5.2 - Enzymes

Enzymes are proteins that play vital roles in almost all biochemical reactions in our body. They act as organic catalysts that help to speed up the reactions in our body. Enzymes react with substances called the substrates. The substrates will bind to the active sites of the enzymes to form enzyme-substrate complex.

Figure 2 - Enzymes are very specific molecules in the body that only bind to substrates that will fit their shapes. Note that the enzyme retains its structure after the reaction.

Enzyme Nomenclature

According to The International Union of Biochemistry and Molecular Biology (IUBMB), the name of an enzyme is derived based on the substrate or the reaction that it catalyzes. The enzymes are named by adding the suffix '-ase' to the name of the substrate that it catalyzes. For example,

• Lactase breaks down lactose
• Penicillinase inactivates penicillin
• Telomerase produces telomere
• Peroxidase breaks down peroxides
• Acetylcholinesterase catalyzes the hydrolysis of acetylcholine

However, not all enzymes are named after this rule. Some have arbitrary names such as trypsin, pepsin and renin.

General Characteristics of Enzymes

1. Enzymes are specific molecules that only bind with specific substrates.
2. Enzymes are very efficient. They can catalyze up to 10,000 molecules of substrates within one second. They can catalyze up to 10,000 molecules of substrates within one second.
3. Enzymes are only needed in small quantities in the reaction. It can be reused after every reaction.
4. Enzymes maintains its structure after every reaction.
5. Most of the enzyme-catalyzed reactions are reversible.
6. Enzymes are biological catalysts that help increase the rate of reactions.
7. Some enzymes require cofactors to function more efficiently such as Vitamin B or magnesium ions.
8. Enzymes can be inhibited or destroyed by inhibitors such as heavy metals (lead or mercury).

Intracellular and Extracellular Enzymes.

Enzymes are capable of functioning both inside and outside of the cells. Intracellular enzymes are enzymes produced that function inside cells. They are responsible in metabolic pathways such as glycolysis and photosynthesis. Meanwhile, extracellular enzymes are enzymes that are secreted by cells and function outside the cells. For example, pancreatic enzymes produced by the pancreas are used to digest food in the duodenum. The process of enzyme production is illustrated as below (Figure 3).

Figure 3 - 1. The process of protein synthesis happens at the ribosomes. 2. The synthesized protein then moves through the lumen of the RER and buds off from the end of the RER, forming a transport vesicle. 3. The transport vesicle carries the synthesized proteins to the Golgi apparatus. It fuses with the Golgi apparatus and releases the proteins for modification. 4. The proteins are modified into enzymes and packed into secretory vesicles which bud off from the tip of the Golgi apparatus. 5. The secretory vesicles move towards the plasma membrane and fuses with it to release the enzymes out of the cell.

Mechanism of Enzyme Action

Enzymes are 3D molecules with specific configurations. The part of the enzymes that bind with the substrates is called the active site. The active site of an enzyme is very specific, which means they will only bind with substrates that are complement to the active site. The substrates have to fit perfectly into the active site for catalysis to occur. This specificity can be explained in the 'lock and key' hypothesis.

Figure 4 - The 'lock and key' hypothesis illustrates the specificity of an enzyme. Just like a key and a lock, only the specific type of key can fit into the specific key for the lock to be opened. 1. Only the substrate that can fit in the active site of enzyme will bind. 2. Binding of substrate to enzyme forms enzyme-substrate complex. 3. The products leave the active site. The enzyme is ready to bind with other substrates.

Figure 5 - All processes have a threshold energy that must be overcome in order for the reaction to start. In the presence of an enzyme, the activation energy is lowered which means the reaction can begin at a faster rate compared to those reaction where enzymes are absent.

Mechanism of Enzyme Action and Factor Changes

There are four main factors that can affect the rate of enzyme-catalyzed reaction.

Temperature

• Higher temperature increases the rate of reaction.
• Kinetic energy of substrate and enzyme increases.
• Higher frequency of effective collision, more enzyme-substrate complex, more products found.
• However, if the temperature continues to increase, denaturation of enzyme will cause the reaction to decrease and eventually stop.

Figure 6 - Each rise 10°C in temperature, the rate of reaction doubles. At the optimal temperature, the rate of enzyme action is at its highest. Most of the enzymes in our body is optimum at 37°C, which is our body temperature.

pH

• Different enzymes have different optimal pH activity.
• Pepsin is optimum between pH 1.5 and 3.5
• Amylase is optimum at pH 6.8
• Trypsin is optimum at around pH 8.5

Figure 7 - The surrounding pH conditions can also affect the activity of enzymes. Enzymes can easily become denatured if their optimum temperature is not met.

Substrate concentration

• Higher concentration of substrates, higher rate of reaction
• More effective collision between substrates and enzymes, more enzyme-substrate complex, more products formed
• However, enzymes become saturated when substrate concentration continues to increase. The rate of reaction becomes constant
• Enzyme is the limiting factor

Figure 8 - When the maximum rate of reaction is reached, it means that all the active sites of the enzymes have been occupied with the substrates. The enzymes are said to have limited the rate of reaction. So, if more enzymes are added, the rate of reaction will further increase.

Enzyme concentration

• Higher concentration of enzymes, higher rate of reaction
• More effective collision between enzymes and substrates, more enzyme-substrate complex, more products formed
• However, the rate of reaction becomes constant when enzyme concentration continues to increase.
• Substrate is the limiting factor

Figure 9 - When the concentration of enzymes is doubled, the rate of reaction is also doubled. At one point, all the substrates are already bound to the enzymes. The substrates are said to be limited. Only adding more substrates can further increase the rate of reaction.