Lecture 2 Flashcards
What are enzymes an what are their general purposes?
They are proteins that are biological catalysts. They control metabolic reactions, each reaction has a highly specific enzyme to do the job. Some enzymes however require metal ions or organic cofactors to become active.
What are the 6 main classifications of enzyme. And what type of reaction do they catalyse?
Oxidoreductases - Oxidation-reduction reactions, Transferases - Transfer of functional groups, Hydrolases - Hydrolysis reactions, Lyases - Group elimination to form double bonds, Isomerases - Isomerisation and Ligases - Bond formation and ATP hydrolysis
What does the complementarity of the enzyme and the substrate depend on?
Non-covalent forces
What are the basic steps to how an enzyme works?
- The substrate enters the enzymes active site
- The enzyme-substrate complex (ESC) is formed
- The enzyme-product complex is formed
- The products of the reaction leave the active site of the enzyme
How does the enzyme speed up the rate of a reaction?
It lowers the activation energy needed for the reaction by providing a lower energy pathway from substrate to product
In an enzyme catalysed reaction, what stays unchanged?
There is no overall effect on the free energy change of the reaction
What does studying enzyme kinetics help us to understand?
- Helps us to understand the mechanisms of enzymes
- Helps us to understand the regulation of biochemical reactions in metabolic pathways
- Helps us to investigate mutations of the metabolic pathways
How does the concentration of substrate affect the initial rate of reaction when the enzyme concentration is constant and low?
Low concentration - The rate of the reaction is dependent on the substrate concentration, so the binding of the substrate is the limiting factor
High concentration - The rate of the reaction is independent of the substrate concentration, the enzymes are fully saturated
What is the Michaelis-Menten kinetic equation?
Vo = (Vmax x [S])/ (Km + [S])
What is the Michaelis constant [Km] and what does it show?
It shows the substrate concentration at which the reaction reaches half of its maximum velocity (Vmax/2)
It also measures the affinity of the enzyme for the substrate
What does Vmax of an enzyme catalysed reaction show?
Shows the initial reaction velocity when the substrate concentration is high compared to the Km
What happened to the Michaelis-Menten equation?
In the 1930s Burk and Lineweaver found limitations to the equation and overcame the limitations of Vmax determination by linearising the equation
What was the Lineweaver-Burk equation and what type of graph did they plot to determine the values needed?
1/Vo = (Km/Vmax) x (1/[S] + 1/Vmax)
They created a ‘double reciprocal’ plot
How are enzymes controlled within an organism?
Through the enzymes availability - Controlling the rate of synthesis and degradation
Through the enzymes activity - Inhibiting or modifying the enzyme
What is the effect of pH on enzyme activity?
- Higher activity at a narrow pH range (optimum pH)
- The R groups of the Amino Acids that make up the enzyme can get changed at different pH levels
- In too low or too high of a pH the tertiary structure of the enzyme is disrupted and the enzymes lose their activity (denatured)
What is the effect of temperature on enzymes?
A range of temperatures are considered optimal for enzymes, most have an optimum temperature of around 37 degrees and if the temperature is too high the enzyme will degrade and become denatured
Why is the inhibition of enzymes studied?
- Many therapeutic drugs are inhibitors of specific enzymes
- We can use inhibition as a probe for understanding the kinetic and properties of enzymes in their uninhibited state
What is the definition of an enzyme inhibitor?
A substance that reduces an enzymes activity by influencing the binding of substrate and/ or its reaction rate
What does the pharmaceutical industry refer to enzyme inhibitors as?
Novel drugs
What are the 3 types of enzyme inhibitors?
- Competitive inhibitors
- Non-competitive inhibitors
- Uncompetitive inhibitors
What is the mechanism of action for a competitive inhibitor?
The inhibitor competes directly with the substrate for the same enzyme binding site
What is the mechanism of action for a non-competitive enzyme. Inhibitor?
Inhibitor binds to the enzyme allosteric site (not the active site) and prevents the binding of the substrate
What is the mechanism of action of an uncompetitive inhibitor on enzymes?
The inhibitor binds to the enzyme however the substrate can still bind to the active site, although nothing will happen as the inhibitor inactivates the enzyme completely
What happens to the Km and the Vmax values of an enzyme catalysed reaction when there is a competitive inhibitor present?
The Km is increased but the Vmax is unaffected
What happens to the Km and the Vmax values of an enzyme catalysed reaction when there is an uncompetitive inhibitor present?
The Km and the Vmax values both decrease
What happens to the Km and the Vmax values of an enzyme catalysed reaction when there is a non-competitive inhibitor present?
The Km value stays unaffected, however the Vmax value decreases
Why are inhibitors used as drugs?
- Some inhibitors can be used to stop enzymes that are necessary for the functioning of a pathogen
- Some can be used for enzymes that are unregulated in a human disease (Disease biomarkers)
- Some can be used to target enzymes that are produced more energetically by tumours than by normal tissues (Anti cancer drugs)
What is similar between the competitive inhibitors and the substrate?
The chemical structure has to be very similar as it needs to be able to bind to the same active site of the enzyme
Enzymes can also be regulated by covalent phosphorylation, how does this happen?
It happens at the -OH group (Ser,Thr and Tyr residues)
Involves using ATP and a Kinase
The inverse process (dephosphorylation) is catalysed by phosphatases
Give an example of an enzyme that is found in the blood and used to diagnose human disease?
PSA (Prostate-specific antigen) high levels in the blood can be a sign of prostate cancer
