An enzyme is a highly selective catalyst that greatly accelerates both the speed and the specificity of metabolic reactions.
What are Enzymes?
Enzymes help speed up chemical reactions in the human body. They bind to molecules and alter them in specific ways. They are essential for respiration, digesting food, muscle and nerve function, among thousands of other roles.
Properties of Enzymes
- Almost all enzymes are proteins, although some catalytically active RNA molecules have been identified.
- Enzyme catalyzed reactions generally take place under relatively mild conditions (temperatures well below 100 ° C, atmospheric pressure, and neutral pH) compared to corresponding chemical reactions.
- Enzymes are catalysts that increase the speed of a chemical reaction without being modified in the process.
- Enzymes are highly specific for the substrates on which they act and the products they form.
- The activity of the enzyme can be regulated, varying in response to the concentration of substrates or other molecules.
- They work under strict temperature and pH conditions in the body.
Ccoenzymes and prosthetic groups
- Many enzymes require the presence of small non-protein units or cofactors to carry out their particular reaction.
- The cofactors can be one or more inorganic ions, such as Zn2 + or Fe2 +, or a complex organic molecule called a coenzyme.
- A metal or coenzyme that covalently binds to the enzyme is called a prosthetic group (heme in hemoglobin).
- Some coenzymes, like NAD +, are bound and released by the enzyme during its catalytic cycle and, in effect, function as co-substrates. Many coenzymes are derived from vitamin precursors.
Holoenzyme and Apo enzymes
A complete catalytically active enzyme along with its coenzyme or metal ion is called a holoenzyme.
The protein part of the enzyme by itself without its cofactor is called an apoenzyme.
Isoenzymes are different forms of an enzyme that catalyze the same reaction, but that exhibit different physical or kinetic properties, such as isoelectric point, optimal pH, substrate affinity, or the effect of inhibitors.
The different forms of isoenzymes of a given enzyme are generally derived from different genes and often occur in different tissues in the body.
An example of an enzyme that has different forms of isoenzymes is lactate dehydrogenase (LDH) that catalyzes the reversible conversion of pyruvate to lactate in the presence of the coenzyme NADH.
LDH is a tetramer of two different types of subunits, called H and M, that have small differences in amino acid sequences. The two subunits can be randomly combined, forming five isoenzymes having the compositions H4, H3M, H2M2, HM3, and M4. All five isoenzymes can be resolved electrophoretically.
The active site of Enzymes
- The active site of an enzyme is the region that binds the substrate and turns it into a product.
- It is generally a relatively small part of the entire enzyme molecule and is a three-dimensional entity made up of amino acid residues that can be widely separated in the linear polypeptide chain.
- The active site is often a cleft or crack in the enzyme surface that forms a predominantly nonpolar environment that improves substrate binding.
- The substrate (s) is (are) bound at the active site by multiple weak forces (electrostatic interactions, hydrogen bonds, van der Waals bonds, hydrophobic interactions; and in some cases by reversible covalent bonds.
Substrate Specificity of Enzymes
- The properties and spatial arrangement of amino acid residues that make up the active site of an enzyme will determine which molecules can bind to and be substrates for that enzyme.
- Substrate specificity is often determined by changes in relatively few amino acids at the active site.
- This is clearly seen in the three digestive enzymes trypsin, chymotrypsin, and elastase.
Mechanism of Action of Enzymes
- The substrate (s) is (are) bound at the active site by multiple weak forces that result in the enzyme-substrate complex.
- Once attached, the active residues within the enzyme’s active site act on the substrate molecule to transform it first into the transition state complex and then into the product, which is released.
- The enzyme is now free to bind to another molecule in the substrate and begin its catalytic cycle again.
Nomenclature of Enzymes
Many enzymes are named by adding the suffix “use” to the name of their substrate.
Example. Urease is the enzyme that catalyzes the hydrolysis of urea, and fructose-1,6-bisphosphatase hydrolyzes fructose-1,6-bisphosphate.
- However, other enzymes, such as trypsin and chymotrypsin, have names that do not denote their substrate.
- Some enzymes have various alternative names.
- To streamline enzyme names, an enzyme naming system has been internationally agreed upon.
- This system places all the enzymes in one of the six main classes according to the type of catalyzed reaction. Each enzyme is uniquely identified with a four-digit classification number.
Example: Trypsin has the Enzyme Commission (EC) number 184.108.40.206, where
- The first number (3) denotes that it is a hydrolase
- The second number (4) that it is a protease that hydrolyzes peptide bonds
- The third number (21) that it is a serine protease with a critical serine
- residue at the active site, and
- The fourth number (4) indicates that it was the fourth enzyme to be assigned to this class.
