Metabolism: Energy & Enzymes CHAPTER 6 Metabolism: Energy & Enzymes

I. Energy & the Cell A. Energy - capacity to perform work 1. Kinetic - energy of motion; that is doing work a. Heat - movement of molecules 2. Potential - stored energy a. Due to location or arrangement 3. Chemical - potential energy of molecule 12/6/2018

II. Laws of Thermodynamics A. Thermodynamics - study of energy transformations that occur in matter 1. Free Energy - energy in a system available for doing work a. First Law of Thermodynamics Total amount of energy in the universe is constant.  Energy cannot be created or destroyed but can be changed from one form into another 12/6/2018

B. 2nd Law of Thermodynamics 1. Energy changes are accompanied by a loss of useable energy and an increase in disorganization. a. Entropy = amount of disorder Example = heat 2. Total entropy of the universe is increasing a. If a particular system becomes more ordered, then its surroundings become more disordered 12/6/2018

3. ∆G = Change in Free Energy a. Measured at 25oC, 1 atm, pH 7.0, [1 mole/liter] Initial Reactants Final Products A + B C + D Gi Gf ∆G = Gf - Gi b. If ∆G is (-) reaction is exergonic c. If ∆G is (+) reaction is endergonic 12/6/2018

III. Reactions & Energy Metabolism - sum of chemical reactions in a cell. 1. Involves turning reactants into products. B. Endergonic Reactions 1. Require a net input of energy 2. Start with reactants with less free energy. 3. Yield products that are high in free energy. Thus, products store energy (∆G is positive) a. Energy stored in covalent bonds 12/6/2018

Carbohydrate Synthesis Example of endergonic reactions 12/6/2018

C. Exergonic Reactions 1. Release energy 2. Start with reactants whose bonds contain more energy 3. Yield products that are lower in free energy (∆G is negative) 4. Releases energy to surroundings a. Example = burning wood 12/6/2018

Carbohydrate Metabolism Example of exergonic reactions 12/6/2018

IV. ATP Shuttles Energy in Cell A. Cells couple exergonic reactions with endergonic ones 1. First obtain energy from an exergonic reaction i.e. Glucose  ATP 2. Then use energy of ATP to drive an endergonic reaction B. Such energy coupling is crucial to cells’ functioning 12/6/2018

C. Structure of ATP (A-P-P~P) 1. ATP is composed of 3 parts: a. adenine, a nitrogenous base b. ribose, a 5-carbon sugar c. a chain of 3 phosphate groups 2. The covalent bonds connecting the 2nd & 3rd phosphate groups are unstable (symbolized by ~) a. These can be readily broken by hydrolysis 12/6/2018

Structure of ATP

D. Functioning of ATP 1. 3 things happen when bond breaks: a. phosphate is removed b. ATP becomes ADP c. energy is released 2. Then 3rd phosphate is transferred to another molecule. This is called phosphorylation. a. Gives energy to new molecule 12/6/2018

The ATP Cycle 12/6/2018

E. 3 Functions of ATP 1. Chemical Work a. Supplies energy to synthesize macromolecules 2. Transport Work a. Gives energy to pump substances across cell membrane 3. Mechanical Work a. Supplies energy to move muscles, cilia, flagella, chromosomes 12/6/2018

F. Coupled Reactions a. In coupled reactions, the energy released by an exergonic reaction is used to drive an endergonic reaction. b. ATP breakdown is often coupled to cellular reactions that require an input of energy. 12/6/2018

Coupled Reactions Figure 6.4 12/6/2018

“A” is Initial Reactant V. Metabolic Pathways A. Reactions usually occur in a series of linked reactions 1. Products of one reaction become reactants of a later reaction Example: AB C D E FG “G” is End Product “A” is Initial Reactant Intermediates 12/6/2018

VI. Enzymes A. Protein molecules that serve as biological catalysts B. Increase the rate of a reaction without itself being changed C. Reactants in enzymatic reactions are called substrates. E. Each enzyme accelerates a specific reaction. E1 E2 E3 E4 E5 E6 A  B  C  D  E  F  G 12/6/2018

VII. How Enzymes Work A. Energy of Activation (Ea) 1. Amount of energy reactants must absorb to start chemical reaction a. Example = energy needed to break bond between 2nd & 3rd phosphate in ATP  ATP molecules don’t spontaneously break down in cells 12/6/2018

(*** Jumping bean analogy here) 2. Enzymes speed up reactions by lowering the energy activation barrier a. They do this by bringing the substrates into contact with one another (*** Jumping bean analogy here) 12/6/2018

Energy of Activation 12/6/2018

B. Enzymes are very selective 1. Enzymes have a unique 3- dimensional shape which determines their specificity 2. Only one small part of the enzyme, called the active site, binds with the substrate. a. Each enzyme recognizes only one specific substrate b. Denatured enzymes lose activity due to change in their shapes 12/6/2018

E + S ----> ES ----> E + P C. Binding Mechanisms 1. “Lock & key” idea (old idea) a. Reactive portion of substrate & active site of enzyme must fit together like a key. b. Come close enough to bond temporarily to form an enzyme- substrate complex (ES) E + S ----> ES ----> E + P 12/6/2018

