2.10 Enzymes Flashcards
2.10 i) Understand the mechanism of action and the specificity of enzymes in terms of their three-dimensional structure. ii) Understand that enzymes are biological catalysts that reduce activation energy. iii) Know that there are intracellular enzymes catalysing reactions inside cells and extracellular enzymes produced by cells catalysing reactions outside of cells.
What are enzymes?
globular proteins that act as biological catalysts by speeding up chemical reactions in the body
What do enzymes catalyse?
metabolic reactions at both a cellular level (e.g. respiration) and for the organism as a whole (e.g. digestion)
Why do metabolic reactions need catalysts?
they would occur very slowly at the temperature within cells without something to speed the reactions up
What is the active site of an enzyme?
the part of the enzyme molecule with the catalytic function (the part of the enzyme where the substrate molecules bind to)
What are the key features of the active site?
- has a specific shape
- is relatively small part of the large protein molecule, with only a few amino acids directly involved in it
What is a substrate?
the substance the enzyme interacts with (it binds to the active site)
Can enzymes affect structures and functions in an organism?
yes
- enzymes are involved in the production of collagen (structure)
- enzymes are involved in respiration (function)
Are enzymes highly specific? What does this mean?
yes, as they usually only catalyse one reaction (e.g. maltase only breaks down maltose)
Why are enzymes highly specific?
only one complementary substrate will fit into the active site
What determines an enzyme’s shape and therefore its function and properties?
the tertiary structure of the enzyme
Explain how the tertiary structure of an enzyme determines the function and properties of the enzyme (5):
- the active site’s shape is determined by the enzyme’s tertiary structure
- the tertiary structure is determined by the primary structure of the enzyme (the sequence of amino acids)
- the sequence of amino acids in the primary structure will determine what bonds form between the amino acid R groups (e.g. hydrogen bonds) and therefore how the polypeptide chain will fold
- this determines the 3D shape of the enzyme protein, and therefore the shape of the active site
- each different enzyme has a different tertiary structure and therefore a differently shaped active site
- meaning enzymes are highly specific and usually only catalyse one reaction, as only one complementary substrate will fit into the specifically shaped active site
Explain how pH and temperature can affect the function of an enzyme (5):
- the active site’s shape is determined by the enzyme’s tertiary structure
- changes in pH and temperature can alter the tertiary structure of an enzyme
- this is because bonds can be broken due to changes in pH and temperature, causing the enzyme’s 3D shape to change, as the protein unfolds
- because the tertiary structure of the enzyme has been altered, the 3D shape of the protein has changed, meaning the shape of the active site has changed
- this means the substrate will not fit into the active site, an enzyme-substrate complex will not be formed, and the enzyme will not be able to carry out its function (to catalyse the reaction)
Explain how mutations in genes can affect the function of an enzyme (5):
- the active site’s shape is determined by the enzyme’s tertiary structure
- the tertiary structure is determined by the primary structure of the enzyme (the sequence of amino acids)
- the primary structure is determined by a sequence of DNA, usually a gene
- therefore mutations in the gene will change the sequence of amino acids in the primary structure
- this will change the type of bonds (e.g. hydrogen bonds) that form between the amino acids, causing the 3D structure of the enzyme to change
- therefore, the tertiary structure of the enzyme has changed, and so the active site’s shape has changed
- this means the substrate will not fit into the active site, an enzyme-substrate complex will not be formed, and the enzyme will not be able to carry out its function (to catalyse the reaction)
If a substrate shape doesn’t match the active site, this means…
an enzyme-substrate complex won’t be formed and the reaction won’t be catalysed
Explain the lock and key theory:
- either a single substrate molecule with a complementary shape, or more than one substrate molecule that together have a complementary shape, can fit into the active site
- these substrate molecules form temporary bonds with the amino acids of the active site to form an enzyme-substrate complex
- the enzyme holds the substrate molecules in such a way that they react more easily (the reaction is catalysed)
- the products are released after the reaction, and the enzyme is left unchanged
- the substrate fits into the enzyme like a key fits into a lock
- the enzyme will only catalyse one specific reaction because only the complementary substrate will fit into its precisely shaped active site
Explain the induced fit theory:
- the active site is often flexible
- when the substrate(s) enters the active site, the enzyme molecule changes shape slightly, fitting more closely around the substrate
- these substrate molecules form temporary bonds with the amino acids of the active site to form an enzyme-substrate complex
- the enzyme holds the substrate molecules in such a way that they react more easily (the reaction is catalysed)
- after the reaction, the products are released, and the enzyme returns to its original shape
- only a specifically shaped substrate will induce the correct change in shape of an enzyme’s active site
- the slight change in shape of the active site enables the substrates to react
How does the induced fit model explain why enzymes are so specific?
not only does the substrate need to have the right shape to fit an enzyme’s active site, it also needs to induce the correct change in shape of the active site
What is an enzyme-substrate complex?
a temporary molecule formed by the temporary bonds that form between the substrate molecule(s) and the amino acids of the active site
How do enzymes speed reactions up by acting as biological catalysts?
they lower the activation energy of a reaction
What is activation energy and how is it often provided?
the minimum amount of energy that must be available to the reactants to break bonds and start the reaction (often provided as heat)
How does lowering the activation energy speed up the reaction?
- lowering the activation energy means reactions can happen at a lower temperature than they could without an enzyme
- this allows reactions to occur without raising the overall temperature of the cell
- this speeds up the reaction as they would occur very slowly at the temperature within cells without enzymes to speed them up
How do enzymes reduce the activation energy?
- the enzyme and substrate form an enzyme-substrate complex
- charged groups on both the surface of the active site and the surface of the substrate interact, where oppositely charged groups attract
- this distorts the shape of the substrate(s), aiding in bond breakage or formation
- if multiple substrate molecules need to be joined, the distortion of the substrates’ shapes holds them closer together, reducing repulsion between the molecules (aiding in bond formation)
- if a substrate molecule needs to be broken, the distortion of the substrate’s shape puts strain on its bonds, allowing the substrate molecule to break up more easily (aiding in bond breakage)
What are the two types of enzymes you need to know?
- intracellular enzymes
- extracellular enzymes
What are intracellular enzymes?
enzymes that catalyse reactions inside cells
