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Energy
The capacity to do work or cause particular changes
Life is sustained by the trapping and use of energy
Use of energy is made possible by the action of enzymes
Energy and work
Chemical work
The synthesis of complex biological molecules from simpler precursors
Energy and work
Transport work
The ability to transport molecules against a concentration gradient (uptake of nutrients, elimination of waste, maintenance of ion balance)
Energy and work
Mechanical work
Changing the location of organisms, cells and structures within cells
The flow of carbon and energy
Source of most biological energy is sunlight
Phototrophs trap light energy during photosynthesis
The flow of carbon and energy
Chemolithoautotrophs derive energy from the oxidation of inorganic molecules
Energy from photosynthesis and chemolithoautotrophy can then be used to transform CO2 into organic molecules
The flow of carbon and energy
Chemoheterotrophs can use organic molecules as carbon and energy sources
Energy is released as organic molecules are oxidized to CO2
Oxidation and reduction
Loss of electrons is oxidation (LEO)
Gain of electrons is reduction (GER)
Aerobic respiration is when O2 acts as the final electron acceptor (O2 H2O)
Adenosine 5´-triphosphate (ATP)
ATP serves as the major energy currency of cells
Contains 2 high energy bonds
ATP ADP + Pi + Energy
Energy + ADP +Pi ATP
Pi = orthophosphate
Energy cycle
The laws of thermodynamics
1. Energy can neither be created or destroyed
The total amount of energy in the universe remains constant (although it can be redistributed)
The laws of thermodynamics
2. Physical and chemical processes proceed in such a way that the randomness of the universe increases to the maximum possible
Entropy
A measure of the randomness or disorder of a system
The greater the disorder the greater the entropy
Free energy and reactions
G = H - T x S
G = change in free energy (amount of energy available to do work)
H = change in enthalpy (heat content)
T = temperature in Kelvin (C + 273)
S = change in entropy
Free energy and reactions
G = H - T x S
A reaction with a large positive change in entropy will result in a negative G value and will occur spontaneously
The change in free energy has an effect on the direction of a reaction
Free energy and reactions
G = H - T x S
When G is determined under standard conditions of, pressure, pH and temperature the G is called the standard free energy change (G )
If the pH is set to 7, the standard free energy change is indicated by the symbol G ´
Free energy and reactions
The change in free energy has an effect on the direction of a reaction
Keq = the equilibration constant
Free energy and reactions
When G ´ is negative, the Keq is greater than 1 and the reaction goes to completion as written
The reaction is said to be exergonic
Free energy and reactions
When G ´ is positive, the equilibrium constant is less than 1 and the reaction is not favored
The reaction is said to endergonic
ATP and metabolism
A major role of ATP is to drive endergonic reactions to completion
ATP links energy-yielding reactions with energy-using reactions
Oxidation-reduction reactions
The release of energy normally involves oxidation reduction reactions (redox reactions)
Electrons move from an electron donor to an electron acceptor
Acceptor +ne- donor
(n = number of electrons transferred)
Oxidation-reduction reactions
2H+ + 2e- H2
The equilibrium constant of a redox reaction is called the standard reduction potential (E)
The reference standard for reduction potentials is the hydrogen system with an E ´ of - 0.42 volts
Each hydrogen atom provides 2 protons and 2 electrons
Oxidation-reduction reactions
Redox couples with more negative reduction potentials will donate electrons to couples with more positive potentials (and a greater affinity for electrons)
Oxidation-reduction reactions
Electron tower with most negative reduction potentials at the top
Electrons move from donors to acceptors from more negative to more positive potentials
Electron carriers
Various carriers serve to transport electrons to different parts of the cell
Example - Nicotinamide adenine dinucleotide
NADH + H+ + 1/2 O2 H2O + NAD+
NAD+/ NADH is more negative than 1/2 O2/ H2O, so electrons will flow from NADH (donor) to O2 (acceptor)
Electron carriers
Structure of NAD
Flavin adenine dinucleotide (FAD)
Proteins bearing FAD (or FMN) are referred to as flavoproteins
Coenzyme Q (CoQ) or ubiquinone
Transports electrons and protons in respiratory electron transport chains
Cytochromes
Cytochromes use iron atoms to transport electrons by reversible oxidation and reduction reactions
Iron atoms in cytochromes are part of a heme group
Nonheme iron proteins carry electrons but lack a heme group (e.g. Ferrodoxin)
Enzymes
Enzymes can be defined as protein catalysts
Increase rate of reactions without being permanently altered
Reacting molecules = substrates
Substances formed = product
Structure of enzymes
Some enzymes are composed purely of protein
Some enzymes contain both a protein and a nonprotein component
The protein component = apoenzyme
The nonprotein component = cofactor
Apoenzyme + cofactor = holoenzyme
Structure of enzymes
Cofactor tightly attached to apoenzyme = prosthetic group
Loosely bound cofactor = coenzyme
Classification of enzymes
Enzymes can be placed in one of six classes
Usually named in terms of substrates and reactions catalyzed
Mechanisms of enzyme activity
Enzymes serve to speed up the rate at which a reaction proceed to equilibrium by lowering the activation energy
Activation energy required to from the transition state (AB)
Mechanisms of enzyme activity
The enzyme may be rigid and shaped to precisely fit the substrate
Binding to substrate positions it properly for reaction
Referred to as the lock-and-key model
Mechanisms of enzyme activity
Some enzymes change shape when they bind their substrate so that the active site surrounds and precisely fits the substrate
Referred to as the induced fit model