- For comparison, chymotrypsin has the EC number 220.127.116.11, and elastase 18.104.22.168.
The Classification of Enzymes
- Catalyze oxidation-reduction reactions where electrons are transferred.
- These electrons are generally in the form of hydride ions or hydrogen atoms.
- The most common name used is a dehydrogenase, and sometimes reductase is used.
- An oxidase is referred to when the oxygen atom is the acceptor.
- Catalyze group transfer reactions.
- The transfer occurs from one molecule that will be a donor to another molecule that will be the acceptor.
- Most of the time, the donor is a cofactor who takes care of the group about to be transferred.
- Example: hexokinase used in glycolysis.
- Catalyze reactions involving hydrolysis.
- It generally involves the transfer of functional groups to water.
- When hydrolase acts on amide, glycosyl, peptide, ester, or other linkages, they not only catalyze the hydrolytic removal of a group from the substrate, but also a transfer of the group to an acceptor compound.
For example: chymotrypsin.
- Catalyze reactions where functional groups are added to break double bonds in molecules or vice versa where double bonds are formed by the elimination of functional groups.
- For example, fructose bisphosphate aldolase used in the conversion of fructose 1,6-bisphosphate to G3P and DHAP by severing the C-C bond.
- Catalyze reactions that transfer functional groups within a molecule to produce isomeric forms.
- These enzymes allow structural or geometric changes within a compound.
- For example, phosphoglucose isomerase to convert glucose 6-phosphate to fructose 6-phosphate. The chemical group in motion within the same substrate.
- They are involved in catalysis where two substrates are joined and in the formation of carbon-carbon, carbon sulfide, carbon-nitrogen and carbon-oxygen bonds due to condensation reactions.
- These reactions are coupled to ATP cleavage.
Significance of Enzymes
- In the absence of an enzyme, biochemical reactions hardly occur, while in its presence the rate can be increased up to 107 times. Therefore, they are crucial for the normal metabolism of living systems.
- In addition to the body, the extracted and purified enzymes have many applications.
The medical applications of enzymes include:
- To treat enzyme related disorders.
- To aid in metabolism
- To assist in the delivery of medications.
- To diagnose and detect diseases.
- In drug manufacturing.
Industrial enzyme applications include:
- Amylase, lactase, cellulase are enzymes that are used to break down complex sugars into simple sugars.
- Pectinase, like the enzymes that act on hard pectin, is used in the manufacture of fruit juices.
- Lipase enzymes act on lipids to break them down into fatty acids and glycerol. Lipases are used to remove grease stains, oils, butter.
- Enzymes are used in detergents and laundry soaps.
- Protease enzymes are used to remove stains from protein nature like blood, sweat, etc.
What do enzymes do?
The digestive system: Enzymes help the body break down larger complex molecules into smaller molecules, such as glucose so that the body can use them for fuel.
DNA replication: Every cell in your body contains DNA. Every time a cell divides, that DNA needs to be copied. Enzymes aid in this process by unwinding the coils of DNA and copying the information.
Liver enzymes: The liver breaks down toxins in the body. To do this, it uses a variety of enzymes.
To ensure that the body’s systems function properly, enzymes sometimes need to be slowed down. For example, if an enzyme is producing too much product, there must be a way to reduce or stop production.
Enzyme activity can be inhibited in several ways:
Competitive inhibitors: A molecule blocks the active site so that the substrate has to compete with the inhibitor to bind to the enzyme.
Non-competitive inhibitors: A molecule binds to an enzyme somewhere other than the active site and reduces how well it works.
Non-competitive inhibitors: The inhibitor binds to the enzyme and the substrate once they have bound each other. The products leave the active site less easily and the reaction slows down.
Irreversible inhibitors: An irreversible inhibitor binds to an enzyme and permanently inactivates it.
Examples of specific enzymes
There are thousands of enzymes in the human body, these are just a few examples:
Lipases: a group of enzymes that help digest fats in the intestine.
Amylase: Helps convert starches to sugars. Amylase is found in saliva.
Maltase: also found in saliva; breaks the maltose sugar into glucose. Maltose is found in foods like potatoes, pasta, and beer.
Trypsin: found in the small intestine, breaks proteins down into amino acids.
Lactase: Also found in the small intestine, it breaks down lactose, milk sugar, glucose, and galactose.
Acetylcholinesterase: breaks down the neurotransmitter acetylcholine in the nerves and muscles.
Helicase: unravels DNA.
DNA polymerase: synthesizes DNA from deoxyribonucleotides.