2. Energetics and Enzymes Flashcards
1st Law of Thermodynamics
Energy can neither be created nor destroyed
Simply converted from one form to another
2nd Law of Thermodynamics
In any isolated system the degree of disorder can only increase.
Entropy
Amount of disorder in a system
Reactions proceed spontaneously towards
products with greater entropy
Gibb’s Free energy
amount of energy within a molecule that could perform useful work at a constant temperature.
Denoted by G with units kJ/mole§
Delta G
measures amount of disorder that results from a particular reaction
Calculate delta G
Free energy (Products) - Free energy (Reactants)
A reaction can only proceed spontaneously if
Delta G is negative
Pathways in cells that synthesise molecules are generally energetically unfavourable, how do they still occur?
Take place because they are coupled to an energetically favourable one.
Providing sum of delta G for overall reaction is still negative, reaction will proceed.
What do the majority of energetically unfavourable biochemical reactions rely on?
Hydrolysis of high-energy phosphate bonds e.g. those in ATP.
Phosphoanhydride bonds have a large negative delta G of hydrolysis
= High energy bonds
Delta G when converting ATP to ADP + Pi
-31 kJ/mol
In a biological setting, some energetically favourable reactions won’t occur at a rate useful for life
Unless catalysed by enzymes
Why is combustion of glucose energetically favourable?
It has a highly negative delta G
Why does glucose not spontaneously combust?
Because it has activation energy
Activation energy
Minimum energy required for a reaction to occur
How can activation energy be overcome?
Enzymes provide an alternative reaction pathway
Lowering activation energy required
Enzyme
A protein that acts a as a catalyst to induce chemical changes in other substances
Itself remaining apparently unchanged by the process
What effect do enzymes have on rate of reaction?
Drastically increase rate of reaction
Lower energy barriers impeding chemical reactions
Enzyme shape
Extremely specific due to their conformation
What do enzymes do to their substrate?
Bend their substrates in such a way that bonds to be broken are stressed
Substrate molecule resembles transition state
Transition state
Particular conformation of substrate in which atoms of the molecule are rearranged geometrically and electronically so the reaction can proceed
Makes them more amenable to reaction with other molecules
How do enzymes work?
Substrate molecule binds tightly to enzyme active site
Arrange substrate in such a way that bonds are strained.
Key residues within enzyme participate in either the making or breaking of bonds by altering arrangement of electrons within substrate in the form of oxidation (-electrons from molecule) or reduction reactions (+ electrons).
When enzymes cause oxidation/ reduction, as cellular environment is generally aqueous
Substrate molecule gains an electron & simultaneously gains a proton
