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.
A Level Biology: Enzymes -
How Temperature Affects Enzyme Activity.
In this A-Level Biology Lesson "Enzymes: How temperature affects Enzyme Activity"
Here we'll learn How enzymes Lower Activation Energy
The effects of temperature on enzyme activity (introduction)
As Temperature increases... The Enzyme as Denatured...
Enzymes have optimal temperatures.
how to Analyse the graph...
Download the A3 Poster (PDF) of the revision notes to follow along the video.
Enzymes have optimum temperatures at which they work best (fastest). Mammalian enzymes have an optimum temperature of about 40°C. Remember though, you're leaning biology - and that includes all life... Thus, there are enzymes that work best at vastly different temperatures than ours, e.g. enzymes from the arctic snow flea have an optimal temperature of about -10°C, and enzymes from thermophilic bacteria such as Thermus aqaticus work have optimal temperatures ranging from 72°C - 95°C (some even higher!)
Taking a look at the graph showing how temperature affects enzymes, you'll see that up to the optimum temperature the rate increases geometrically with temperature (i.e. it's a curve, not a straight line). The rate increases because the enzyme and substrate molecules both have more kinetic energy so collide more often, and because these molecules have sufficient energy to overcome the energy barrier which of course due to the enzyme is greatly reduced.
The increase of enzyme activity rate as temperature increases can be quantified as a Q10, which is the relative increase for a 10°C rise in temperature.
Q10 is usually 2 - 3 for enzyme catalysed reactions (meaning the rate of reaction doubles wit every 10°C increase in temperate). Q10 is typically less than 2 for non-enzyme reactions.
It is important that you understand also, that the rate of enzyme activity is not zero at 0°C, meaning enzymes still work at 0°C (and lower, but slower) e.g. food in the fridge still goes 'off', its just that the enzymes breaking down the food work much more slowly. Enzymes can even work in ice, albeit the rate of reaction is extremely slow due to the extremely slow diffusion of enzyme and substrate molecules through the ice lattice.
Above the optimum temperature the rate of reaction decreases because more and more of the enzymes denature. The heat energy breaks the hydrogen bonds holding the secondary and tertiary structure of the enzyme together, so the enzyme (and especially the active site) loses its shape to become a random 'coil'. The substrate can no longer bind, and the reaction is no longer catalysed.
A Level Biology: Enzymes -
How pH Affects Enzyme Activity.
Enzymes have optimum pH levels at which they work best. For most enzymes this is about pH 7 - 8 (which is pH of most cells). Some enzymes though have their optimal pH in the extreme, like protease enzymes (in the stomach), which have an optimum of pH 1.
pH affects the 'charge' of the amino acids at the active site, e.g. Carboxyl acid R groups are uncharged a low pH (COOH), but are 'charged' at high pH (COO-). This behaviour affects the properties of the active site - altering it which results in the substrate no longer being able to bind.
A Level Biology: Enzymes -
How Enzyme and Substrate Concentration Affects Enzyme Activity.
As the enzyme concentration increases the rate of the reaction increases linearly.
Why? Because there are more enzyme molecules available to catalyse the reaction.
At very high enzyme concentration the substrate concentration may become rate-limiting, so the rate stops increasing - and is shown graphically as the levelling out of enzyme activity. Enzymes however, are typically present in cells at low concentrations.
As the substrate concentration increases, the rate of reaction increases.
Why? Because there are more substrate molecules which collide with enzyme molecules, thus, more enzyme-substrate-complexes form and more enzyme dependent reactions take place.
Graphically, the rate of an enzyme-catalysed reaction shows slight curved dependence on substrate concentration.
At higher concentrations the enzyme molecules become saturated with substrate, so there are fewer free enzyme molecules, so adding more substrate doesn't make any real difference to the rate of reaction (however, it will increase the rate of E-S collisions).
Enzymes have varying affinities (the tendency for the enzyme to bind to its substrate).
So, Enzymes have varying affinities for their substrates, and the affinity for an enzyme to bind to its substrates can be described by an enzyme's Km
What is Km and Vmax?
Km (or Michaelis Constant) is the substrate concentration at which half the amount of an enzymes active sites are occupied by its substrate.
A high Km means lots of substrate must be available in order to saturate the enzyme - meaning the enzyme has a low affinity for its substrate.
