The work of our body is an extremely complex process in which millions of cells, thousands of the most diverse substances are involved. But there is one area that completely and completely depends on special proteins, without which the life of a person or an animal will be completely impossible. As you probably guessed, we are now talking about enzymes.
Today we will consider the enzymatic function of proteins. This is an important area of biochemistry.
Since these substances are primarily based on proteins, they themselves can be considered them. You need to know that for the first time enzymes were discovered back in the 30s of the 19th century, only scientists needed more than a century to come to a more or less uniform definition for them. So what is the function of protein enzymes? You will learn about this, as well as their structure and reaction examples from our article.
You need to understand that not every protein can be an enzyme, even theoretically. Only globular-form proteins are capable of catalytic activity against other organic compounds. Like all natural compounds of this class, enzymes are composed of amino acid residues. Remember that the enzymatic function of proteins (examples of which will be in the article) can be performed only by those whose molar mass is not less than 5000.
What is an enzyme, modern definitions
Enzymes are catalysts of biological origin. They have the ability to accelerate reactions due to the closest contact between two substances (substrates) participating in the reaction. We can say that the enzymatic function of proteins is a process of catalysis of some biochemical reactions that are characteristic only of a living organism. Only a small fraction of them can be reproduced in a laboratory.
It should be noted that in recent years a certain breakthrough has been outlined in this direction. Scientists are gradually coming close to creating artificial enzymes that can be used not only for the purposes of the national economy, but also medicine. The development of enzymes that can effectively destroy even small areas of incipient oncological diseases.
What parts of the enzyme are directly involved in the reaction?
Note that not the whole body of the enzyme comes into contact with the substrate, but only its small area, which is called the active center. This is their main property, complementarity. This concept implies that the enzyme is ideally suited to the substrate in form and in its physicochemical properties. We can say that the function of protein enzymes in this case is as follows:
- Their water shell is coming off the surface.
- A certain deformation occurs (polarization, for example).
- After which they are located in a special way in space, simultaneously approaching each other.
It is these factors that accelerate the reaction. Now let's make a comparison between enzymes and inorganic catalysts.
Compare feature | Enzymes | Inorganic catalysts |
Faster forward and reverse response | The same | The same |
Specificity (complementarity) | Only suitable for a specific type of substance, high specificity | They can be universal, accelerating several similar reactions at once. |
Speed reaction | They increase the reaction intensity by several million times. | Acceleration of hundreds and thousands of times |
Reaction to heat | The reaction is "no" due to the complete or partial denaturation of the proteins involved in it | When heated, most catalytic reactions are accelerated many times |
As you can see, the enzymatic function of proteins suggests specificity. We also add from ourselves that many of these proteins also have species specificity. Simply put, human enzymes are unlikely to be suitable for guinea pigs.
Important enzyme structure information
In the structure of these compounds, three levels are distinguished at once. The primary structure can be identified by those amino acid residues that make up the enzymes. Since the enzymatic function of proteins, examples of which we repeatedly cite in this article, can be carried out only by certain categories of compounds, it is quite possible to determine them by this criterion.
As for the secondary level, membership in it is determined using additional types of bonds that can occur between these amino acid residues. These bonds are hydrogen, electrostatic, hydrophobic, as well as Van der Waals interactions. As a result of the stress that these bonds cause, α-helices, loops and β-strands are formed in various parts of the enzyme.
The tertiary structure appears as a result of the fact that relatively large sections of the polypeptide chain simply fold. The resulting cords are called domains. Finally, the final formation of this structure occurs only after a stable interaction is established between different domains. It should be remembered that the formation of the domains themselves occurs in an absolutely independent order.
Some characteristics of domains
Typically, the polypeptide chain from which they are formed consists of approximately 150 amino acid residues. When domains interact with each other, a globule is formed. Since the active centers on their basis perform the enzymatic function, the importance of this process should be understood.
The domain itself is characterized in that numerous interactions are observed between amino acid residues in its composition. Their number is much larger than those for reactions between the domains themselves. Thus, the cavities between them are relatively “vulnerable” to the action of various organic solvents. Their volume is about 20-30 cubic angstroms, which fit several water molecules. Different domains most often have a completely unique spatial structure, which is associated with the fulfillment by them of completely different functions.
Active centers
As a rule, active centers are located strictly between domains. Accordingly, each of them plays a very important role in the course of the reaction. Due to this arrangement of domains, significant flexibility, mobility of this region of the enzyme is revealed. This is extremely important, since only those compounds that can correspondingly change their spatial position perform the enzymatic function.
There is a direct link between the length of the polypeptide bond in the enzyme body and how complex functions it performs. The complication of the role is achieved both due to the formation of an active reaction center between two catalytic domains, and due to the formation of completely new domains.
Some enzyme proteins (examples are lysozyme and glycogen phosphorylase) can vary greatly in size (129 and 842 amino acid residues, respectively), although they catalyze the cleavage reaction of the same types of chemical bonds. The difference is that more massive and larger enzymes are able to better control their position in space, which ensures greater stability and reaction rate.
The main classification of enzymes
Currently, the standard classification is common and widespread throughout the world. According to her, six main classes are distinguished, with corresponding subclasses. We will consider only the basic ones. Here they are:
1. Oxidoreductases. The function of protein enzymes in this case is the stimulation of redox reactions.
2. Transferase. They can carry out transfer between substrates of the following groups:
- Monocarbon residues.
