Enzymes

​​In a nutshell

Enzymes are large molecules of protein that act as biological catalysts. The most important part of an enzyme is the active site, which is where reactants (also called a substrate) bind to the enzyme. Since enzymes are proteins, they are affected by small changes in the body such as changes in pH or temperature, which then affects the reaction rate. Reaction rates are also affected by enzyme and substrate concentration. 

Enzymes

Enzymes are proteins that work as biological catalysts, which means that they can speed up chemical reactions without being used in the reaction itself. This ensures that they can be reused a lot. 

Note: Enzymes do not make reactions happen; they only speed up reactions that already happen.

Enzymes can help to make big molecules from smaller ones, or even break down big molecules into smaller ones. Enzymes are used in many processes such as respiration, digestion and photosynthesis. 

Enzyme action

Enzymes catalyse specific reactions by reducing the activation energy of a reaction. The activation energy is the minimum amount of energy needed for the reaction to take place. By reducing the activation energy, the enzyme allows reactions to take place at a lower temperature. 

Since enzymes are proteins, they are formed of chains of amino acids that fold over each other to give the enzyme a specific shape. The active site of the enzyme is a small region in the enzyme with a specific shape. This is where the action happens: the substrate binds to the active site and is converted into the products of the reaction. The substrate is the molecule that will bind to an enzyme's active site. 

Lock and key model

Enzymes are very specific to the reaction they catalyse due to their unique shape. The unique sequence of amino acids gives the active site a very specific shape that matches the shape of the substrate. The shape of the active site also only matches the shape of one particular substrate. In this way, the enzyme and substrate interact using the lock and key model: the enzyme's active site is like a lock that will only fit one key - the substrate. Therefore, each enzyme will only catalyse one particular reaction. 

Changing enzyme reaction rates 

The rate of reactions catalysed by enzymes can be altered by a few things. These include pH, temperature, substrate concentration and enzyme concentration.

pH 

The shape of the active site is affected by pH and can change if the pH is not right. This change means that bonds in the enzyme are altered and so the enzyme is denatured. It cannot catalyse the reaction anymore, as the shape of the substrate and shape of the active site do not match anymore and the substrate cannot bind. Therefore, all enzymes work at an optimum pH, which is the pH that lets their active site stay in the right shape. 

Temperature

As temperature increases, the rate of reaction will increase but only up to a certain point. Once it gets too hot, the enzyme will denature as high temperatures break the bonds in the enzyme, preventing the reaction from being catalysed. Therefore, enzymes can work at a range of temperatures but they will have an optimum temperature where they work best and the rate of reaction is highest.

Substrate concentration

If there is less substrate than enzymes, then as the substrate concentration increases the rate of reaction will keep increasing. This is because at low substrate concentrations, there will be active sites that are unoccupied as they do not have a substrate bound to them. Therefore, adding more substrate will just mean that more enzymes are being utilised. 

However, once there is more substrate than enzyme, all the enzyme active sites are occupied; the enzyme concentration becomes the limiting factor of the reaction. The reaction cannot go any faster because there is no more enzyme available, it can only continue once an active site is freed up. At this point, increasing substrate concentration will not affect rate of reaction.

Enzyme concentration

As with substrate concentration, increasing enzyme concentration will increase rate of reaction up to a point. As we add more enzymes, we are introducing more unoccupied active sites - therefore, substrates can bind to these active sites so that more is being turned into products at any given time. 

However, once there is more enzyme than there is substrate, the rate of reaction cannot increase anymore as all the substrate is already bound to an enzyme active site and is being converted into products. The substrate concentration is now the limiting factor. At this point, increasing enzyme concentration will not affect rate of reaction. 

Interpreting graphs of enzymatic reactions

Reaction rates and enzyme activity are often presented graphically. We can see how the above factors and their impact on reaction rate looks on a graph. 

pH

There is a peak at the optimal pH where enzyme activity and rate of reaction is highest, with a very narrow range either side. This is because enzymes do not work over a range of pHs, rather only at one optimal pH.

Temperature

Although enzymes have an optimum temperature, they may work over a range of temperatures. After the enzyme has denatures it will not work at all - at this point, the activity drops to zero. 

Substrate concentration

At the beginning, rate of reaction increases as the substrate concentration increases, as more and more enzyme active sites are being occupied. However, after all the active sites have been occupied and enzyme concentration becomes the limiting factor, the rate of reaction plateaus - it has reached its maximum. 

Enzyme concentration

At the beginning, rate of reaction increases as enzyme concentration increases, as more and more substrates are binding to active sites and being converted into products. However, after all the substrate has been bound or converted and the substrate concentration becomes the limiting factor, the rate of reaction plateaus - it has reached its maximum. 

Enzymes in digestion

Enzymes are key in breaking down carbohydrates, lipids and proteins in food. 

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Digestive enzymes all have their own optimum pH; that is why carbohydrases and lipases are found in the saliva and small intestine (where the pH is fairly neutral) and proteases are found in the stomach (where the pH is fairly acidic due to stomach acid). They also have an optimum temperature of $$37\degree C$$​, which is our internal body temperature. Bile helps in maintaining optimum pH in the small intestine, as it can neutralise food that has become acidic after passing through the stomach.