enzymes Flashcards
Enzymes
enzymes are globular proteins composed of one or more polypeptides, and their 3D structure is affected by changes in temperature and pH.
Enzymes as catalysts
- Students should understand the benefit of increasing rates of reaction in cells.
- The rate of reaction is the amount of product produced per unit time. It is the speed at which a biochemical reaction is taking place.
Role of enzymes in metabolism
- Students should understand that metabolism is…
- Many different enzymes are required by living organisms, and control over metabolism can be exerted through these enzymes.
- Metabolism is the complex network of interdependent and interacting biochemical reactions occurring in living organisms.
- Enzymes are specific to their substrate (the reactant), so a large number of enzymes are required in organism.
- And all the different metabolic reactions (anabolic and catabolic) can carefully be controlled with the help of enzymes.
Anabolic and catabolic reactions
- Examples of anabolism should include
the formation of macromolecules from monomers by condensation reactions including protein synthesis, glycogen formation and photosynthesis.
Examples of catabolism should include
hydrolysis of macromolecules into monomers in digestion and oxidation of substrates in respiration.
Compare and contrast anabolic and catabolic rxns
Enzymes as globular proteins with an active site for catalysis
- Define substrate, enzyme, ES complex
- Include that the active site is composed of a few amino acids only, but interactions between amino acids within the overall three-dimensional structure of the enzyme ensure that the active site has the necessary properties for catalysis.
- An enzyme has an active site – a region of polar amino acids on the surface of the enzyme to which substrates bind and which catalyzes the reaction.
- The substrate is a reactant in a biochemical reaction. Together with the enzyme it forms the enzyme-substrate complex.
- The enzyme-substrate complex is a temporary structure formed when a substrate binds to the active site of an enzyme.
- When the substrate is bound to the active site, it is converted into products. The products are then released, leaving the active site free to catalyse another rxn
Interactions between substrate and active site to allow induced-fit binding
- Students should recognize that both substrate and enzymes change shape when binding occurs.
- Define induced-fit
- See diagram on slide 14 ( 3 stages)
- The overall three-dimensional structure and the enzyme’s amino acid sequence of the active site is specific to the substrate. This ensures that the active site binds to the substrate molecule, holds on to it and lowers the activation energy for the reaction.
- The enzyme is highly specific and the induced shape change of the enzyme and substrate upon binding results in a proper alignment of the catalytic groups on its surface which allows the enzymatic reaction to take place.
- definition: when substrate attaches to enzyme, both the shape of the substrate and enzyme change, resulting in the proper alignment of catalytic groups, enabling the catalysis of the rxn
Role of molecular motion and substrate-active site collisions in enzyme catalysis
- Movement is needed for a substrate molecule and an active site to come together.
- Sometimes large substrate molecules are immobilized while sometimes enzymes can be immobilized by being embedded in membranes.
- For a substrate and an active site to come together molecular movement is needed so that substances can collide. The greater the kinetic energy of enzyme and substrate, the greater the chance of collisions.
- Immobilized enzymes are also used as cell-free preparations industrially in the preparation of lactose free milk. Enzymes are attached to an insoluble material or membrane with the substrate moving over it. This is convenient, as it allows reuse of the enzyme and avoids having to add the enzyme to the sample.
Formation of enzyme-substrate complex depends on 3 important aspects
- The molecular motion of both, enzyme and substrate, which causes random collisions between molecules. All metabolic reactions occur in an aqueous solution, which enables the molecule’s continual motion.
- The correct alignment and angle between substrate and enzyme affects the success of the collision.
- The speed of movement which is affected by the molecule`s size – substrates are usually smaller than enzymes, so their movement is faster.
Relationships between the structure of the active site, enzyme–substrate specificity and denaturation
- Students should be able to explain these relationships.
- Since enzymes are proteins, their structure can be altered by changes in pH or temperature. If the shape of the active site is changed considerably it is said to have become denatured. It will not function anymore, and the enzyme-substrate specificity is compromised.
- High temperature causes denaturation as the extra energy leads to increased vibration with the molecule, breaking intra-molecular bonds within the protein.
- Changes in pH lead to a higher or lower concentration of hydrogen ions in the solution – as a consequence, hydrogen bonds within the enzyme are broken.
Effects of temperature, pH and substrate concentration on the rate of enzyme activity
- The effects should be explained with reference to collision theory and denaturation.
- As temperature increases, the rate of reaction increases as molecules have more kinetic energy.
Enzyme and substrate molecules move faster, increasing the chances of colliding more frequently with the active site of the enzyme. Enzyme activity is increasing.
When enzymes are heated above optimum, the chance of bonds breaking within the molecule increases. The structure changes irreversibly, denaturing the enzyme.
Measurements in enzyme-catalysed reactions
- Students should determine reaction rates through experimentation and using secondary data.
substrate concentration
- Increasing substrate concentration increases the rate of reaction, because the chances of molecular (successful) collisions between active site of the enzyme and substrate have increased.
- At the optimum concentration of substrate molecules, all active sites are full and working at maximum efficiency.
- Any increase in concentration beyond the optimum will have no added effect as there are no extra active sites to be used – a greater and greater proportion of substrate-active site collisions are blocked.
Effect of enzymes on activation energy
- Students should appreciate that energy is required to break bonds within the substrate & that there is an energy yield when bonds are made to form the products of an enzyme catalysed reaction. Students should be able to interpret graphs showing this effect.
- With the help of enzymes reactions can be carried out with a lower investment into the activation energy. This will allow more product to be produced – the rate of reaction is increased.
- In any reaction, energy is required to reach a transition state before completing a reaction.
- The transition state is an intermediate state before being converted into products, e.g. bonds need to be broken or weakened in a substance. This is what the activation energy is used for.
- An enzyme lowers the activation energy by weakening bonds and therefore less energy is needed for the reaction to occur. After the products have been formed, energy is released.
