Enzymes Flashcards
What are enzymes?
Biological catalysts that increase the rate of reaction without being used up.
Specific to one substrate
Where are enzymes found in cells?
Often bound to membranes within cells, such as the Endoplasmic reticulum, chloroplasts, mitochondria and Golgi apparatus
What type of proteins are enzymes, and what sort of size are they?
Globular, so are relatively large
Why are enzymes so important?
-Reduce activation energy
-Increase speed of reactions
-No need to increase temperature of reactions
What two environmental factors can affect enzyme activity?
pH and temperature
What two types of reaction can enzymes catalyse?
Anabolic (where larger molecules are formed from smaller ones) and catabolic (where larger molecules are broken down into smaller ones)
What is needed for a reaction to occur?
-Collision of reactants
-Activation energy to break existing bonds and form new ones (the more energy needed, the slower the reaction)
What does intracellular mean?
Inside the cell
What does extracellular mean?
Outside of the cell
Examples of extracellular enzymes
Amylase: digests polysaccharides in mouth, found in saliva
Trypsin: found in pancreas, digests proteins
Examples of intracellular enzymes
Catalase: found in nearly all organisms exposed to oxygen, protects cells from damage from reactive oxygen by breaking hydrogen peroxide down into water and oxygen
Why are enzymes able to catalyse so many reactions?
-Enzymes have very specific shapes
-Only catalyse one reaction, which is often able to be reversed
-Proteins can form the many different combinations possible and needed to fit all of these shapes
Basic step by step of lock and key/induced fit model?
-Enzyme and substrate collide successfully
-Substrate enters complementary active site, forming enzyme-substrate complex, and then enzyme-product complex
-Product leaves the active site
-Enzyme becomes available and is unchanged
Explain the Lock and Key model
-Substrate and enzyme successfully collide
-Substrate binds/enters active site
-Active site is complementary to the substrate (has a complementary 3D, specific shape), forming an enzyme substrate complex
-Enzyme catalyses the reaction forming on enzyme product complex
-Product leaves the active site
-Enzyme is left unchanged
Evaluation of the Lock and Key hypothesis
-Explains the specificity of enzymes, but fails to sufficiently explain how strain is put on bonds, or explain the enzyme substrate complex
Explain the induced fit model
Works the same as the lock and key method
(-Substrate and enzyme successfully collide
-Substrate binds/enters active site
-Active site is complementary to the substrate (has a complementary 3D, specific shape), forming an enzyme substrate complex
-Enzyme catalyses the reaction forming on enzyme product complex
-Product leaves the active site
-Enzyme is left unchanged)
HOWEVER,
-active site is not complementary
-substrate induces the fit, and causes a temporary conformational change to the shape.
-puts strain on the bond
-Active site returns to original shape.
Effect of changing the pH on the rate of enzyme activity
either side of the optimum pH, the rate decreases. optimum is the fastest rate, but on either side, the shape is lost and the active site is no longer complementary as the charge is changed, so the substrate no longer bonds.
Effect of changing temperature on enzyme activity
As temperature increases, so does the rate.
-kinetic energy increases
-more successful collisions, which results in a higher number of enzyme-substrate complexes
-the volume and mass of the product increases
-H bonds holding tertiary structure begin to break once past a certain point
-active site changes shape
-substrate no longer fits active site
-PERMANENT change
Effect of increasing substrate concentration
-As substrate concentration increases, so does the number of collisions between enzyme and substrate
-More enzyme substrate complexes form
-therefore more product is formed
-rate of reaction increases
-occurs to a certain point when enzymes are forming enzyme-substrate complexes as fast as possible
-all active sites become saturated, rate plateaus.
