CAPE 1: Enzymes and Enzyme Activity

What are enzymes?

Enzymes are globular proteins (biological catalysts). They speed up catalyse) chemical reactions in all living things, and allow them to occur more easily.

They are too small to be seen either when they are inside cells or after they have been released from them, for example in the digestive system.

Each particular enzyme has a unique, 3-dimensional shape shared by all its molecules. Within this shape there is an area called the active site where the chemical reactions occur. 

The active site makes an enzyme specific as it fits only one type of substrate.

How do enzymes work?

Enzymes work by 2 mechanisms:

1. The Lock and Key Model 

    --The enzyme is like a lock with a specific shape into which the key (substrate) fits.

    --Enzymes are usually larger than the substrates that they act on.

     --Once formed, the products cannot fit into the active site and are thus released, leaving the site free.

Click the link to see: the lock and key model in action

2. Induce Fit Model

     --The active site is not rigid and there is no exact fit, instead it can be modified as the substrate interacts with it.

      --The active site is moulded into the shape of the substrate on contact, improving the fit (makes a tighter fit).

Click the link to see: the induce fit model in action

What do enzymes do?

Enzymes lowers the amount of energy required for a chemical reaction to take place. (This energy is called the activation energy.) This causes a reaction involving enzymes to speed up in other words it takes a shorter time for this reaction to form products.

Some enzymes help to break down large molecules. Others build up large molecules from small ones. While many others help turn one molecule into another.

Properties of Enzymes ?

• Enzymes are catalysts → speed up chemical reactions
• Reduce activation energy required to start a reaction between molecules
• Substrates (reactants) are converted into products
• Reaction may not take place in absence of enzymes (each enzyme has a specific catalytic action)
• Enzymes remain unchanged at the end of a reaction. [E + S → ES → P + E]
• Enzymes are specific 

Factors affecting enzyme activity

Enzyme activity is  how fast an enzyme is working and is also called the "Rate of Reaction". It is affected by the following factors Temperature, pH, substrate concentration and enzyme concentration.

Although they can do fantastic things they are sensitive and work best under specific conditions.

Each type of enzyme has its own specific optimum conditions under which it works best.

Enzymes work best when they have a high enough substrate concentration for the reaction they catalyse. If too little substrate is available the rate of the reaction is slowed and cannot increase any further.

Sometimes, if too much product accumulates, the reaction can also be slowed down. So it is important that the product is removed.

The pH must be correct for each enzyme. If the conditions are too alkaline or acidic then the activity of the enzyme is affected (it slows down). This happens because the enzyme's shape, especially the active site, is changed. It is denatured, and cannot hold the substrate molecule.

Graph of enzyme activity verses temperature

As the temperature rises, reacting molecules have more and more kinetic energy. This increases the chances of a successful collision and so the rate increases. There is a certain temperature at which an enzyme's catalytic activity is at its greatest (see graph above). This optimal temperature is usually around human body temperature (37.5 oC) for the enzymes in human cells.

Above this temperature the enzyme structure begins to break down (denature) since at higher temperatures intra- and intermolecular bonds are broken as the enzyme molecules gain even more kinetic energy. At very low temperature enzymes are inactive.

Concentration of enzyme and substrate

Graph of enzyme activity verses enzyme concentration           Graph of enzyme activity verses substrate concentration

           

The rate of an enzyme-catalysed reaction depends on the concentrations of enzyme and substrate. As the concentration of either is increased the rate of reaction increases (see graphs).

For a given enzyme concentration, the rate of reaction increases with increasing substrate concentration up to a point, above which any further increase in substrate concentration produces no significant change in reaction rate. This is because the active sites of the enzyme molecules at any given moment are virtually saturated (occupied) with substrate. The enzyme/substrate complex has to dissociate before the active sites are free to accommodate more substrate. (See graph above on the right)

Provided that the substrate concentration is high and that temperature and pH are kept constant, the rate of reaction is proportional to the enzyme concentration. (See graph above on the left).

Inhibition of Enzyme Activity

Some substances reduce or even stop the catalytic activity of enzymes in biochemical reactions. They block or distort the active site. These chemicals are called inhibitors, because they inhibit reaction.

Inhibitors that occupy the active site and prevent a substrate molecule from binding to the enzyme are said to be active site-directed (or competitive, as they 'compete' with the substrate for the active site).

Inhibitors that attach to other parts of the enzyme molecule, perhaps distorting its shape, are said to be non-active site-directed (or non competitive).

