Catalysts increase the rate of spontaneous reactions (reactions with –ΔG). They do not change ΔG
values and they are not used up by catalysis. In biology, these are proteins called enzymes. Enzymes
cannot catalyse reaction with a positive ΔG (non-spontaneous), they only catalyse spontaneous
Enzymes work by lowering the activation energy, and they are crucial in our body, because at 37
degrees, most spontaneous reactions do not proceed, because the activation energy barrier cannot
The transition state is characterized by a lower activation energy, which speeds up the reaction. At
the transition state, a covalent bond is half formed and a covalent bond is half broken.
When reactants (called substrate) bind to the active site of the enzyme, they form an enzyme-
substrate complex. Then, the enzyme brings reactants closer to the transition state. The enzyme
does this by undergoing a small conformational change (a shape change) which brings the
reactants(substrates) into a transition state. As soon as the products leave the active site, the
enzyme reverts to its original shape. Inducing this transition state can happen in 3 ways:
1) by binding substrates in just the correct orientation
The enzyme places the reactants together, therefore enhancing the possibility that they will react
2) by exposing the reactants to altered charge environments that promote catalysis
A positive or a negative charge can sometimes speed up the reaction.
*pH is important: This example demonstrates the importance of a constant pH: with a different pH,
the enzyme would be inactive, because e.g. a negative charge might no longer be there.
3) by inducing a strain on the substrate that facilitates breaking of a covalent bond
It puts the reactants closer, so they will react
Many enzymes require cofactors in order to catalyze a reaction: these are small organic molecules
or ions that are not amino acids, and are associated more or less tightly with the enzyme. Enzymes are saturated when all active sites are occupied; a further increase in substrate
concentration will no longer increase the rate of product formation; this is the maximum speed of
reaction or turnover rate, which varies widely from enzyme to enzyme.
Metabolic pathways are nearly always down-regulated by the final product to avoid
overproduction. This is called end-product inhibition. Just-in-time production in modern factories
copies that process. Ensyme regulation: 2 ways
Enzymes can be regulated competitively, with the regulator binding the active site, or allosterically,
with the regulator binding somewhere else on the enzyme (allosteric = other site).
1) Competitive inhibition: regulatory molecule competes with the substrate molecule because they
bind at the same site. So, we need a lot more regulatory(competitor) molecules than substrate
2) Allosteric inhibition (non-competitive): the regulator binds to another site than the substrate.
Allosteric inhibition is much more efficient, because less inhibitor molecules are required (you only
need more allosteric regulators than enzymes).
Normally, in nature, we find only allosteric inhibition because it is more efficient. Competitive
inhibition is seen as a side effect or toxic. Cooperative allosteric transition: Cooperativity occurs with two or more identical enzyme subunits
forming an enzyme complex. In that case, when the regulator(competitor) molecule binds to the
allosteric site of one of the two subunits, it induces a conformation change (shape change) in the
subunit it binds and it induces a partial conformation change in the other subunit. This partial
conformation change facilitates binding of the second regulator molecule.
You therefore get a sigmoid (S- shaped) inhibition curve with increasing inhibitor concentration
leading to a switch- like behavior of multi-subunit enzymes. This happens because, the more
subunits there are, the more hard it is for a regulatory molecule to bind first, but then the other
subunits bind faster to the regulatory molecule. Catabolic pathways are long and complex in order to release energy slowly. That is the only way for
the cell to capture some of the energy. Burning glucose to carbon dioxide and water releases all the
free energy in one reaction, without capturing any energy for the cell.
Better to split this reaction up into many steps that release only a small amount of free energy, just
enough to keep the reactions going, and are coupled to energy storage, e.g. in the form of ATP. The
complete oxidation of glucose in cells releases -686 kcal/mol. The oxidation of glucose is very
favorable (big negative delta G). Half of that is captured as ATP, the other half is released as heat to
drive the reactions.
All the food we eat comes directly or indirectly from photosynthetic organisms, and therefore