A low Km means only a small amount of substrate is needed to saturate the enzyme - meaning the enzyme has a high affinity its substrate.
The maximum rate of reaction at infinite substrate concentration is called is maximum velocity or Vmax.
Vmax and Km are useful for characterising enzyme affinity and thus enzyme activity (rates of reaction). For example, a good enzyme will have a high Vmax and a low Km Meaning the enzyme dependent reactions will reach maximum velocity (Vmax) (maximum rate of reaction) with low concentrations of substrate (low Km).
A Level Biology: Enzymes -
How Inhibitors Affects Enzyme Activity.
Inhibitors inhibit (prevent) the activity of enzymes, reducing the rate of their reactions. They are found naturally, but are also used artificially, e.g. antibacterial drugs and pesticides.
There are two kinds of inhibitors.
A competitive inhibitor molecule has a similar structure to the normal substrate molecule, and it can fit into the active site of the enzyme. As such, the competitive inhibitor “competes” with the substrate for the active site, the result is a slower rate of reaction.
Competitive inhibitors increase Km for the enzyme, but have little effect on Vmax. Meaning the rate of reaction can approach a normal rate (normal Vmax) if the substrate concentration is increased high enough to saturate the enzyme (i.e. the Km is increased).
A non-competitive inhibitor molecule has a different structure from the substrate molecule and does not fit into the active site.
Non-competitive inhibitors binds to another part of the enzyme, resulting in a shape change of the enzyme, including the active site. Meaning the substrate can no longer bind to its enzymes active and no Enzyme-Substrate-Complexes are formed.
So, non-competitive inhibitors reduce the amount of active enzyme available (just like decreasing enzyme concentration).
Non-competitive inhibitors decrease Vmax, but have no effect on Km.
Non-competitive inhibitors that bind weakly to the enzymes can be ‘washed out’ and are sometimes called reversible inhibitors, while those that bind tightly and cannot be ‘washed out’ are called irreversible inhibitors.
Poisons like cyanide, heavy metal ions and some insecticides are examples of non-competitive inhibitors.
Check Your Exam Specification
★ AQA Specification Reference: - 22.214.171.124 Many proteins are enzymes. The properties of an enzyme relate to the tertiary structure of its active site and its ability to combine with complementary substrate(s) to form an enzyme-substrate complex. Explain the mode of enzyme action - the lock and key model and the induced-fit model of enzyme action. The properties of an enzyme, structure of its active site and its ability to combine with complementary substrate(s) to form an enzyme-substrate complex. The specificity of enzymes. Students should be able to: appreciate how models of enzyme action have changed over time. Each enzyme lowers the activation energy of the reaction it catalyses. Students should be able to: appreciate that enzymes catalyse a wide range of intracellular and extracellular reactions that determine structures and functions from cellular to whole-organism level.
Factors that affect enzyme action - Temperature, pH, enzyme and substrate competition. How competitive and non-competitive inhibits affect enzyme action.
★ CIE Specification Reference: - 2.3 (Biological Molecules) Proteins: An understanding of protein structure and how it is related to function. explain the mode of action of enzymes in terms of an active site, enzyme/substrate complex, enzyme specificity (the lock and key hypothesis and the induced fi t hypothesis should be included). An understanding that an enzyme is a biological catalyst that increases the rate of a reaction and remains unchanged when the reaction is complete. 3.1 Mode of action of enzymes: a) explain that enzymes are globular proteins that catalyse metabolic reactions b) state that enzymes function inside cells (intracellular enzymes) and outside cells (extracellular enzymes). Factors that affect enzyme action - Temperature, pH, enzyme and substrate competition. How competitive and non-competitive inhibits affect enzyme action.
★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - 2.10 (Topic 2: Genes and Health) Understand the mechanism of action and the specificity of enzymes in terms of their three-dimensional structure. Understand that enzymes are biological catalysts that reduce activation energy. Know the structure of enzymes as globular proteins. Understand the concepts of specificity and the induced fit hypothesis. Know that there are intracellular enzymes catalysing reactions inside cells and extracellular enzymes produced by cells catalysing reactions outside of cells.
★ Edexcel (Biology B) Specification Reference: - 1.5 (Enzymes) Know the structure of enzymes as globular proteins, (ii) Enzymes Understand the concepts of sp