hi everyone and welcome to learn a level biology for free with miss estrich in this video it's part one of enzymes which is enzyme action if you are new here just click subscribe to make sure you don't miss any of the latest videos so the first bit of information that you need to know about enzymes is actually quite similar to the level of detail of gcse there are some additional features though so for example knowing that enzymes are tertiary structure proteins and if you haven't already learned about the different levels of structure in proteins then just click here and you can see my video on protein structure their function is to catalyze different reactions so enzymes are relatively large molecules compared to other biological molecules that you learn about however it's only a small part of an enzyme which is involved in catalyzing the reaction and that is the active site which is a term you would have heard of before at gcse so that's what we can see in this diagram it's only a small section the active site and that is where the substrate which is complementary in shape will bind and you get enzyme substrate complexes so the active site is a specific and unique shape and this is where it links to your a level knowledge you need to link it to what you know about protein structure and in particular the tertiary structure of a protein so knowing that the tertiary structure is determined by the sequence of amino acids in the primary structure that determines where bonds form how the polypeptide chain folds and therefore the final 3d shape or tertiary structure that you get and that unique 3d shape is what creates a unique shape active site which is complementary to one particular substrate and that is why they're able to bind together to create enzyme substrate complexes and this term here is key so that term enzyme substrate complex will nearly always get a mark in any question which is to do with explaining something to do with enzymes so at a level as well you need to know there's two models of explaining how enzymes catalyze reactions now you probably knew already that they catalyze a reaction by lowering the amount of energy needed for the reaction to occur and we call that the activation energy so enzymes lower the activation energy that is required for an end for the reaction and that's what this graph here is showing you so we can see here the amount of energy the reactants have and if you didn't have an enzyme you would need this level of energy for the reaction to occur if you add in an enzyme then it lowers the amount of energy needed for the reaction to occur or in other words it lowers the activation energy so the two models that we're going to have a look at are the lock and key which you learned about at gcse but also the induced fit model so the locking key is the one that you did know from gcse and this model is suggesting that the enzyme is like a lock and the substrate is like a key that fits into the lock and only a key which is exactly complementary in shape to the lock can fit in to turn the lock and that's what this model is suggesting that the enzyme's active site is an exact fixed shape just like a lock would be and it's due to random collisions between the enzyme and the substrate they can collide they then combine because they're complementary in shape and enzymes substrate complex forms once they are then combined in this enzyme substrate complex that will cause the substrate to slightly distort in shape and that distortion can lower the activation energy the products are then released the enzyme active site is empty and it can be reused again so that's model one the lock and key model model two is induced fit and this model is more like an analogy of a glove and a hand so in this model it suggests that the enzyme is like a glove and the substrate is like your hand so an empty glove is not exactly complementary to the 3d shape of your hand however when you put your hand inside of a glove the glove molds around your hand to become completely complementary so what that suggests in terms of enzymes is the enzyme active site is induced or in other words it slightly changes shape so that it can completely mold around the substrate that is almost complementary to and because the active site does change shape and slightly molds around the substrate when it binds that puts strain on the bonds and it weakens the bonds and that is the bonds in the substrate and because the bonds have been weakened by that strain that is why less energy is needed for the reaction to occur or in other words the activation energy has been lowered the product will then again be removed but the active site will then go back to its original shape so just to clarify the key differences in the induced fit model the active site is almost complementary to the substrate however it will slightly change shape when they collide so that the active site will then mold around the substrate that will then start to pull and put tension on the bonds in the substrate making them easier to break and therefore lowering activation energy the products can then be removed they're released and the active site returns to its original shape the enzyme can be used again now this is the accepted model for how enzymes function so it is actually that the active site does slightly change shape to mold around the substrate so this is the accepted model and that is it for knowing how enzymes work and how they lower the activation energy [Music] you