CLICK HERE FOR ANIMATION: see for yourself how changing factors affect enzyme activity

• All enzymes are globular proteins and round in shape
• They have the suffix "-ase"
• Intracellular enzymes are found inside the cell
• Extracellular enzymes act outside the cell (e.g. digestive enzymes)
• Enzymes are catalysts → speed up chemical reactions
  • Reduce activation energy required to start a reaction between molecules
  • Substrates (reactants) are converted into products
  • Reaction may not take place in absence of enzymes (each enzyme has a specific catalytic action)
  • Enzymes catalyse a reaction at max. rate at an optimum state
• Induced fit theory
  • Enzyme's shape changes when substrate binds to active site
  • Amino acids are moulded into a precise form to perform catalytic reaction effectively
  • Enzyme wraps around substrate to distort it
  • Forms an enzyme-substrate complex → fast reaction
  • E + S → ES → P + E
• Enzyme is not used up in the reaction (unlike substrates)

Changes in pH

• Affect attraction between substrate and enzyme and therefore efficiency of conversion process
• Ionic bonds can break and change shape / enzyme is denatured
• Charges on amino acids can change, ES complex cannot form
• Optimum pH
  • pH 7 for intracellular enzymes
  • Acidic range (pH 1-6) in the stomach for digestive enzymes (pepsin)
  • Alkaline range (pH 8-14) in oral cavities (amylase)
• pH measures the conc. of H+ ions - higher conc. will give a lower pH

Enzyme Conc. is proportional to rate of reaction, provided other conditions are constant. Straight line
Substrate Conc. is proportional to rate of reaction until there are more substrates than enzymes present. Curve becomes constant.

Increased Temperature

• Increases speed of molecular movement → chances of molecular collisions → more ES complexes
• At 0-42 °C rate of reaction is proportional to temp
• Enzymes have optimum temp. for their action (varies between different enzymes)
• Above ≈42°C, enzyme is denatured due to heavy vibration that break -H bonds
  • Shape is changed / active site can't be used anymore

Decreased Temperature

• Enzymes become less and less active, due to reductions in speed of molecular movement
• Below freezing point
  • Inactivated, not denatured
  • Regain their function when returning to normal temperature
• Thermophilic: heat-loving
• Hyperthermophilic: organisms are not able to grow below +70°C
• Psychrophiles: cold-loving

Inhibitors

• Slow down rate of reaction of enzyme when necessary (e.g. when temp is too high)
• Molecule present in highest conc. is most likely to form an ES-complex
• Competitive Inhibitors
  • Compete with substrate for active site
  • Shape similar to substrates / prevents access when bonded
  • Can slow down a metabolic pathway
• [EXAMPLE] Methanol Poisoning
  • Methanol CH3OH is a competitive inhibitor
  • CH3OH can bind to dehydrogenase whose true substrate is C2H5OH
  • A person who has accidentally swallowed methanol is treated by being given large doses of C2H5OH
  • C2H5OH competes with CH3OH for the active site
• Non-competitive Inhibitors
  • Chemical does not have to resemble the substrate
  • Binds to enzyme other than at active site
  • This changes the enzyme's active site and prevents access to it
• Irreversible Inhibition
• Chemical permanently binds to the enzyme or massively denatures the enzyme
• Nerve gas permanently blocks pathways involved in nerve message transmission, resulting in death
• Penicillin, the first of "wonder drug" antibiotics, permanently blocks pathways certain bacteria use to assemble their cell wall component (peptidoglycan)

End-product inhibition

• Metabolic reactions are multi-stepped, each controlled by a single enzyme
• End-products accumulate within the cell and stop the reaction when sufficient product is made
• This is achieved by non-competitive inhibition by the end-product
• The enzyme early in the reaction pathway is inhibited by the end-product

The metabolic pathway contains a series of individual chemical reactions that combine to perform one or more important functions. The product of one reaction in a pathway serves as the substrate for the following reaction.

Curves for reaction rates against substrate concentrations & How to read them

This graph is showing how substrate concentration affects the rate of reaction which is also the speed or velocity of the reaction. The maximum velocity is called Vmax.



The graph levels off at a maximum reaction rate of Vmax. This happens when all the enzyme molecules are working as fast as they can. Once that happens, increasing the concentration of the substrate can't make the reaction go any faster.

What happens to the graph in the presence of a competitive inhibitor?


At a relatively low substrate concentration, the rate of the reaction will be less in the presence of a competitive inhibitor. The competitive inhibitor is just getting in the way of the substrate by attaching itself to some of the active sites.

However, as the substrate concentration increases, the substrate out-competes the inhibitor. If there is a lot more substrate than inhibitor, the chances are far higher that a substrate molecule will hit an active site than an inhibitor molecule will.

That means that at a high enough substrate concentration, the maximum reaction rate will again be Vmax.



What happens to the graph in the presence of a non-competitive inhibitor?


This is quite different.

Once a non-competitive inhibitor has attached itself to the enzyme, that particular enzyme molecule won't work any more. The attachment may be reversible, but even if it is, there will be a proportion of the enzyme molecules which are out of action at any one time.

That means that however much you may increase the concentration of the substrate, you will never reach the original Vmax.

So a graph involving a non-competitive inhibitor looks like this:



There is now a new Vmax, lower than the one where there was no inhibitor present.

Combining these two graphs



In order to get the full mark for the non-competitive line, it had to start off more steeply than the other one, and then it may or may not cross - as I have shown in this diagram. This is so because "The initial rate of a non-competitively inhibited reaction could be greater or less than that of a competitively inhibited reaction. If greater, the curves will cross; if less, they won't cross.”