A macromolecule is a molecule that has a high molecular weight. Its structure is presented in the form of multiple repeating units. Consider the features of such compounds, their importance for the life of living beings.
Features of the composition
Biological macromolecules are formed from hundreds of thousands of small starting materials. For living organisms, three main types of macromolecules are characteristic: proteins, polysaccharides, nucleic acids.
The starting monomers for them are monosaccharides, nucleotides, amino acids. A macromolecule is almost 90 percent of the cell mass. A specific protein molecule is formed depending on the sequence of amino acid residues.
Those substances that have a molar mass of more than 103 Da are considered high molecular weight.
History of the term
When did the macromolecule appear? This concept was introduced by the Nobel laureate in chemistry Hermann Staudinger in 1922.
The polymer ball can be considered as a tangled thread, which was formed during random unwinding throughout the room of the coil. This ball systematically changes its conformation, this is the spatial configuration of the macromolecule. It is similar to the trajectory of Brownian motion.
Such a coil is formed due to the fact that at a certain distance the polymer chain “loses” direction information. One can speak of a ball in the case when the high molecular weight compounds are much longer in length than the length of the structural fragment.
Globular configuration
A macromolecule is a dense conformation in which the volume fraction of a polymer can be compared with a unit. The globular state is realized in those cases when, under the mutual action of the individual polymer units, mutual attraction arises between themselves and the external environment.
A replica of the structure of a macromolecule is the part of water that is integrated as an element of such a structure. It is the nearest hydrated environment of the macromolecule.
Characterization of the protein molecule
Protein macromolecules are hydrophilic substances. When a dry protein is dissolved in water, it initially swells, then a gradual transition to a solution is observed. During swelling, water molecules penetrate into the protein, and its structure is bound by polar groups. In this case, the dense packing of the polypeptide chain is loosened. The swollen protein molecule is considered a reverse solution. With the subsequent absorption of water molecules, a separation of protein molecules from the total mass is observed, and the dissolution process is also underway.
But swelling of a protein molecule does not in all cases cause dissolution. For example, collagen after absorption of water molecules remains in a swollen state.
Hydration theory
High-molecular compounds according to this theory do not just adsorb; electrostatic binding of water molecules to polar fragments of the side radicals of amino acids that have a negative charge, as well as of the basic amino acids that carry a positive charge, occurs.
Partially hydrated water binds to peptide groups forming hydrogen bonds with water molecules.
For example, polypeptides that have nonpolar side groups swell. When bound to peptide groups, it spreads polypeptide chains. The presence of interchain bridges does not allow protein molecules to completely come off, go into the form of a solution.
The structure of macromolecules is destroyed when heated, as a result, the polypeptide chains break and release.
Gelatin Features
The chemical composition of gelatin is similar to collagen, it forms a viscous liquid with water. Among the characteristic properties of gelatin, one can single out its ability to gel.
These types of molecules are used as hemostatic and plasma substituting agents. The ability of gelatin to form gels is used in the manufacture of capsules in the pharmaceutical industry.
The solubility of macromolecules
These types of molecules have different solubilities in water. It is determined by the amino acid composition. In the presence of polar amino acids in the structure, the ability to dissolve in water increases significantly.
Also, this feature is affected by the peculiarity of the organization of the macromolecule. Globular proteins have higher solubility than fibrillar macromolecules. In the course of numerous experiments, the dependence of dissolution on the characteristics of the solvent used was established.
The primary structure of each protein molecule is different, which gives the protein an individuality of properties. The presence of cross-links between polypeptide chains reduces solubility.
The primary structure of protein molecules is formed due to peptide (amide) bonds; during its destruction, protein denaturation occurs.
Salting out
To increase the solubility of protein molecules, solutions of neutral salts are used. For example, in a similar way it is possible to carry out selective precipitation of proteins, to carry out their fractionation. The resulting number of molecules depends on the initial composition of the mixture.
A feature of proteins that are obtained by salting out is the preservation of biological characteristics after complete removal of salt.
The essence of the process is the removal by the anions and cations of the salt of the hydrated protein shell, which ensures the stability of the macromolecule. The maximum number of protein molecules is salted out using sulfates. This method is used for the purification and separation of protein macromolecules, since they differ significantly in charge size and hydration shell parameters. Each protein has its own salting out zone, that is, for it you need to select a salt of a given concentration.
Amino acids
Currently, about two hundred amino acids that are part of protein molecules are known. Depending on the structure, they are divided into two groups:
- proteinogenic, which are part of macromolecules;
- non-proteinogenic, not actively involved in the formation of proteins.
Scientists were able to decipher the sequence of amino acids in many protein molecules of animal and plant origin. Among the amino acids, which are often quite a part of protein molecules, note serine, glycine, leucine, alanine. Each natural biopolymer is characterized by the presence of its own amino acid composition. For example, protamines include about 85 percent arginine, but they do not have acidic, cyclic amino acids. Fibroin is a protein molecule of natural silk, which contains about half of the glycine. Collagen contains such rare amino acids as hydroxyproline, hydroxylysine, which are absent in other protein macromolecules.
The amino acid composition is determined not only by the characteristics of amino acids, but also by the functions and purpose of protein macromolecules. Their sequence is determined by the genetic code.
Levels of structural organization of biopolymers
There are four levels: primary, secondary, tertiary, and also quaternary. Each structure has its own distinctive characteristics.
The primary structure of protein molecules is a linear polypeptide chain of amino acid residues linked by peptide bonds.
It is this structure that is the most stable, since it contains peptide covalent bonds between the carboxyl group of one amino acid and the amino group of another molecule.
The secondary structure involves the folding of the polypeptide chain using hydrogen bonds in a spiral form.
The tertiary type of biopolymer is obtained by spatial folding of the polypeptide. Spiral and layered-folded forms of tertiary structures are subdivided.
Globular proteins are characterized by an ellipsoidal shape, and an elongated shape is characteristic of fibrillar molecules.
If the macromolecule contains only one polypeptide chain, the protein has only a tertiary structure. For example, it is a muscle tissue protein (myoglobin) necessary for oxygen binding. Some biopolymers line up from several polypeptide chains, each of which has a tertiary structure. In this case, the macromolecule has a quaternary structure, consisting of several globules combined into a large structure. Hemoglobin can be considered the only quaternary protein that contains about 8 percent histidine. It is he who is the active intracellular buffer in erythrocytes, which allows maintaining a stable pH value of the blood.
Nucleic acids
They are high molecular weight compounds that are formed by nucleotide fragments. RNA and DNA are found in all living cells, it is they that perform the function of storing, transmitting, and also implementing hereditary information. Nucleotides act as monomers. Each of them contains the remainder of the nitrogenous base, carbohydrate, as well as phosphoric acid. Studies have shown that the principle of complementation (complementarity) is observed in the DNA of various living organisms. Nucleic acids are soluble in water but not soluble in organic solvents. These biopolymers are destroyed by increasing temperature, ultraviolet radiation.
Instead of a conclusion
In addition to various proteins and nucleic acids, macromolecules are carbohydrates. Polysaccharides in their composition have hundreds of monomers, which have a pleasant sweet taste. Huge molecules of proteins and nucleic acids with complex subunits can be cited as examples of the hierarchical structure of macromolecules.
For example, the spatial structure of a globular protein molecule is the result of a hierarchical multilevel organization of amino acids. There is a close connection between the individual levels, elements of a higher level are associated with the lower layers.
All biopolymers perform an important similar function. They are the building material for living cells, responsible for the storage and transmission of hereditary information. Each living creature is characterized by specific proteins, so biochemists have a difficult and responsible task, solving which they save living organisms from certain death.