Enzymes Flashcards
What are enzymes?
Enzymes are biological catalysts.
How are enzymes named?
By adding the suffix -ase to the substrate that they catalyse e.g. urease
What are the classification of enzymes?
Hydrolases - hydrolysis reactions
isomerases - transfer of groups within a molecule
ligases or synthases - bond formation coupled by ATP hydrolysis
How do enzymes increase the reaction rate?
Enzymes lower the activation energy by binding the substrate and forming an enzyme-substrate complex
What is the mechanism of an enzyme with a substrate?
An enzyme is specific - binds to only one substrate. It is a lock and key mechanism.
How do enzymes decrease the activation energy?
The enzymes hold the substrates at certain positions and angles to improve the reaction rate. Enzymes formation of an enzyme-substrate complex cause slight changes in the 3D shape which can also promote activity. In multisubstrate enzyme-catalysed reactions (enzymes with more than one active site) enzyme can hold substrate so that reactive regions of substrates are close to each other (proximity effect).
How does the enzyme activity change with temperature?
As the temperature increases up to a certain limit, the rate of enzyme catalysed reactions increase. This is because the number of successful collisions and the number of molecules with sufficient activation energy increases. Beyond this limit, the enzyme activity decreases with temperature.
What causes the enzyme activity to decrease above a certain temperature?
The enzyme becomes denatured. The active site will change shape and can no longer bind to the specific substrate.
Up until the peak of enzyme activity, what is the equation for the rate of enzyme conversion of substrate?
Vm = k2 * E0
What is the rate of enzyme conversion above this temperature
Vm = k2 * E
What is the rate constant given by according to the Arrhenius equation?
k2 = Ae^(-Ea/RT)
Does a small rise in temperature affect enzyme activation or enzyme denaturation more?
It will affect Enzyme denaturation much more
What are the two reasons for the change of enzyme activity with pH?
- The ionic groups on the active site of the enzyme will change
- The three-dimensional shape of the enzyme will change
Is the activity of enzymes limited by the pH?
Yes, there is an optimum pH at which an enzyme will function
What is enzyme immobilisation?
The restriction of enzyme mobility in a fixed space
What are the advantages of enzyme immobilisation?
Enzyme reutilisation, elimination of enzyme recovery and purification processes, product purity is usually improved, effluent handling problems are minimised by immobilisation
Why is catalyst reuse critical for many processes?
Enzymes are expensive
For which enzymes in the body are immobilised membranes a good model system?
Some intracellular enzymes are membrane bound, the enzymes in this process are able to mimic and understand the action of membrane-bound intracellular enzymes.
Disadvantages of immobilised enzymes
Many immobilised enzymes exhibit lower activity compared to free enzymes, more expensive to prepare than free enzymes, mass transfer limitations due to immobilisation methods
The two categories of immobilisation
- Entrapment (Matrix-Entrapped and Membrane- Entrapped)
- Bound (Adsorbed and Covalently Bound)
What is matrix entrapment?
The enzyme solution is mixed with polymeric fluid that solidifies into various forms, depending on application. The polymeric fluid is semi-permeable. Large molecular weight enzymes cannot diffuse out, but smaller substrate and product molecules can
Give some examples for matrices fro enzyme entrapment
Agar, polyacrylamide, collagen
What is membrane entrapment?
Enzyme solutions may be confined between thin semi-permeable membranes.
Give some examples of membrane materials
Nylon, cellulose, polyacrylate
What is surface adsorption?
The attachment of enzymes to stationary solids by weak physical forces e.g. van der Waals forces. The active site of the enzyme is normally unaffected and nearly full activity is observed
What is surface immobilisation by covalent bonding?
The retention of enzyme on support surfaces by covalent bonding between functional groups on the enzyme and those on the support surface.
What are the functional groups on the enzyme?
amino, carboxyl, hydroxyl, sulfhydryl
What is the condition for the surface immobilisation by covalent bonding on the enzyme?
The active site cannot participate in covalent bonding
How can the covalent bonding of the active site to the immobilisation surface be prevented?
Enzyme inhibitors can be used.
Why are there diffusional limitations on immobilised enzyme systems?
The substrate must diffuse from the bulk solution up to the surface of the immobilised enzyme prior to reaction.
What is the Damkohler number?
Da = maximum rate of reaction/ maximum rate of diffusion = (Vm/ kL[Sb})
where kL is the mass transfer coefficient
[Sb] is the substrate concentration in bulk liquid
Why is Damkohler number important?
Determines whether limitations on intrinsic enzyme kinetics are observed or not
What happens if Da»1 ?
Diffusion rate is limiting the observed rate.
What happens if Da«1?
The reaction rate is limiting, the observed rate is not being affected by rate of diffusion.
