Chapter 8- Energy and Metabolism
Energy - ability to do work
 Potential energy - stored energy (bonds, gradients)
 Kinetic energy - energy of motion (electron transfer)
 Many forms, mechanical, chemical, light, heat
 Can change forms
 Governed by the Laws of Thermodynamics
Metabolism - the sum total of all chemical reactions in a living system
 Anabolic reactions - synthesis
 Catabolic reactions - decomposition
 Regulated by enzymes
 Law of Mass Action
Review of Basic Thermodynamics
 First Law of Thermodynamics:
o Energy can be neither created nor destroyed but may change form or be
transferred. The amount of energy in the universe is constant. (examples:
potential to kinetic, light to chemical, chemical to mechanical)
 Second Law of Thermodynamics:
o No transfer of energy is 100% efficient because there is always a loss of energy as
heat to surroundings.
o Disorder (entropy) in the universe is continuously increasing.
o Energy transformations proceed spontaneously to convert matter from a more
ordered, less stable form to a less ordered, more stable form
o This predicts direction of reaction
o See Page 146 in textbook
Free Energy & Endergonic & Exergonic Reactions
 Free Energy:
o Energy available to break or form bonds, i.e. available to do “work”
 Endergonic Reaction:
o Free Energy of the reactants is LESS than the products
o Requires input of energy to proceed
o Low entropy
 Exergonic Reaction:
o Free energy of the reactants is GREATER than the products
o Proceeds spontaneously and releases energy
o High Entropy
Activation Energy - see page 148 in textbook
 Energy required to destabilize existing chemical bonds and initiate a chemical reaction
 All chemical reactions require the input of at least some activation energy
 Rate of a reaction depends on the activation energy necessary to initiate it.
 Does NOT change the direction of the reaction


Catalysis speed up the RATE of a reaction by lowering the activation energy
Enzymes are organic catalysis used in biological system
“Waste Not Want Not”
Law of Mass Action - Le Chatelier’s Principle
 When all other conditions are constant, the rate of reaction is proportional to the
concentrations of the reactants and products
 Direction of a chemical reaction can be determined by change in temperature, pressure,
OR concentration of reactants and products (as in most biological systems)
 Basis of reversible reactions
A+B
C+D
equilibrium
Coupled Reactions and the Transfer of ENERGY - see page 145 in textbook and handout
1. Oxidation-Reduction Reactions
a. Chemical energy stored in bonds can be transferred by its electrons
b. Transfer of electrons/hydrogen = transfer of energy
c. Molecules lose energy when oxidized (transfer electrons)
d. Molecules gain energy when reduced (gain electrons)
e. Usually occurs in sequential, step-down reactions, from high to low energy levels,
releasing energy in a controlled manner
f. Energy, electrons, and/or hydrogen are transferred to energy storing molecules
i. ex: NAD, NADP, FAD (co-enzymes)
2. Linking an exergonic reaction to an endergonic reaction
a. to create an overall exergonic, spontaneous reaction
b. Role of ATP in driving endergonic reactions - release 7.3 kcal/mole when
hydrolyzed into ADP and P (exergonic) (page 154, fig. 8.14 ATP)
c. ATP cycle
d. Since 7.3 kcal is more than most endergonic reactions require, the coupled
reaction is over exergonic and proceeds spontaneously.
3. Substrate-Level Phosphorylation
a. Transfer of phosphate group from ATP (exergonic) to another molecule (a
reactant) to raise its potential energy to drive and endergonic reaction
b. Phosphorylated molecule (Reactant/Substrate) has more energy than the
products; therefore overall reaction is exergonic and proceeds spontaneously.
c. Example: Cellular respiration: Glucose + ATP  Glucose 6-P
“Not reactive”
Reactive
Low energy
“energized”
Enzymes - see page 150 and handout 153
 Organic Catalysts (globular proteins)
 Speed up rate of reaction by lowering activation energy
 Do not change direction of reaction
 The remain changed and REUSABLE
 Active site = binding site- holds substrate

Catalytic site = stresses bonds of substrate, lowering activation energy, causes reaction
to occur faster
Induced Fit Mechanism - page 150 in textbook
Binding of the substrate causes configurationally change in active site, bringing the catalytic site
into closer proximity to the bonds in the substrate(s) to make or break bonds.
Enzymes Take Many Forms
1. Multienzyme Complexes - see page 155
a. Several enzymes grouped together to catalyze a biochemical pathway
b. Efficient because the product of one reaction is present as reactant for the next
reaction without diffusing away
c. Prevents unwanted side reactions
d. Controlled as one unit
e. Part of other structures, like c.m.
f. Not free floating like most enzymes.