2. Induced-fit hypothesis a. Many enzymes undergo conformational changes during bonding, which improve the fit and make the ES more reactive b. Enzyme & substrate must be able to make numerous & precise weak bonds with each other c. After reaction the product is released and active site returns to original state. 12/6/2018

Induced Fit Model 12/6/2018

Degradation vs. Synthesis 12/6/2018

D. Factors Affecting Enzymes : 1. Substrate Concentration a. Molecules must collide to react b. Enzyme activity increases with substrate concentration c. More collisions between substrate molecules and the enzyme d. As more substrate molecules fill enzyme active sites, more product can be made per unit time. e. Eventually all sites are filled and rate cannot increase any more. 12/6/2018

a. Affects molecular motion 2. Temperature a. Affects molecular motion b. Optimal temperature produces highest rate of contact between substrate and enzyme c. Higher temperatures denature the enzyme, altering its shape d. Humans: 35-40oC optimum 12/6/2018

Effect of Temperature on Enzyme Activity 12/6/2018

3. Salt concentration & pH a. Salt ions interfere with chemical bonds in proteins b. Extra hydrogen ions can interfere with bonding patterns  Optimal pH is usually near 7, between 6 and 8  Outside this range, normal functioning may be impaired 12/6/2018

Effect of pH on Enzyme Activity 12/6/2018

● which enzymes are present ● concentration of enzymes in cell 4. Enzyme Concentration a. Cells regulate: ● which enzymes are present ● concentration of enzymes in cell ● which enzymes are active or inactive 5. Phosphorylation a. Enzymes known as kinases activate enzymes by phosphorylating them (adding a phosphate group) 12/6/2018

Phosphorylation 12/6/2018

E. Cofactors & Coenzymes 1. Many enzymes need inorganic ions or nonprotein helpers in order to function. a. The inorganic ions are called cofactors. Examples: zinc, iron, copper b. If the cofactor is an organic nonprotein molecule it is called a coenzyme. Vitamins needed for synthesis of some coenzymes. 12/6/2018

F. Enzyme inhibition 1. Inhibitor a. Chemical that interferes with enzyme’s activity b. There are 2 types:  Competitive inhibitors  Noncompetitive inhibitors 12/6/2018

2. Competitive Inhibitor a. Resembles enzyme’s normal substrate b. Competes with substrate for active site on enzyme  Blocks substrate from entering  Prevents enzyme from acting c. This is usually reversible  Depends on concentration d. Example = CO poisoning 12/6/2018

3. Noncompetitive Inhibitor a. Does not enter active site b. Binds to enzyme at some other site called the allosteric site.  Binding changes shape of enzyme so that active site no longer fits substrate  Called an allosteric change  Enzyme exists in two forms:  active &  inactive 12/6/2018

4. Reversibility a. Irreversible  When covalent bonds form between enzyme & inhibitor b. Examples: Cyanide blocks ATP production Penicillin lethal to bacteria Mercury and lead Nerve gases 12/6/2018

5. Importance of Inhibitors a. Inhibitor is sometimes the substance the reaction produces  Example: When cell has too much ATP, ATP acts as a noncompetitive inhibitor:  prevents its own production b. This is called feedback inhibition, or negative feedback. 12/6/2018

Feedback Inhibition 12/6/2018

VIII. Oxidation-Reduction Reactions A. Electrons pass from one molecule to another. 1. Oxidation is the loss of electrons a. The molecule loses an electron has been oxidized. 2. Reduction is the gain of electrons b. The molecule gaining an electron has been reduced. Both take place at the same time. One molecule accepts electron given up by another. 12/6/2018

B. Summary of Redox 12/6/2018

C. Photosynthesis 1. Equation for Photosynthesis 6CO2 + 6H2O + energy --> C6H12O6 + 6O2 a. Water is oxidized since it loses hydrogen atoms b. Carbon dioxide is reduced since it gains hydrogen atoms c. NADP+ is a coenzyme that accepts electrons and passes them to reactions that form glucose 12/6/2018

D. Cellular Respiration 1. Equation for Aerobic Respiration C6H12O6 + 6O2 --> 6CO2 + 6H2O + ATP a. Glucose is oxidized since it loses hydrogen atoms b. Oxygen is reduced since it gains hydrogen atoms c. NAD+ is a coenzyme that accepts hydrogen ions and electrons 12/6/2018

E. Electron transport chains 1. Ordered groups of molecules embedded in membranes of the mitochondria and chloroplasts a. Electrons are passed along membrane-bound carriers  Everytime an electron is transferred to a new carrier, energy is released  The energy is used to produce ATP 12/6/2018

Electron Transport Chain 12/6/2018

c. Establishes an electrochemical (H+) gradient across the membrane. b. Electron energy is used to pump hydrogen ions (H+) to one side of membrane. c. Establishes an electrochemical (H+) gradient across the membrane. d. Special ATP synthase complexes span the membrane. Each one allows H+ ions to flow down their gradient. The flow of ions provides energy to produce ATP from ADP. ♣ This is called chemiosmosis. 12/6/2018

Chemiosmosis 12/6/2018