Theories About Enzymes Flashcards
What are enzymes?
Biological substances that speed up chemical reactions in the body by acting as catalysts.
Two fundamental properties of enzymes
1) They increase the rate of chemical reactions, without being consumed or permanently altered by the reaction.
2) They increase reaction rates without altering the chemical equilibrium between reactants and products.
How enzymes work
•Enzymes bind to specific substrates at their active sites.
•Lower the activation energy needed for a reaction to occur by stabilising the transition state.
•May change shape of their substrates to help them reach transition state.
•Amino acid side chains in active site may react with substrate.
•Enzymes allow the product to separate from the enzyme surface.
Lock & key mechanism
A theory of enzyme action that explains how enzymes fit their substrate. The active site of an enzyme is structured to fit a specifically shaped substrate. Once the substrate binds to the active site, the enzyme will facilitate the reaction and release products of the reaction.
‘Induced fit’ theory
Current research supports that as the enzyme and substrate come together, their interaction causes a mild shift in the enzyme’s structure, enabling an ideal binding arrangement between the enzyme & substrate. This dynamic binding maximises the enzyme’s ability to catalyse its reaction.
Induced fit process
•substrate enters active site of enzyme
•enzyme changes shape slightly to allow substrate to bind to active site
•this creates the enzyme/substrate complex
•the substrates then divide into the products creating the enzyme/products complex
•the separate products then leave the active site of the enzyme
Activation energy
The minimum amount of energy required for the biochemical reaction to take place to reach the transition state - called the activation energy. It creates a barrier, limiting the rate of reaction. Enzymes reduce the activation energy, increasing the rate of reactions.
Enzyme-substrate complex
When an enzyme binds its substrate, it forms an enzyme-substrate complex. This complex lowers the activation energy of the reaction, and promotes its rapid progression. This process can contort the substrate molecules and facilitate bond-breaking.
Environment at active site
Active site of enzyme creates an ideal environment, such as slightly acidic or non-polar, which enables the reaction to occur. Enzyme will always return to its original state at completion of reaction - enzymes remain unchanged by the reactions they catalyse. After enzyme has catalysed a reaction, it releases products (substrates broken down).
Presence of inhibitors
Enzyme inhibitors are compounds which modify the catalytic properties of the enzyme - therefore slow down reaction rate (sometimes stop reaction from occurring). These inhibitors block or distort the active site, preventing the substrate from binding as quickly/easily.
Competitive inhibition
Substrate is prevented from binding to active site by a molecule that is structurally similar to the substrate. Inhibitor molecule and substrate ‘compete’ for same active site. Inhibitor prevents the enzyme from recognising/binding to substrate by masking the substrate, or blocking the enzyme’s active site.
Non-competitive inhibitors
Bind to an enzyme at a site other than the active site, reducing the enzyme’s activity. Non-competitive inhibitors change the shape of the enzyme’s active site, preventing the substrate from binding and decreasing the reaction rate. (E.g. cyanide, mercury, silver, and ibuprofen).
Irreversible inhibitors
A substance that permanently blocks the activity of an enzyme by forming a covenant bond with it. This creates a new enzyme structure, and the process is irreversible because of this. Often used in cancer treatment (chemo) to block enzymes that cancer cells need to grow.
