Chapter 5 Flashcards
What are enzymes?
Enzymes are organic catalysts that speed up, or catalyse, biochemical reactions by lowering the activation energy required to initiate a given reaction.
Enzymes bind to a molecule called a substrate, the substrate is the reactant undergoing a reaction.
They are reusable, they usually only bind to one specific substrate, most enzyme catalysed reactions are often reversible, they catalyse reactions but don’t create new ones, enzymes have an active site which is the one area the substrate always binds to and the reaction occurs- the corresponding area on a substrate is called a binding site, Most enzymes are proteins, all enzymes are catalysts but not all catalysts are enzymes, enzymes frequently influence entire biochemical pathways by catalysing each step,enzyme names typically end with the suffix ‘ase’.
What do enzyme catalysed reactions look like?
The active site is a pocket-like area of the enzyme’s tertiary structure where the substrate fits and binds to the enzyme. Due to the compatibility of the complex three dimensional structures, we say that an enzyme’s active site and substrate are complementary in shape.
When a substrate binds to an enzyme’s active site, together they form an enzyme- substrate complex. Upon binding, the active site undergoes a conformational change to accomodate for the substrate, and the substrate undergoes a small change in turn.
Many chemical bond ( hydrogen bonds, hydrophobic interactions) hold the substrate and active site together in the enzyme-substrate complex. The reaction proceeds and the substrate is converted into the product
The product disassociates away from the complex and is released. The enzyme is then ready to bind to another substrate and repeat the process.
Define the lock and key model
The lock and key model also called Fisher’s theory is one of two models which describe the enzyme-substrate interaction. The lock and key model assumes that the active site of the enzyme and the substrate are equal shaped. It supposes that the substrate fits perfectly into the active site of the enzyme.
Define induced fit model
A theory describing the model of the enzyme- substrate complex where a change in the conformation of the active site occurs when the substrate binds
Define denature
To irreversibly change a protein’s tertiary structure
This causes a conformational change in the enzyme’s active site, meaning that the substrate can no longer fit and the reaction can’t proceed, even if the conditions return to normal.
What effect does temperature have on Enzymes?
As with many chemical reactions, the rate of an enzyme-catalysed reaction increases as the temperature increases because the molecules have greater kinetic energy and collide into one another more frequently.However, at high temperatures the rate decreases again because the enzyme becomes denatured and can no longer function. An optimum activity is reached at the enzyme’s optimum temperature.
Below the optimum temperature the enzyme activity also decreases because the molecules collide less frequently. When it becomes too cold, enzymes experience little to no activity and can freeze. However, when reheated the enzymes can regain functionality as significant denaturation does not occur at low temperatures.
What effect does Ph have on Enzymes?
The pH scale measures the acidity or alkalinity of a solution. Acids have low pH values ( under 7), and alkaline/ basic solutions have a high pH ( above 7). Just like with temperatures, enzymes have a specific pH at which they function optimally. Unlike with temperatures however, the denaturation of an enzyme occurs if it is exposed to an environment that is either above or below the optimal pH. The pH range of enzymes varies greatly depending on where the enzyme is located ( ie digestive enzymes have an optimum of around 1-2 ph). Because denaturation occurs at both extremes, plotting enzyme activity ( or reaction rate) against pH results in a symmetrical, bell shaped curve.
What effect does substrate concentration have on enzymes?
If the enzyme concentration remains constant while the substrate concentration increases then the reaction rate will also increase. This is because there are more reactants available to undergo the reaction. However, a point will be reached where there is so much substrate that every active site is constantly occupied and the reaction rate will no longer increase with more substrate. This is called the saturation point, as the enzymes are saturated with substrate. This results in the plateau when gra[hing substrate concentration against the reaction rate
What effect does Enzyme concentration have on Enzymes?
If the concentration of enzymes is high, then the reaction rate will be high and will occur quickly. This is due to the large number of active sites available for substrates to use.
If enzyme concentration rises while the substrate concentration is kept constant, the reaction rate will increase until enzymes are in excess, at which point the reaction rate will remain the same regardless of any continued increase in enzyme concentration. However in biological systems, there are typically far more substrates than enzymes so an increase in enzyme usually always increases reaction rate.
Define enzyme inhibitors
Enzyme inhibitors are molecules that bind to an enzyme and prevent it from performing it’s function. When an inhibitor is bound to an enzyme, the enzyme can either no longer function, or function is greatly reduced
What is competitive inhibition ?
Competitive inhibition occurs when molecules very similar to the substrate molecules bind to the active site and prevent binding of the actual substrate.
What is noncompetitive inhibition?
Occurs when an inhibitor binds to an enzyme at a site other than the active site ( an allosteric site), This causes a conformational change in the active site that prevents the substrate from binding and the reaction from occuring
Define irreversible inhibition
Irreversible inhibitors permanently prevent the substrate from binding the active site.
This is because irreversible inhibitors usually react with the enzyme such that it is changed chemically so that the active site can no longer bind the substrate or permanently occupy the active site so that the substrate cannot bind.
Define reversible inhibtion
Reversible inhibitors form weak bonds with the enzyme. They are often used to control enzyme activity. An example of this is feedback inhibition in which the build up of the end product of a metabolic pathway serves to deactivate an enzyme.
What are conenzymes and cofactors?
Some enzymes are inactive until they bind with other molecules or ions that change their conformation (shape) so that its active site is better able to bind substrates and catalyse reactions.
Cofactors are small inorganic substances, such as metal ions. They sometimes bind to help the enzyme fold into its tertiary structure or bind the active site to assist the reaction.
Coenzymes are non-protein organic substances, such as vitamins, that are required for enzyme activity. Other examples include NADH and NADPH, which are essential for cellular respiration and photosynthesis, respectively.