- Residues of aldehydes as well as ketones.
- Acyl and glycosyl components.
- Alkyl residues (as an exception cannot tolerate CH3).
- Nitrous bases.
- Phosphorus groups.
3. Hydrolases. In this case, the significance of the enzymatic function of proteins consists in the breakdown of the following types of compounds:
- Esters.
- Glycosides.
- Ethers as well as thioethers.
- Peptide type bonds.
- CN type bonds (except for the same peptides).
4. Liases. They have the ability to disengage groups with the subsequent formation of a double bond. In addition, they can perform the reverse process: joining individual groups to double bonds.
5. Isomerase. In this case, the enzymatic function of proteins is to catalyze complex isomeric reactions. The following enzymes belong to this group:
- Racemases, epimerases.
- Cystransisomerase
- Intramolecular oxidoreductases.
- Intramolecular transferases.
- Intramolecular lyases.
6. Ligases (otherwise known as synthetases). Serve to cleave ATP with the simultaneous formation of certain bonds.
It is easy to see that the enzymatic function of proteins is incredibly important, since they to one degree or another control almost all the reactions that occur in your body every second.
What remains of the enzyme after interaction with the substrate?
Often an enzyme is a protein of globular origin, the active center of which is represented by its amino acid residues. In all other cases, the center includes a prosthetic group firmly connected with it or coenzyme (ATP, for example), whose connection is much weaker. The whole catalyst is called a holoenzyme, and its residue, formed after the removal of ATP, is an azo enzyme.
Thus, on this basis, enzymes are divided into the following groups:
- Simple hydrolases, lyases and isomerases, which do not contain coenzyme base at all.
- Protein enzymes (examples are some transaminases) containing a prosthetic group (lipoic acid, for example). This group also includes many peroxidases.
- Enismism, for which coenzyme regeneration is required. These include kinases, as well as most oxidoreductases.
- Other catalysts, the composition of which is not yet fully understood.
All substances that make up the first group are widely used in the food industry. All other catalysts require very specific conditions for their activation, and therefore work only in the body or in some laboratory experiments. Thus, the enzymatic function is a very specific reaction, which consists in stimulating (catalysis) certain types of reactions under strictly defined conditions of the human or animal organism.
What happens in the active center, or why do enzymes work so efficiently?
We have repeatedly said that the key to understanding enzymatic catalysis is their creation of an active center. It is there that specific binding of the substrate takes place, which under such conditions reacts much more actively. In order for you to understand the complexity of the reactions carried out there, we give a simple example: for
fermentation of glucose to occur
, 12 enzymes are needed right away! Such a difficult interaction becomes possible solely because the protein that performs the enzymatic function has the highest degree of specificity.
Types of specificity of enzymes
It is absolute. In this case, specificity for only one, strictly defined type of enzyme is manifested. So, urease interacts only with urea. With milk lactose, it does not react under any conditions. This is the function of protein enzymes in the body.
In addition, absolute group specificity is often found. As the name implies, in this case there is a “susceptibility” to strictly one class of organic substances (esters, including esters, alcohols or aldehydes). So, pepsin, which is one of the main enzymes of the stomach, is specific only for the hydrolysis of the peptide bond. Alcohol dehydrase interacts exclusively with alcohols, and lactic dehydrase does not cleave anything but α-hydroxy acids.
It also happens that the enzymatic function is characteristic of a certain group of compounds, but under certain conditions, enzymes can act on substances that are quite different from their main “purpose”. In this case, the catalyst “gravitates” to a certain class of substances, but under certain conditions it can also break down other compounds (not necessarily similar). True, in this case, the reaction will go many times slower.
The ability of trypsin to act on peptide bonds is widely known, but few people know that this protein, which performs an enzymatic function in the gastrointestinal tract, may well interact with various ester compounds.
Finally, specificity is optical. These enzymes can interact with a wide range of completely diverse substances, but only on condition that they have strictly defined optical properties. Thus, the enzymatic function of proteins in this case is in many respects similar to the principle of action not of enzymes, but of catalysts of inorganic origin.
What factors determine the effectiveness of catalysis?
Today it is believed that factors that determine the extremely high degree of enzyme efficiency are:
- The effect of concentration.
- The effect of spatial orientation.
- Multifunctionality of the active center of reaction.
In general, the essence of the concentration effect is no different from that in the inorganic catalysis reaction. In this case, the concentration of the substrate is created in the active center, which is several times higher than the same value for the entire other volume of the solution. At the center of the reaction, the molecules of the substance that should react between themselves are selectively sorted. It is easy to guess that it is this effect that leads to an increase in the rate of a chemical reaction by several orders of magnitude.
When a standard chemical process takes place, it is extremely important which part the interacting molecules will collide with each other. Simply put, the molecules of a substance at the time of a collision must necessarily be strictly oriented relative to each other. Due to the fact that in the active center of the enzyme such a turn is performed in a forced manner, after which all the components involved line up in a certain line, the catalysis reaction is accelerated by approximately three orders of magnitude.
In this case, multifunctionality is understood as the property of all the constituent parts of the active center to act simultaneously (or strictly in concert) on the molecule of the “processed” substance. Moreover, it (the molecule) is not only properly fixed in space (see above), but also significantly changes its characteristics. All this together leads to the fact that it becomes much easier for enzymes to act on the substrate in the necessary way.