2.4 - enzymes *

Question 1

what are enzymes?

Answer 1

biological catalysts that increase the rate of reaction by lowering the activation energy of the reaction they catalyse

Question 2

what is an active site?

Answer 2

• the area of the enzyme where the reaction with the substrate takes place
• enzymes are specific to substrates they bind to meaning that only one type of substrate fits into the active site of the enzyme

Question 3

describe the lock and key model?

Answer 3

• enzyme is like a lock and that the substrate is like a key that fits into it due to their complementarity in shape
• the enzyme active site is a fixed shape and that due to random collision, the substrate can collide and attach to the enzyme forming an enzyme-substrate complex
• once the enzyme-substrate complex has formed, the charged groups within the active site distort the substrate and lower the activation energy, releasing products

Question 4

describe the induced fit model?

Answer 4

• the enzyme active site is induced or slightly changes shape to mould around the substrate
• when the enzyme-substrate complex occurs, it puts strain on the bonds and lowers the activation energy
• the products are removed and the enzyme active site returns to its original shape

Question 5

describe the effect of enzyme concentration on enzyme controlled reactions?

Answer 5

• rate of reaction increases as enzyme concentration
  increases so there are more active sites for substrates to bind to
• but increasing
  the enzyme concentration beyond a certain point has no effect on the rate of reaction as there are more active sites than substrates so substrate concentration becomes the limiting factor

Question 6

describe the effect of substrate concentration on enzyme controlled reactions?

Answer 6

• as concentration of substrate increases, rate of reaction increases as more enzyme-substrate complexes are formed
• but beyond a
  certain point the rate of reaction no longer increases as enzyme concentration
  becomes limiting factor

Question 7

describe effect of temperature on enzyme controlled reactions?

Answer 7

• rate of reaction increases up to the optimum temperature, which is
  the temperature at which enzymes work at their maximum rate
• rate of reaction
  decreases above the optimum temperature

Question 8

what is a cofactor?

Answer 8

• a non-protein compound required for the enzyme’s activity to occur
• there are three types of cofactors: coenzymes, activators and prosthetic groups

Question 9

what is a coenzyme?

Answer 9

• organic cofactors which do not bind permanently
• they facilitate the
  binding of substrate to enzyme
• many coenzymes are vitamin derived
• an example is NAD derived from niacin, which acts as a hydrogen acceptor

Question 10

what is an activator?

Answer 10

• inorganic metal ions that temporarily bind to the enzyme and alter its
  active site, making the reaction more feasible
• an example is a magnesium ion which is involved in processes such as shielding negative charge

Question 11

what is a prosthetic group?

Answer 11

• permanently attached to the enzyme
• for example, haemoglobin
  contains a prosthetic haem group which contains iron that serves as a means of binding oxygen

Question 12

what is an inhibitor?

Answer 12

• a substance which slows down or stops a reaction by affecting the binding of
  substrate to the enzymes
• inhibitors can either be reversible and irreversible

Question 13

what are two examples of irreversible inhibitors?

Answer 13

• heavy metal ions such as mercury and silver
  which cause disulphide bonds in protein structure to break, causing shape of active site to change, affecting protein activity
• cyanide, a nerve gas that covalently binds to the active site, preventing the
  binding of the substrate

Question 14

what is a reversible inhibitor?

Answer 14

• they bind to the active site through hydrogen bonds and weak ionic
  interactions so don’t bind permanently
• can either be competitive or non-competitive

Question 15

what is a competitive inhibitor?

Answer 15

• they bind to the active site of the enzyme, decreasing its activity as they compete with substrate for the enzyme
• amount of product formed remains same, but rate at which product formation occurs decreases
• the higher the concentration of competitive inhibitor the
  lower the reaction rate
• increasing the substrate reverses the effect of competitive inhibitors by outcompeting them

Question 16

what does a non-competitive inhibitor do?

Answer 16

• binds at another site on the enzyme called the allosteric site
• this changes shape of the active site preventing the binding of the substrate
• increasing the
  concentration of substrate has no effect on non-competitive inhibition

Question 17

give two examples of drugs which are inhibitors?

Answer 17

• penicillin: used to fight bacterial infections, it’s an inhibitor of enzyme transpeptidase which plays important role in cell wall formation
• Ritonavir: an antiretroviral drug used to treat HIV which inhibits HIV protease which is responsible for assembly of new viral particles and
  spread of infection