Enzymes are biological catalysts. There are approximately 40,000 different enzymes in human cells, each controlling a different chemical reaction. Enzymes increase the rate of reactions allowing the chemical reactions that make life possible to take place at optimal temperatures and pH levels. Enzymes were discovered in fermenting yeast in 1900 by Buchner, and the name enzyme actually means "in yeast". In addition to catalysing all the metabolic reactions of cells (such as respiration, photosynthesis and digestion), Enzymes also play roles as ‘motors’, ‘membrane pumps’ and ‘receptors’.
You already know by now that Enzymes are Proteins, and as proteins, their function is determined by their complex 3D structure.
Enzymes catalyse reactions which take place in a small part of the enzyme known as the active site.
The amino acids around the active site attach to the substrate molecule and hold it in position while the reaction takes place. This makes the enzyme specific for one reaction only, since other molecules are not complementary to its active site.
Many enzymes may also need cofactors (or coenzymes) to work properly.
Coenzymes can be metal ions (such as Fe2+, Mg2+, Cu2+) or organic molecules (such as haem, FAD, NAD or coenzyme A). Many coenzyme are derived from dietary vitamins, which is why vitamins are such important in our diet! Also, just in case you come across this terminology:
A complete active enzyme with its cofactor is called a holoenzyme.
Whilst, just the protein part of the enzyme without its cofactor is called the apoenzyme.
So, How do enzymes work?
There are 3 ways to think about enzyme as catalysis. They all describe the same process, but in different ways (you must know each of them).
A Level Biology: Enzymes - A summary of basic enzyme properties
In this A-Level Biology Lesson “A Summary of Basic Enzyme Properties" we'll build directly on what you already know from GCSE and re-cap the important features of Enzymes. You'll probably remember that Enzymes are Specific and that the Active Site of an Enzyme is its Catalytic Centre. You'll see diagrammatically how the The Enzyme-Substrate-Complex (ESC) forms. Here we can take the opportunity to re-enforce that enzymes are proteins and that the Primary sequence of amino acids determines the 3D conformational shape of the enzyme. This is very important to know (and apply). For example, factors affect enzyme structure and function - and you'll have to know this! You'll also be expected to apply your knowledge of basic enzyme properties by understanding that If the substrate and active site are not complementary NO Enzyme-Substrate-Complex can form. Finally we'll quickly summarise this lesson and once you're happy with all this you'll be ready to move on to the lock and Key V's Induced fit models of enzyme action.
When you've watched this lesson and you're confident with the topic covered you’ll be ready to download the knowledge check PDF and test your knowledge regarding the basic properties of enzymes. When you’ve answered all the questions, compare your answers to the ones I’ve written in the back of the work booklet - and of course here you can see exactly how to write answers in a way that gains maximum marks in the exams.
A Level Biology: Enzymes -
The Models of Enzyme Action - Lock and Key & Induced Fit.
In this A-Level Biology Lesson "The Models of Enzyme Action - Lock and Key & Induced Fit" we once again build upon a common topic in biology - an easy topic that bridges the gap from GCSE to A-Level biology pretty seamlessly. So we being with a few key points. We know by now that enzymes are very specific, meaning enzymes only bind to their complementary substrates. You must be able to define this specificity of enzymes. We take the opportunity next to emphasise that over time scientific knowledge changes, and because of this hypothesis are continually tested and new hypothesis formed. Enzymes action showcases this notion - that as the scientific community gather more evidence and information, theories can be updated and change slightly over time. So the lock and key model of enzyme action was “updated” to a more current idea, The induced fit model of enzyme action. (This idea that scientific theories can be updated over time is a key principle that you may have to apply to unfamiliar situations - another good example is the central dogma of molecular biology and the enzyme reverse transcriptase). So, by the end of this lesson you’ll know about the lock and key model of enzyme action and how that idea has been updated to the induced fit model of enzyme action. You must be able to describe and illustrate both models of enzyme action (compare and contrast if needed) and of course understand that over time theories can change (or more appropriately be updated based upon the new evidence gathered by the scientific community).
