Enzymes for S4 2011-IGCSE LEVELPresentation Transcript
E N Z Y M E S
What are they?
What do they do?
How do they work?
What can affect how an enzyme works?
An enzyme is a biological catalyst.
This means it is a protein (biological molecule) that speeds up chemical reactions (a catalyst).
A substrate is the molecule on which an enzyme acts.
WHAT ENZYMES DO
Enzymes act in chemical reactions that build up (join simple molecules to make a bigger, complex compound) or break down (digest) big complex molecules to small simple substances.
Amino acids are joined to make proteins (building up or anabolic reactions )
Starch is broken down to glucose (breaking down or catabolic reactions )
SOME TERMS TO REMEMBER
SUBSTRATE : the molecule upon which an enzyme acts.
Example : The enzyme amylase breaks down starch. So starch is the substrate for amylase.
ACTIVE SITE (of the enzyme) part of the enzyme that binds to the substrate during a chemical reaction.
PRODUCT : the molecule obtained as a result of the enzyme’s action.
RATE OF REACTION : how an enzymes works. It is measured by recording either the disappearance of substrate or the appearance of product. In other words, it records the changes in concentration of either the substrate or the product as the reaction proceeds.
PROPERTIES OF ENZYMES
Enzymes are specific . They will react only with a one given type of substrate. Example : an enzyme that breaks down proteins will not break down fats.
Enzymes are affected by extreme temperatures and pH. Each enzyme has an optimum pH and temperature at which it works best.
Enzymes are not affected by the chemical reaction they help take place. This means that the same enzyme can be reused again, and that a small number of enzymes can carry out a great number of chemical reactions to produce a lot of products.
Enzymes are affected by poisons, such as toxins and heavy metals, and by high concentrations of salts.
HOW ENZYMES WORK
A) lock and key model : the substrate has a similar shape to the one in the active site – an enzyme-substrate complex forms – a product is obtained and the unchanged enzyme is free to start again.
B) induced fit model
The substrate does not have a similar shape to the active site in the enzyme; instead, when the enzyme approaches the substrate, the enzyme’s shape changes so that the substrate can now “fit” in the active site.
FACTORS AFFECTING ENZYME-CONTROLLED REACTIONS
Cofactors and coenzymes (these increase the rate of reaction if present, they are necessary for the enzyme to work; e.g. vitamins)
Concentration of substrate
Concentration of enzymes
HOW TEMPERATURE AFFECTS ENZYMES At low temperatures , enzymes are inactive or work very slowly. As the temperature increases, enzymes become more active and the rate of reaction increases until the optimum temperature , when the highest rate of reaction is achieved. At higher temperatures , the enzymes are denatured (destroyed) so they can no longer bind to the substrate and no reaction takes place.
HOW pH AFFECTS ENZYMES
Very high or very low pH affects how enzymes work, as extreme pH will denature the enzyme.
Remember each type of enzyme has its own optimum pH at which it works best.