2. Non Protein Enzymes
a. Ribozymes - RNA catalysts
b. Two types of ribozymes:
i. Folded Structure and acts on itself, editing out portions
ii. Edit other RNA molecules, especially mRNA
c. Their discovery helps answer the question “which came first proteins or nucleic
acids?”
d. Maybe RNA evolved first & catalyzed protein synthesis
Factors that affect enzyme activity
1. Co-enzymes and Co-factors
a. Molecules that are associated with enzymes that help with its catalysis of a
reaction
b. Co-enzymes - organic molecules, vitamin-based, loosely attached/movable,
increases the activity of the enzyme
examples: NAD, NADP help dehydrogenases by accepting the removed H+ and
electrons (see page 153, fig 8.13 NAD)
c. Co-factors - inorganic molecules or ions (minerals) help stabilize enzyme’s
tertiary structure or assist with reaction
Examples: Zn+, Ca+2, Mg+2 work with kinases to help transfer phosphates
d. Holoenzymes - enzymes with “helper” molecules to assist with substrate binding
2. Temperature - See page 152 and textbook
a. Environmental factor
b. Optimum temperature maintains most stable configuration of active site for
best rate of catalysis
c. Optimum temperature is specific for each enzyme, usually 37°C
d. Below optimum i. Slows rate of Enzyme-Substrate collisions
ii. Enzyme becomes more rigid no induced fit
iii. Slows rate of catalysis
iv. Reversible
e. Above optimum i. Denaturation of protein (enzyme) by disrupting H-bonds over 40°C
ii. Irreversible over 60°C
3. pH - see textbook page 152 and handout
a. Environmental factor
b. Stabilizes protein structure at optimum pH
c. Change in pH (H+) upsets balance of charges and breaks ionic and H-bonds
changing shape of enzyme’s active site (denaturation)
d. Optimum pH varies with each enzyme
e. Change in salinity (salts) in environment has same effect as change in pH
4. Effect of Enzyme and/or Substrate Concentrations
a. Enzymes exhibit saturation
i. When all are occupied*, rate of reaction remains constant if there is excess
substrate
ii. *Maximum number of enzyme-substrate complexes formed
iii. Number of enzyme molecules is a limiting factor of reaction rate
iv. Keeping substrate concentration fixed also levels the rate of reaction
Regulation of Enzyme Activity
1. Zymogens
a. Pre-enzymes produced inactive form with an amino acid chain blocking the
active site
b. Requires another enzyme/molecule to cleave off the amino acid chain when
enzyme is needed (signaling)
c. ex: proteases in stomach & small intestine secreted as zymogens to protect
lining
Pepsinogen (inactive) + HCL  Pepsin (active form)
zymogen
removes; blocking amino acid chain from active site
2. Competitive Inhibition - see Page 153 and handout 156
a. Two molecules (substrate & analog/competitive inhibitor) compete for the
active site
b. Analog fits into active site and blocks substrate
c. Increasing substrate reverses inhibition
d. Antibiotics are competitive inhibitors of bacterial enzymes that synthesize cell
walls
e. Sometimes end products of biopathway may be an analog to early enzyme to stop
reaction - Feedback Inhibition, to conserve reactants, energy
3. Non-Competitive Inhibition
a. Inhibitor molecules attaches to enzyme somewhere other than the active site
(non-specific)
b. Causes configurational change in enzyme’s active site
c. Reversible if inhibitor can be drawn away (Not by adding more substrates)
d. Irreversible if inhibitor is permanently bonded and denatures protein (ex: lead
poisoning, nerve has)
4. Allosterism
a. A type of non-competitive inhibition when the enzyme has a specific binding site
for the allosteric effector (molecule that effects change in enzyme)
b. Allosteric enzymes have 2 binding sites - Active site and Allosteric site
c. Allosteric effector molecule binds to the allosteric site and changes shape of
active site
i. Allosteric inhibitors - “close” active site, blocking substrate from binding
ii. Allosteric activator - “open” active site, attracting the substrate
d. Enzymes alternate between being activated or inactivated depending on the need
of the cell - Homeostastis Regulation!!
e. Allosteric enzymes are regulatory enzymes for certain biopathways by feedback
inhibition
5. Cooperativity
a. More than one active site per enzyme
b. Binding of one substrate induces fit of all the others
c. Efficiency