THE ROLE OF ENZYMES
Proteins have many structures, resulting in a wide range of functions
Enzymatic proteins regulate metabolism by acting as catalysts.
Catalysts – chemical agents that selectively speed up chemical reactions
without being consumed by the reaction.
An enzyme can perform its function over and over again, so it keep cells
running by carrying out the processes of life.
Polypeptides – polymers of amino acids linked by peptide bonds.
A protein consists of one or more polypeptides, each folded and coiled
into a specific 3D structure.
Amino acid monomers
Amino acids are organic molecules possessing both carboxyl and amino
Alpha carbon – the centre of the amino acid.
An amino acid comprises of an amino group, a carboxyl group, a hydrogen
atom, and a variable group (R).
The R group is also called the side chain, differs with each amino acid.
The physical and chemical properties of the side chain determine the
unique characteristics of a particular amino acid, thus affecting its
functional role in a polypeptide.
Acidic amino acids are those with side chains that are generally negative
in charge owing to the presence of a carboxyl group, which is dissociated
at cellular pH.
Basic amino acids have amino groups in their side chains that are
generally positive in charge.
Because they are charged, acidic and basic side chains are hydrophilic.
Amino acid polymers
When two amino acids are positioned so that the carboxyl group of one is
adjacent to the amino group of the other, they can become joined by a
dehydration reaction, with the removal of a water molecule.
Protein structure and function
The specific activities of proteins result from their detailed 3D structure,
the simplest level of which is the sequence of their amino acids.
It is the amino acid sequence of each polypeptide that determines what
3D structure the protein will have.
When a cell makes a polypeptide, the chain folds spontaneously.
The function of a protein depends on its ability to recognise and bind to
some other molecule. A change in primary structure
A simple change in protein structure can have a devastating effect on
What determines protein structure?
A polypeptide chain of a given amino acid sequence can spontaneously
arrange itself into a 3D shape determined and maintained by the
interactions responsible for secondary and tertiary structure.
Protein structure also depends on the physical and chemical conditions of
the protein’s environment.
If the pH, salt concentration, temperature, or other aspects of its
environment are altered, the protein may unravel and lose its native
shape – denaturation.
Most proteins become denatured if they are transferred from an aqueous
environment to an organic solvent, ether or chlorofoam.
Other denaturation agents include chemicals that disrupt the hydrogen
bonds, ionic bonds, and disulphide bridges that maintain a protein’s
Denaturation can also result from excessive heat, which agitates the
polypeptide chain enough to overpower the weak interactions that
stabilize the structure.
The activation energy barrier
Activation energy –the energy required to contort the reactant
molecules so the bonds can break.
Transition state – the reactants are in an unstable condition.
Activation energy is often supplied in the form of heat that the
reactant molecules absorb from the surroundings.
The bonds of the reactants break only when the molecules have
absorbed enough energy to become unstable – to enter the transition
The absorption of thermal energy increases the speed of the reactant
molecules, so they collide more often and more for