Effect of increasing enzyme concentration
-Increase in enzyme concentration results in more active sites being available
-More successful collisions
-more enzyme-substrate complexes form per unit time, so faster rate
-enzyme concentration is limiting factor, as it increases so does rate
-as graph plateaus, substrate concentration becomes rate limiting
-rate increase if substrate level increases
Prosthetic group
-Carbonic anhydrase contains a zinc ion permanently bound to the active site
-found in RBC
-vital to enable carbon dioxide to be taken away from the respiring cells
Cofactor
-some enzymes work better in presence of ions which aren’t permanently bound
-some act as co-substrates, they and substrate together form correct shape to bind to active site
-some change the charge distribution either on the substrate or active site to make bonds in enzyme substrate complex easier to form
-E.g, amylase digests starch to maltose, works better in presence of chloride ions.
Coenzyme
-small, organic, non-protein
-coenzymes can be slightly chemically changed by the reaction, and need to be recycled to original state before being reused
-many are derived from vitamins, and so a vitamin deficiency means lack of coenzymes
-eg, NAD, FAD, CoA used in respiration
-NASP used in photosynthesis
Competitive Inhibition
-molecule of similar shape to substrate
-fits into active site, forming an enzyme inhibitor complex preventing enzyme from binding
-more inhibitor molecules mean greater inhibition as more in active site
-increasing substrate concentration lessens the effect
Examples of Competitive Inhibitors
-statins are competitive inhibitors of an enzyme used in the synthesis of cholesterol
-aspirin irreversibly inhibits the active site of COZ enzymes, preventing the synthesis of the chemicals responsible for pain and fever
What do inhibitors do?
-control enzyme activity
-regulate product production
-prevent enzymes carrying out functions
Non-competitive inhibition
-Binds to a site other than the active site, called the allosteric site
-causes tertiary structure of enzyme to change shape, changing active site, so less product
-increasing substrate concentration has no effect
Example of a non-competitive inhibitor
-Organophosphates in insecticides and herbicides
-Irreversibly inhibit the enzyme acetylcholinesterase, which is needed for nerve impulse transission
-toxic to humans
What is a precursor?
Many enzymes exist in the form of inactive precursors, which is activated when another enzyme removes part of of the molecule, allowing the active site to assume its correct shape
Examples of precursors
-trypsin exists in cells as trypsinogen and needs to be activated when secreted to trypsin in order to digest proteins
-pepsin exists as a pepsinogen and is activated to pepsin by the action of hydrochloric acid in the stomach
-this protects the cell from digestion before the enzyme is secreted into the digestive system
Pathway of end product inhibition
-enzyme inhibition where product inhibits the enzyme producing it
-after catalysing reaction reaches completion, product molecule may stay tightly bound to enzyme
-enzyme cannot form more of the product than the cell needs
-example of negative feedback
Examples of metabolic posions
Inhibit enzyme action
Cyanide (Potassium Cyanide KCN)
-inhibits aerobic respiration
-KCD hydrolysed to produce hydrogen cyanide, which dissociate into H+ and CN-, which binds irreversibly to enzymes in the mitochondria, inhibiting the final stage of aerobic respiration
Snake Venom
-can cause paralysis as movement depends on muscles being able to contract
-venom of green mamba snake inhibits enzyme acetylcholinesterase, which is used at nerve synapses
Examples of medicinal drugs that inhibit enzyme action
Aspirin
-contains salicylic acid which inhibits enzymes prostaglandins, which make nerve cells sensitive to pain, and are produced when tissues are infected or damaged, increase swelling when damaged.
-can also reduce risk of blood clots
ACE Inhibitors
-drugs that inhibit the angiotensin converting enzyme (ACE), which can increase blood pressure
-used to lower blood pressure in patients with hypertension, treat heart failure, or minimise the risk of a second heart attack
Protease inhibitors
-examples include amprenavir or ritonavir
-used to treat viral infections
-prevent the replication of virus particles within host cells, and inhibit protease enzymes so viral coats cannot be made
-often done through competitive inhibition
Nucleotide reverse transcriptase inhibitors
-antiviral drugs such as zidovudine and abacavir can be used to treat patients who are HIV positive, and are nucleoside reverse transcriptase inhibitors, which means they inhibit enzymes involved in making DNA using viral RNA as a template
