Option B.7 – Enzymes
B.7.1 – Describe the characteristics of biological catalysts (enzymes)
Enzymes are globular proteins that have a specific structure that allows them to catalyse a particular reaction. The shape of the active site on the molecule is crucial for maintaining the specificity of enzymes to their substrate molecules. The active site is where the substrate molecules bind and the reaction takes place. According to induced fit theory, the active site can alter its shape slightly, allowing some enzymes to catalyse more than one reaction. The biological reactions catalysed by enzymes would otherwise proceed too slowly to maintain life.
Where E=Enzyme, S=Substrate and P=Product
Enzymes, like any catalyst, provide an alternative reaction pathway that has a lower activation energy, allowing the reaction to take place more rapidly. The presence of an enzyme means that a greater proportion of the substrate molecules will be at or above the activation energy.
Some enzymes also require addition co-factors, or non-protein molecules, to help with their activity. These include vitamins and metal ions.
However, enzymes only speed up reactions to a certain extent. When the concentration of substrate molecules is above a certain level, all the enzymes present are saturated, and increasing the substrate concentration will not increase the rate of reaction.
B.7.2 – Compare inorganic catalysts and biological catalysts (enzymes)
B.7.4 – Determine Vmax and the value of the Michaelis constant (Km) by graphical means and explain its significance
Experimental data shows that the rate of reaction decreases as the substrate concentration increases. This is because, in low substrate concentrations, there are sufficient enzymes available to bind to the molecules. However, when more substrate is added, more of the enzymes will have occupied active sites and are not available to react. Eventually, and the substrate concentration continues to increase, the rate of reaction will plateau, as the enzymes are all saturated and cannot speed up the reaction further.
Each enzyme has a different saturation point, which can be calculated using the Michaelis-Menten equation. This takes into account the maximum velocity (Vmax), or rate of reaction, and the Michaelis constant (Km). The Vmax is affected by temperature and pH, and is also called the turnover number. The Km is the substrate concentration at . This constant tells
us about the attraction between the enzyme and the substrate. Low Km values mean that the reaction goes quickly at low substrate concentrations, whilst a high value means that the reaction proceeds more slowly, as there is lower affinity to the substrate.
B.7.5 – Describe the mechanism of enzyme action, including enzyme substrate complex, active site and induced fit model
The substrate particles are the reactants. The bind to the enzyme at the active site and the reaction takes place. Substrate specificity is determined by the chemical structure of the active site, meaning that an enzyme will only work with a certain substrate.
However, according to the induced-fit model, the active site can still alter slightly, allowing enzymes to bind to a number of similar substrates.
The substrate particles bind to the enzyme by intermolecular forces. The activation energy is reduced because the enzyme:
- Holds the reactants in the correct orientation
- Helping the bonds of the reactants to break
The enzyme-substrate complex is formed when the enzyme binds to the substrate.
B.7.6 – Compare competitive inhibition and non-competitive inhibition
Competitive inhibition is when the inhibitor and substrate must compete for the active site. The inhibitor is structurally similar to the substrate, and it prevents the substrate from binding. Examples are:
- O₂ competing with CO₂ for the active site of RuBisCo
- Malonate competing with succinate for the active site of succinate dehydrogenase
Competitive inhibition is reversible, and does not cause any permanent structural change to the enzyme. If the concentration of the substrate is increased, then the competition will be overcome. The Vmax of the reaction does not change, although the Km increases.
Non-competitive inhibitors bind to another place on the enzyme, causing the active site to change shape and become distorted. The substrate cannot bind. This is reversible, and once the inhibitor is removed, the enzyme will be able to bind with the substrate. Examples are:
- Cyanide ions blocking cytochrome oxidase in terminal oxidation in cell aerobic respiration
- Nerve gas Sarin blocking acetyl cholinesterase in synapse transmission
In this case, the Vmax will increase, but the Km will stay the same.
B.7.7 – State and explain the effects of heavy metal ions, temperature changes and pH changes on enzyme activity
Temperature -Each enzyme has an optimal temperature for function. At this temperature, the enzyme will work at its peak. After the temperature reaches its optimum level, the reaction rate abruptly declines. Many enzymes are adversely affected by high temperatures, at which point denaturation occurs. Many enzymes only have a narrow range of conditions under which they operate properly.
pH -Enzymes also have an optimal pH. At this point, the reaction will happen the fastest, as the enzyme is the most active. Extremes in pH will usually result in a complete loss of activity for most enzymes, or denaturation. The optimum pH for each enzyme varies greatly. For example, pepsin has an optimum pH of 1.5, but lipase has an optimum pH of 8.0.
Metal ions can form covalent bonds with sulfur atoms in enzymes. This causes the enzyme to change shape and lose its function. The active site will change shape, and the substrate will not be able to bind to it. This is a form of non-competitive inhibition.