Chapter 6 - Metabolism: Energy & Enzymes

Diffusion: passive process where molecules naturally spread due to random motion, no energy needed

Active transport: requires ATP to move molecules against concentration gradient, often facilitated by carrier proteins in cell membrane

Osmosis: refers to movement of water across semi permeable membrane, always moving from region with low solute to high solute

Equilibrium: equal concentration on both sides of the membrane 

Energy is the ability to do work or bring about a change

Kinetic energy: energy of motion, mechanical (water going over waterfall, avalanche, breaking dam)

Potential energy: stored energy, chemical (food)

Two laws of thermodynamics-

First law: Law of Conservation of Energy

• energy is not created or destroyed, but transformed

Second law: Law of Entropy (Disorder)

• energy is lost as heat during transformations

Solar energy gets transformed into chemical energy by plants. Some energy gets lost as heat. 

Energy transformations/reactions increase entropy (disorder of the universe)

• Celluar metabolic reactions are energy transformed

• Every reaction in cells increases the total entropy in the universe, leaving less energy available to do useful work

• In a reaction, high entropy reactants may form low entropy

• Although the products become more ordered and less stable, the universe will increase in entropy or disorder or the energy due to heat being lost in energy conversions

Metabolism

• the sum of cellular chemical reactions in a cell

• reactants participate in reaction

• products form as a result of a reaction

Free energy is the amount of energy available to preform work

Exergonic reactions- products have less free energy than reactants (release energy) (occurs spontaneously) (-ΔG)

Endergonic reactions- products have more free energy than reactants(require energy input) (nonspontaneous) (+ΔG)

ATP: Energy for cells

• Adenosine triphosphate (ATP)

• high energy compound used to drive metabolic reactions

• ATP not stored by cells, constantly being generated from adenosine disphosphate (ADP)

Composed of adenine, ribose (together= adenosine), and three phosphate groups

Coupled reactions:

• energy released by an exergonic reaction is captured in ATP

• ATP is used to drive endergonic reaction 

ATP is unstable and has high potential energy. 

AMP→ ADP → ATP

Hydrolysis of ATP: ΔG = -7.3 kcal/mole

• reaction favors information of products

• energy liberated can drive a variety of cellular processes

ATP during muscle contraction: 

1. myosin assumes its resting shape when it combines with ATP

2. ATP splits into ADP and P, causing myosin to change its shape anda llowing it to attach to actin

3. Release of ADP and P causes myosin to again change shape and pull against actin, generating force and motion

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Chapter 6 - Cont.

Reactions normally occur in a sequence called a metabolic pathway

1. Begins with a reactant/substrate

2. As reactant/substrate is changed by enzyme, it proceeds through several intermediate structures and terminates with a particular end product

3. The intermediate product of a prior reaction serves as the substrate of the next reaction until the end product is made. A new enzyme is needed to change each intermediate. 

4. In other words, the product of the first reaction is the substrate of the second and so on, except in the case of G.

A→B→C→D→E→F→G

Enzymes:

1. Proteins that function as catalysts

2. Each enzyme accelerates a specific reaction by lowering the activation energy needed to start that reaction!!

3. The reactants of an enzymatically catalyzed reaction are substrates

4. Enzymes are specific and fit like a lock and key to the substrates shape and size

5. Each reaction in a metabolic pathway requires a specific enzyme to bind to a specific substrate

6. The substrates are changed to products

7. The end will not be formed unless ALL enzymes are in the pathway are present and function

8. The enzyme is unchanged and ready to participate in another reaction

Enzymes:

• catalyst

• proteins

• lower EA

• unchanged by reaction

• specific

• “lock and key” → induced fit model

• whatever enzyme acts upon is considered substrate

Induced fit model:

• the enzyme is induced to undergo a slight alteration to achieve optimum fit for the substrates
  
  

Enzymatic reactions

1. Degradation (Catabolic)

• enzyme binds t substrate molecule

• substrate is broken apart into 2 product molecules which are released

2. Synthesis (Anabolic)

• enzymes binds 2 substrate molecules

• substrates are joined together and released as single product molecule

• Energy must be added to at least one reactant to initiate the reaction: 

Energy of Activation

• Ea (the energy os activation) prevents molecules from spontaneously degrading in the cell

Enzyme operation:

• Enzymes catalyze reactions by lowering the energy of activation

• This brings substrates into contact with one another

• This influences the rate of reaction by allowing the reaction to start sooner

Factors affecting enzymatic speed/activty

1. Cells - regulate the presence/absense of an enzyme

2. Substrate concentration - enzyme activity increases w/ substrate concentration due to more frequent collisions between substrate molecules and the enzymes

3. Temperature - enzyme activity increases with temp.

4. pH - most enzymes are optimized for a specific pH

5. Cofactors and coenzymes - binding to the active site of enzyme

Enzyme Saturation: plateau where nearly all active sites are occupied by a substrate

Vmax = maximal rate of reaction

Km = substrate concentration at half the maximal rate (Km = ½ of Vmax)

• If working w/ an enzyme with a high Km, a sigher concentration of a substrate should be used

Enzyme activity increases with temperature.

Warmer temperatures cause more effective collisions between substrates.

However, hot temperatures can denature and destroy enzymes.

Enzyme cofactors-nonprotein:

• molecules required to activate enzyme

• FAD and NAD+ are cofactors in cellular respiration

• NADP+ are cofactors in photosynthesis

Coenzymes are nonprotein organic molecules

• Vitamins are small organic compounds required in the diet for synthesis of coenzymes

• Some enzymes require cofactors to be active

• Cofactors are a nonprotein component of an enzyme

Substrance known as an inhibitor binds to an enzyme and decreases its activity

• Competitive inhibition - the substrate and the inhibitor are both able to bind to the active site and they compete with one another

• Noncompetitive inhibition- the inhibitor does not bind at the active site, but at an allosteric (other) site

• A change in shape initiated by inhibitor binding to the allorteric site changes the shape of the active site, making it unable to bind substrate

Competitive:

• molecules binds to active site

• inhibits ability of substrate to bind enzyme

• more substrate needed, competition can be overcome

Noncompetitive:

• inhibitor binds to alloristic site, not the active site 

• Molecules that irreversaibly inhibit an enzyme are poisons 

1. Cyanide inhibited enzymes required for ATP production

2. Sarin inhibits an enzyme located at the neuromuscular junction

3. Warfin inhibits an enzyme responsible for the blood clotting process

Positive feedback: one direction, up or down, almost always bad (ex: blood clotting, childbirth)

Negative feedback: make a correction, up down up (homeostasis), (ex: blood sugar regulation)

• In feedback inhibition, the accumulation/concentration of product can turn a metabolic pathway off or on

• Positive feedback causes a cell to synthesize increased product

• Negative feedback inhibition prevents a cell from wasting chemical resources by not synthesizing more product than is needed

Oxidation-reducation (redox) reactions

• Electrons (energy) passes from one molecule to another

• Oxidation: loss of electron

• Reduction: gain of electron

• OILRIG (oxidation is loss, reduction is gain)

• Both take place at the same time

Example: NaCI- sodium is oxidized and chlorine is reduced