Proteins – macromolecular (molecular weight from about 10,000 to several million Dalton) biopolymers, or rather biological polycondensates, made of amino acid residues connected together by peptide bonds -CONH-. They are found in all living organisms and viruses. Protein synthesis takes place with the help of special cellular organelles called ribosomes.

The main elements included in the proteins are C, O, H, N, S, also P and sometimes metal cations Mn2 +, Zn2 +, Mg2 +, Fe2 +, Cu2 +, Co2 + and others.
This composition does not coincide with the composition of amino acids. This is because most proteins (they are called compound proteins or proteids) have various other molecules attached to their amino acid residues. The rule is the attachment of sugars, and in addition, many different organic compounds acting as coenzymes and metal ions can be attached covalently or by hydrogen bonds.

Protein structure
The protein chain synthesized in the cell resembles a “thread” floating in solution, which can take any shape (in biophysics it is called a statistical bundle), but undergoes the process of so-called protein folding to create a more or less rigid spatial structure, called “native” protein structure or conformation. Usually only molecules that have collapsed into such a structure can perform the proper biochemical role of a given protein; however, there are proteins without a tertiary structure that are an exception to this rule.

Due to the spatial scale, the full structure of the protein can be described at four levels:
• Primary protein structure (amino acid sequence, primary protein structure) – order of amino acids in the polypeptide chain
• Secondary protein structure – spatial arrangement of fragments of polypeptide chains. The secondary structures include:
– alpha helix (α helix)
– beta sheet (β sheet)
– beta bend (omega loops) (β hairpin)
• Tertiary structure of the protein – the mutual location of elements of the secondary structure.
• Quaternary structure of the protein – mutual location of polypeptide chains and possibly non-protein structures  (prosthetic group):
– sugars in glycoproteins
– lipids in lipoproteins
– nucleic acids in nucleoproteins
– chromoprotein dyes
– phosphoric acid residue in phosphoproteins.

Most often, the elemental composition of proteins is presented as follows:
Coal – 50-55%
Oxygen – 19-24%
Nitrogen – 15-18%
Hydrogen – 6–8%
Sulfur – 0.3–3%
Phosphorus – 0-0.5%.

Physical and chemical properties
Proteins do not have their own melting point. When heated in solution, and even more so in solid state, they undergo a irreversible denaturation (shear fiber cutting) above a certain temperature – a change in the structure that makes the protein biologically inactive (the daily example of such denaturation is frying or boiling an egg). This is due to the irreversible loss of tertiary or quaternary protein structure. For this reason, to obtain a dry but undenatured sample of a given protein, a freeze-drying method is used, i.e. evaporation of water or other solvents from the frozen sample under reduced pressure. Protein denaturation can also occur under the influence of heavy metal salts, strong acids and bases, low molecular weight alcohols, aldehydes and irradiation. The exception are simple proteins that can also undergo the reverse process, the so-called renaturation – after removing the factor that caused this denaturation. A small portion of proteins undergoes permanent denaturation under the influence of increased salt concentration in the solution, but the salting out process is in most cases fully reversible, thanks to which it is possible to isolate or separate proteins.
Proteins are generally water soluble. Proteins insoluble in water include fibrillar proteins found in the skin, tendons, hair (collagen, keratin) or muscles (myosin). Some of the proteins may dissolve in dilute acids or bases, others in organic solvents. The solubility of proteins is affected by the concentration of inorganic salts in the solution, while the low salt concentration has a positive effect on the solubility of proteins. However, at higher concentration, damage to the solvation shell occurs, which causes proteins to fall out of the solution. This process does not affect the protein structure, so it is reversible and is called protein salting out.
Proteins have the ability to bind water molecules. This effect is called hydration. Even after receiving a dry protein sample, it contains bound water molecules.

Proteins, due to the presence of basic NH2 and acidic COOH groups, have a binary character – depending on the pH of the solution, they will behave as acids (in alkaline solution) or as bases (in acidic solution). Thanks to this, proteins can act as a pH stabilizing buffer, e.g. blood. However, the difference in pH cannot be significant, as the protein may be denatured. The net charge of protein depends on the amount of acid and basic amino acids in the molecule. The pH value at which the positive and negative charges of the amino acids balance off is called the protein isoelectric point.
Proteins play an essential role in all biological processes. They participate in the catalysis of many transformations in biological systems (enzymes are proteins), participate in the transport of many small molecules and ions (e.g. 1 hemoglobin molecule carrying 4 oxygen molecules), serve as antibodies and participate in the transmission of nerve impulses as receptor proteins. Proteins also perform a mechanical and structural function. All proteins are made of amino acids. Some proteins contain unusual, rare amino acids that complement their basic set. Many amino acids (usually over 100) linked together by peptide bonds form a polypeptide chain in which two distinct ends can be distinguished. At one end of the chain is an unblocked amino group (the so-called N-terminus), at the other is an unblocked carboxyl group (C-terminus).

