Enzymes Flashcards
What is an enzyme?
A globular protein which is a biological catalyst
What features of a biological catalyst differ from a chemical catalyst?
Catalyse very high reaction rates, shows great reaction specificity, work in mild temp/pH conditions, can be regulated
What are ribozymes?
Catalytic RNA molecules with no protein component
What is a cofactor?
A non-protein component needed to help an enzyme with activity
What is a coenzyme?
A complex organic molecule, usually produced from a vitamin
What are examples of coenzymes?
FAD, NAD+ and Coenzyme A
What is a prosthetic group?
A cofactor covalently bound to the enzyme or very tightly associated with the enzyme
What is an apoenzyme?
The protein component of an enzyme that contains a cofactor
What is a holoenzyme?
The whole enzyme (the apoenzyme plus the cofactor)
What is a substrate?
The molecule acted on by the enzyme
What is the active site?
The part of the enzyme in which the substrate binds and is acted upon
What three things to enzymes do?
- Increase rates of spontaenous reactions
- Lower the activation energy of biochemical reactions
- Accerlerate movement towards reaction equilibrium
What do enzymes not do?
They do NOT move reaction equilibria and do NOT make a non-spontaneous reaction spontaneous
What ΔG value does a spontaneous reaction have?
Negative (-ΔG)
What is the energy barrier?
The energy required to position chemical groups correctly, bond rearrangements, electron arrangements etc.
What happens at the transition state?
Chemical bonds are formed and broken
What happens if there is no enzyme to the activation energy?
It is a high activation energy
How do enzymes reduce the activation energy?
- Entrophy reduction
- Desolvation
- Induced fit
What is entrophy reduction?
Enzymes force the substrate to be correctly orientated by binding them in the formation they need to be in for the reaction
What is desolvation?
Weak bonds between substrate and enzyme replace H-bonds between subsrate and aqeous solution
What is induced fit?
Conformational changes occur in protein structure when substrate binds
What techniques can be used to analyse enzymes?
Enzyme kinetics, 3D structure, mutagenesis
What happens if you add more substrate to a reaction
More substrate = higher initial rate of reaction
What is Vmax in enzyme kinetics?
Vmax occurs when all enzyme active sites are saturated with substrate and you reach the maximum reaction velocity
How is the Michaelis constant calculated?
From the hyperboli reaction curve as half of the Vmax
When is the rate limiting step of an enzymatic reaction?
The breakdown of the ES complex to give free enzyme and product
What is the definition of Km?
Km is equivalent to the substrate concentration at which the initial reaction rate is half of the maximum reaction rate
What is a better way to find out Vmax and Km?
Draw a Lineweaver-Burk plot using same data as used to draw M-M hyperbolic curve
What does a large Km value tell you?
Indicates a less stable ES complex
What does a smaller Km value indicate?
Indicates a more stable ES complex
What can Vmax tell you?
Vmax can tell you how fast a reaction is proceeding when the enzyme is saturated with substrate AND is a measure of the enzymes catalytic rate
What does a high Vmax indicate?
Fast enzyme
What is the M-M equation?
E+S ES –> E+P
What is Km?
Km is measure of affinity of enzyme for S
What are isozymes?
Different enzymes/proteins but catalyse the same reaction
What do both glucokinase and hexokinase catalyse?
Glucose + ATP –> Glucose-6-phosphate + ADP
Why are enzymes important in disease?
Enzymes in wrong place can tell us about disease e.g. How it is spreading
What are examples of isoenzymes?
Glucokinase and hexokinase
What does electrophoresis do?
Separates enzymes into different plasma proteins
Catalysing a reaction with two or more substrates involves transfer of groups from one substrate to another, how does this occur?
Random order OR ordered with a ternary complex OR no ternary complex formation
How does an ordered sequential mechanism work?
The order of binding and release of substrate ad enzyme is important; the enzyme exists in ternary complex, first with substrates of the reaction and then with the products of the reaction
What is an example of an ordered sequential mechanism?
Lactate dehydrongenase and the catalysis of pyruvate to lactate
How does a random sequential mechanism?
The order of binding and release of substrate and product is random, but the formation of a ternary complex still occurs first wth subsrates and then the products of the reaction
What is an example of the random sequential mechanism?
Creatine kinase catalysing the formation of phosphocreatine from creatine
What does a ping-pong reaction mean?
The substrates bounce on and off the enzyme as they are catalysed
What are allosteric enzymes?
Enzymes made up of many subunits which contain many active sites
What is cooperative binding?
When one substrate binding to an ezyme subunit can cause changes in other active sites on other subunits
What happens to an enzyme when there is an increase in temperature?
An increase in molecule collisions, an increase in internal energy of molecules, and increase temp will eventually denature enzyme
How will a pH change affect an enzyme?
Changes the charge of amino acids, change to active site can mean the enzyme will stop functioning, can denature
What will affect an enzyme?
Temp, pH, inhibitors
What are different types of enzyme inhibitors?
Competitive, uncompetitive, non-competitive, irreversible inhibitors, feedback inhibition
How do competitive inhibitors work?
They bind to enzymes non-covalently and resembles substrate molecule, therefore competing with active site
What effect do competitive inhibitors hav on Km and Vmax values?
Increases the Km values (decrease in affinity) but Vmax remains unchanged
What are transition stage analogues?
An inhibitor that mimics the transition state (as maximal interaction occurs between the enzyme and transition state, rather than substrate)
How do non-competitive inhibitors work?
Bind to enzymes non-covalently and will usually attach to a site other than active site of enzyme, substrate still binds so Km remains unchanged, Vmax will decrease
How do irrversible inhibitors work?
Bind covalently to enzyme (irreversible)
What are the two main ways regulatory enzymes modulate reactions?
- Allosteric enzymes
2. Covalently modified enzymes
What is feedback inhibition?
A build up of the end product of a reaction can then bind to a different active sit and this will block the enzymes action and the product will stop being made
What is an example of non-competitive inhibition?
Allosteric effectors
How do allosteric effectors work?
Bind non-covalently to site on enzyme (not active site), changes enzymes structure and will increase or decrease the efficiency of substrate binding and processing
What is a concerted model?
When each subunit can exist in two different conformations and one binds substrates well and the other doesn’t, with no substrate the enzyme will flip between the two conformations
How do allosteric activators and inhibitors affect the concerted model?
Allosteric activators will stabilise the open conformation so substrate can bind, allosteric inhibitors will stabilise closed conformation and make it difficult for substrate to bind
What is a sequential model?
No flipping between conformational states, subunits can always bind substrate and binding causes conformational change, subsrate binding causes change in ONE subunit but then this sensitises the enzyme to bind more
What is one of the most important covalent modifications?
Phosphorylation
What do multiple phosphorylation sites allow?
Very fine control of enzyme function depending on requirement of the particular enzyme at given time
What is a proprotein/proenzyme?
An enzyme that can exist as an inactive precursor protein
What happens when you cleave a proprotein?
Gives an active enzyme
What cleaves proproteins?
Proteases
How can enzymes be modulated?
Allosterically, covalent modifications, proteolytic cleavage
