy) to be converted into the kinetic energy of muscle contractions (mechanical energy)

8. Laws of Energy Conversions
1st Law of Thermodynamics
2nd Law of Thermodynamics
Energy can be transferred and transformed, but it cannot be created or destroyed
In any conversion of energy, the total energy before and after the conversion is the same.
Every energy transfer or transformation increases the entropy of the universe
Entropy is a measure of disorder
Disorder (entropy) tends to increase

9. Ex) when we eat (chemical energy) some is converted into kinetic energy to make us move but most is converted into heat, which is energy associated with the random motion of atoms or molecules.
In other words, no physical process or chemical reaction is 100 percent efficient; some of the released energy is lost to a form associated with disorder
Think of disorder as a kind of randomness due to the thermal motion of particles; this energy is of such a low value and so dispersed that it is unusable

10. ∆G = G final state – G initial state
Free Energy
Free energy (G) refers to energy that can perform work
Scientists study free energy to determine if chemical reactions will occur spontaneously
This is done by comparing the amount of free energy before the reaction occurs to the amount of free energy after the reaction has occurred
Change in free energy = ∆G
∆G = G final state – G initial state

11. Unstable systems have high G (able to do a lot of work)
Stable systems have low G (not able to do a lot of work)
This means systems tend to change from unstable conditions to stable conditions OR
Systems change from more organized to less organized
Remember: 2nd law of thermodynamics – the universe follows an unstoppable trend toward disorder (entropy)

12. Exergonic and Endergonic Reactions
Releases free energy
Spontaneous (-∆G)
Absorbs free energy
Non-spontaneous (+∆G)

13. Systems at Equilibrium
Reactions in a closed system eventually reach equilibrium and then do no work
Cells are not in equilibrium; they are open systems experiencing a constant flow of materials
A defining feature of life is that metabolism is never at equilibrium

14. Energy Coupling A cell does three main kinds of work:
Chemical
Transport
Mechanical
To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one
Most energy coupling in cells is mediated by ATP

15. ATP Structure
ATP (adenosine triphosphate) is the cell’s energy shuttle
ATP is composed of ribose (a pentose sugar), adenine (a nitrogenous base), and three phosphate groups
The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis
Energy is released from ATP when the terminal phosphate bond is broken

16. How ATP Performs Work
In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction
Overall, the coupled reactions are exergonic
ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant

18. ATP Fun Facts
The total quantity of ATP in the human body is about 0.1 mole (6.02x1023 molecules).
The energy used by human cells requires the hydrolysis of 200 to 300 moles of ATP daily.
This means that each ATP molecule is recycled 2000 to 3000 times during a single day.
ATP cannot be stored, so its consumption must closely follow its synthesis

19. Enzymes An enzyme is a catalytic protein
A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction
Enzymes catalyze reactions by lowering the activation energy barrier
The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA)
Activation energy is often supplied in the form of heat from the surroundings
Enzymes do not affect the change in free energy (∆G); instead, they speed up reactions that would occur eventually

20. How Enzymes Work
The reactant an enzyme acts on is called a substrate. The substrate will bind to a spot on the enzyme called the active site forming an enzyme-substrate complex.
An enzyme is very specific; it will recognize its specific substrate (reactant) even among closely related compounds, such as isomers.
As the substrate binds to the active site, interactions between the chemical groups of the substrate and the R-groups of the active site cause the enzyme to change shape slightly so the active site fits more snugly around the substrate; this is called an induced fit.

23. Conditions that Effect Enzyme Activity
Temperature – enzymes function best in specific temperature ranges. When temperatures get too hot, the enzyme denatures
The active site may change shape which prevents the substrate from entering
Other conditions include pH
Salt concentration
Inhibitor molecules

24. Enzyme Inhibitors
Competitive inhibitors bind to the active site of an enzyme, competing with the substrate
Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective

25. Allosteric Regulation
Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site
Most allosterically regulated enzymes are composed of more than one polypeptide, each with its own active site
The entire complex is usually activated at all times or inactivated
All subunits are effected by the attachment of a single activator or inhibitor

26. Feedback Inhibition
In feedback inhibition, the end product of a metabolic pathway shuts down the pathway
When enough product is produced, the product actually stops the production of more…genius!