Protein breakdown
There are many criteria for protein breakdown. Due to the structure and composition, we divide proteins into simple and complex.

• Simple proteins (proteins) are made exclusively of amino acids. We divide them into the following groups:
protamines – they are strongly alkaline, have a high arginine content and lack of sulfur-containing amino acids. They are well soluble in water. The most famous protamines are: clupeine, salmine, cyprinin, ezocin, gallin.
histones – like protamines are strongly alkaline and dissolve well in water; components of cell nuclei (in combination with deoxyribonucleic acid), i.e. they are also present in erythroblasts. They contain a large amount of such amino acids as lysine and arginine.
Albumin – neutral proteins that perform a number of important biological functions: they are enzymes, hormones and other biologically active compounds. They dissolve well in water and diluted salt solutions, they coagulate easily. They are found in muscle tissue, blood plasma and milk.
globulins – they contain all protein amino acids, except that aspartic acid and glutamic acid in larger quantities; unlike albumin, they are poorly soluble in water, whereas they are well in dilute salt solutions; have similar properties to them. They are found in large quantities in body fluids and muscle tissue.
prolamines – these are typical vegetable proteins found in seeds. A characteristic property is the ability to dissolve in 70% ethanol.
glutelins – like prolamines – are typical vegetable proteins; have the ability to dissolve in dilute acids and bases.
scleroproteins – proteins characterized by a high content of cysteine ​​and basic amino acids as well as collagen and elastin, as well as proline and hydroxyproline, insoluble in water and diluted salt solutions. These are typical proteins with a fibrous structure, thanks to which they perform support functions. Keratin belongs to this group of proteins.

• Complex proteins (formerly – proteids):
chromoproteins – composed of simple proteins and a prosthetic group – a dye. These include hemoproteins (hemoglobin, myoglobin, cytochromes, catalase, peroxidase) containing heme system and flavoproteins.

phosphoproteins – contain about 1% phosphorus in the form of phosphoric acid residues. These proteins include: milk casein, egg yolk vitellin, ichthulin fish eggs.
nucleoproteins – consist of basic proteins and nucleic acids. Ribonucleoproteins are primarily located in the cytoplasm: in ribosomes, microsomes and mitochondria, in small amounts also in cell nuclei, and outside the nucleus only in mitochondria. Viruses are made almost exclusively of nucleoproteins.

lipidoproteins – combinations of proteins with simple or complex fats, e.g. steroids, fatty acids. Lipoproteins are cholesterol carriers (LDL, HDL, VLDL). For example, they are part of the cell membrane.
glycoproteins – their prosthetic group are sugars, including mucopolysaccharides (saliva). Glycoproteins also occur in the ocular substance and fluid of joint capsules.

metalloproteins – they contain metal atoms as a cofactor (copper, zinc, iron, calcium, magnesium, molybdenum, cobalt). Metal atoms are the active group of many enzymes.
We also divide proteins due to their nutritional properties – selection and deficiency proteins are distinguished.

• Selected proteins (wholesome) – those that contain all exogenous amino acids. Such proteins include, for example, albumin, collagen, egg white, milk and meat proteins.

• Deficiency (Deficient) proteins – those lacking at least one exogenous amino acid. An example of such a protein is Gelatin.

Protein function

Proteins have the following functions

enzymatic catalysis – from carbon dioxide hydration to chromosome replication – – transport – hemoglobin, transferrin
storage – ferritin
membrane permeability control – regulation of metabolite concentration in the cell
ordered movement – muscle contraction, movement – e.g. actin, myosin
production and transmission of nerve impulses
buffers
growth and differentiation control
immunological – e.g. immunoglobulins
building, structural – e.g. & -keratin, elastin, collagen
cell adhesion (e.g. cadherin)
regulatory (hormonal regulation and regulation of genetic processes) – regulates biochemical processes – e.g. growth hormone, insulin, transcription factors and others.

Protein digestion
In humans, protein digestion begins only in the stomach, where the main cells of the glandular cells of the stomach secrete the inactive enzyme pepsinogen. Cladding cells secrete hydrochloric acid, in the presence of which pepsinogen is transformed into the active form – pepsin. Trypsin and chymotrypsin act in the duodenum, which break down polypeptide molecules into tripeptides and dipeptides. These, in turn, are broken down by the peptidases of the walls of the small intestine into amino acids, which are absorbed into the blood by means of appropriate conveyors located in the brush limb and the portal vein migrate to the liver. From there, most amino acids continue to enter the body’s cells with blood. The surplus is deprived of amino residues, which results in the formation of ammonia and keto acids. Ammonia is transformed into less toxic urea, which is transported with blood to the kidneys. On the other hand, keto acids can be used for the synthesis of sugars and some amino acids, used for energy purposes or transformed into spare fats.