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CHM1022: Exam Revision - Enzymes

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Monash University

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  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 groups.  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 protein function. 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 shape.  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 state.  The absorption of thermal energy increases the speed of the reactant molecules, so they collide more often and more for
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