Download the A3 Poster (PDF) of the revision notes to follow along the video.
1. Reaction (forming the Enzyme-Substrate-complex).
In any chemical reaction, a substrate (S) is converted into a product (P):
*Note: There may be more than one substrate and more than one product, but that doesn't matter right now).
In an enzyme-catalysed reaction, the substrate first binds to the active site of the enzyme (forming an enzyme-substrate (ES) complex). Next the substrate is converted into a product while still held in position by the enzymes active site. Finally the product is released.
This reaction mechanism can be shown as: -
Once the product is released by the enzymes active site the Enzyme is then free catalyse the reaction again.
2. Geometry (Lock and Key / Induced Fit model of enzyme action).
The substrate molecule fits into the active site of the enzyme molecule like a key fitting into a
lock (hence the lock and key model of enzyme action). However, the enzyme changes shape slightly, distorting the molecule in the active site, and making it more likely to change into the product. e.g. if a bond in the substrate is to be broken, that bond might be 'forced' by stretching or twisting by the enzyme, making it more likely to break. Alternatively the enzyme can make the local conditions inside the active site quite different from those outside (i.e. pH), so that the reaction is more likely to happen. It is a bit more complicated than that in truth. In fact the active site doesn't really fit the substrate at all, but instead they "kind of fit" - this is called the transition state. When a substrate (or product) binds, the active site changes shape and fits itself around the molecule, distorting somewhat and forming the transition state, thus speeding up the reaction. This called the induced fit model of enzyme action.
A Level Biology: Enzymes Lower Activation Energy
In this A-Level Biology Lesson "Enzymes Lower Activation Energy" we'll continue our learning of these important proteins as we begin to understand that these biological catalysts play important biological roles as intracellular and extracellular enzymes. It's good to note here that learning all about enzymes as a 'synoptic topic' is really useful since there are so many topics in your A-level biology that can be 'connected' though "enzymes" by means of a well written synoptic essay (a skill you want to start practicing as soon as possible). So, chemical reactions need energy and here we'll see how enzymes lower activation energy, note that activation energy is usually provided in the form of Heat. We go on to explain the graph showing that the peak - is the transition state. You'll need to be able to recognise and describe graphs a lot in your A-level biology, so by understanding these now will stand you in good stead for the rest of your studies! You'll need to able to compare graphs to show the amount of activation energy needed without an enzyme and graphs to show the amount of activation energy needed WITH an enzyme.
When you've watched this lesson and you're confident with how enzymes lower activation energy and can explain the graphs shown you’ll be ready to download the knowledge check PDF and test your knowledge regarding the basic properties of enzymes. When you’ve answered all the questions, compare your answers to the ones I’ve written in the back of the work booklet - and of course here you can see exactly how to write answers in a way that gains maximum marks in the exams.
3. Energy Changes (Enzymes lower activation energy).
The way enzymes work can also be shown by considering the energy changes that take place during a chemical reaction. Consider a reaction where the product has a lower energy than the substrate, so the substrate naturally turns into product.
Before it can change in the product, the substrate must overcome a "energy barrier" known as Activation Energy (EA).
The larger the Activation Energy (EA), the slower the reaction.
Why? Well, because only a few substrate molecules will by chance have sufficient energy to overcome the "energy barrier". Imagine pushing boulders over up hill before they can roll down the other side of the hill without any effort on your part... and you'll get the idea.
In reality most physiological reactions have large "energy barriers", that is they have large EA.
Enzymes dramatically lower the amount of activation energy required so that a reaction can take place - Enzymes lower activation energy so that substrate molecules can easily get over the activation energy barrier and quickly turn into product.
e.g. For the catalase reaction (2H2O2 -> 2H2O + O2) the EA is 86 kJ mol-1 with No catalyst.
62 kJ mol-1 with an inorganic catalyst (e.g. iron filings), and just
1 kJ mol-1 with the enzyme catalase.
The activation energy (EA) is actually the energy required to form the transition state, so enzymes lower the EA by stabilising the transition state, enzymes do this by changing the conditions within the active site of the enzyme.