Lesson 2

 Repro  Enzyme question  Slides 17-22

 Jot down the key ideas from last lesson on to some scrap/spare paper.  What were some of the key words and phrases?  Use them in a sentence/paragraph to describe what enzymes do...

Some enzymes need the help of...

 Some enzymes can only work if another small, non-protein molecule is attached to them (non permanent).  The presence of cofactors such as certain ions may help the formation of the enzyme-substrate complex.  Some cofactors are free and can even join with the substrate to make the correct, complementary shape required (co-substrates).  Some cofactors change the charge distribution on the surface of the substrate or enzyme and make the temporary bonds in the ES-complex easier to form. e.g. Amylase will only digest starch in the presence of chloride ions cofactor enzyme active site substrate

 Carbonic anhydrase with zinc ion permanently bound to its active site.  Found in red blood cells  Catalyses conversion of carbon dioxide and water to carbonic acid, which then breaks down into protons and hydrogencarbonate ions.  Important for the removal of CO 2 from respiring tissues to the lungs

 Along with cofactors and prosthetic groups, coenzymes are another small molecule that helps the enzyme-substrate complex form.  Coenzymes bind temporarily to the active site of enzymes.  Many vitamins act as coenzymes.  Vitamin C is a very important coenzyme. Unlike prosthetic groups and other cofactors, coenzymes are changed in a reaction. What’s the implication of this? need to be recycled or need a source of more

 Nicotinamide (NAD) is a very important coenzyme needed by cells. The RDA for humans is 18mg. The amount of NAD used in metabolic reactions is a great deal more than 18mg. Suggest why the RDA is so low. The NAD is constantly recycled, which means that there is a always a supply of it. Therefore not much is needed in the diet.

 Have a similar shape to that of the substrate molecule  Complementary shape to the active site  Inhibitor occupies the active site, forming enzyme- inhibitor complexes.  Does not lead to the formation of products  Most do not bind permanently  They bind for a short period of time and then leave.  Their action is described as reversible, as the removal of the inhibitor form the reaction mixture leaves the enzyme molecule unaffected.

The level of inhibition depends on the concentrations of inhibitor and substrate. As the number of substrate molecules is increased, the level of inhibition decreases because a substrate molecule is more likely than an inhibitor molecule to collide with the active site.

 Does not compete with substrate molecules for a place in the active site.  Instead, they attach to the enzyme, molecule in a region away from the active site.  This distorts the tertiary structure of the enzyme molecule, leading to the shape of the active site changing.  This means that the substrate no longer fits into the active site  Enzyme-substrate complexes cannot form  The reaction rate decreases.  Most non-competitive inhibitors bind permanently to the enzyme molecule. The inhibition is irreversible

 The level of inhibition depends on the number of inhibitor molecules present.  If there are enough inhibitor molecules to bind to all of the enzyme molecules present, then the enzyme controlled reaction will stop.  Changing the substrate concentration will have no effect

Example 1: Snake Venom Inhibitor name: A protein called fasciculation is found in snake venom. Function: Inhibits Acetylcholinesterase which is an enzyme used to degrade a neurotransmitter called Acetylcholine (Serotinin is another example of a neurotransmitter that you should have heard of from GCSE). How: Fasciculation acts as a competitive inhibitor preventing the acetylcholine from being broken down by Acetylcholinesterase after an impulse transmission. Effect: In skeletal muscle fasciculations stop nerve impulses from being transmitted and hence stop muscle contraction. Eventually this will lead to flaccid paralysis. Normal (no venom) After venom

Example 2: Cyanide poisoning Inhibitor name: Potassium cyanide Function: Inhibits a vital respiratory enzyme called cytochrome oxidase (found inside mitochondria) How: Cytochrome oxidase normally combines oxygen and hydrogen together to form water and allows ATP creation. Cyanide non competitively inhibits chytochrome oxidase changing the shape of its active site meaning no ATP creation. Effect: Any reactions requiring ATP are no longer supplied. The body eventually has no energy supply causing total cell failure … and death even though all products for respiration still present.

Example 1: HIV Protease inhibitors Inhibitor name: Protease inhibitors (many variations all under research) Function: Competitively inhibits HIV virus protease enzymes. Normally the virus uses this to cut viral RNA into smaller pieces so as into implant genes into the host cells DNA and hence replicate). How: The inhibitor binds specifically with the HIV protease enzymes active site preventing longer viral RNA pieces from bindings, as a result the RNA is not cut into smaller pieces so it cannot be implanted into the host cells DNA = no replication. Effect: A host cell can be infected by HIV but it cannot be ‘hijacked’ into making more HIV copies as a result of DNA implantation by the virus Normal (no inhibitor) With protease inhibitor (red)

Example 1: Suspected antifreeze poisoning treatment Inhibitor name: Ethanol (alcohol!) Function: Ethylene glycol is found in antifreeze, if ingested can be broken down by alcohol dehydrogenase (liver) forming extremely toxic oxalic acid = death. Ethanol if taken as a treatment can prevent this. How: Ethanol competitively inhibits alcohol dehydrogenase so give the patient a massive dose of ethanol so as to prevent ethylene glycol from interacting with alcohol dehydrogenase. Effect: Less oxalic acid is produced allowing the harmless ethylene glycol to be excreted. Better to be drunk than dead!! Ethylene glycol Broken down by alcohol dehydrogenase into Oxalic acid Ethanol (inhibitor) Massive ethanol dosage Ethylene glycol excreted

 Product of an enzyme binds to the enzymes and inhibits its action  Way to regulate metabolism  Form of negative feedback  E.g. regulation of ATP formation by phosphofructokinase (an enzyme in glycolysis)  ATP inhibits phosphofructokinase, so that when ATP levels are high, glucose is not broken down

 Some enzymes produced in inactive precursor form  E.g. Trypsin produced in the small intestine as Trypsinogen  After they’re made some amino acids are removed by another enzyme  Thus completing the shape/or exposing the active site  E.g. trypsinogen turned into trypsin

 Discuss with a partner: The difference between intracellular and extracellular enzymes, and examples of each. The similarities and differences between: Cofactors, Prosthetic Groups and Coenzymes.

The role of enzymes in catalysing both intracellular and extracellular reactions  To include catalase as an example of an enzyme that catalyses intracellular reactions and amylase and trypsin as examples of enzymes that catalyse extracellular reactions. The need for coenzymes, cofactors and prosthetic groups in some enzyme-controlled reactions  To include Cl – as a cofactor for amylase, Zn 2+ as a prosthetic group for carbonic anhydrase and vitamins as a source of coenzymes.