Proteins

Origin and metabolism

Organic compounds make up an average of 20 to 30% of the total cell mass. At the same time, the chemical profile depends on the type of organism: plant cells are dominated by complex carbohydrates (polysaccharides), whereas animal cells are characterized by a predominance of proteins and fats. Despite this difference, the functional role of these compounds is universal for all forms of life.

The life cycle of protein molecules varies across different organisms:

• In plants: molecules are synthesized directly inside cells on ribosomes. Amino acids containing various chemical radicals serve as the building blocks.
• In animals: initial proteins come from outside with food. In the digestive tract, they are broken down into elementary monomers—amino acids. Then these monomers are used by cells to synthesize their own proteins, specific to the given organism, the structure of which is strictly dictated by the genetic code.

Classification by size and composition

Based on the number of amino acid units in the chain, the following are distinguished:

• Oligopeptides: short chains comprising from 2 to 10 amino acids.
• Polypeptides: contain 10 or more units. Strictly speaking, molecules with a length of 10 to 100 amino acids are called polypeptides (these include many hormones). Molecules numbering from 100 to several million units are classified as true proteins.

According to their chemical composition, molecules are divided into two large groups:

1. Simple proteins (proteins). Their composition consists exclusively of amino acid residues. These include protamines and histones (which regulate genome activity as part of nucleoproteins), plant prolamins and glutelins (which form gluten), as well as animal albumins and globulins (found in eggs, muscles, and milk).

1. Complex proteins (proteinaceous compounds / conjugated proteins). In addition to the protein chain, they contain an additional non-protein part called the prosthetic group.

Levels of spatial organization

A protein molecule is not just a chaotic thread, but a complex spatial structure formed in several stages:

1. Primary structure. This is the basic linear sequence of amino acids. The units are held together by strong covalent peptide bonds. It is this structure that is encoded in the DNA molecule; it is strictly individual for each protein and predetermines all its further properties.
2. Secondary structure. The linear chain begins to organize in space due to the emergence of numerous hydrogen bonds. Three folding variants are possible:
3. Alpha-helix: twists to the right, bonds are formed between CO and NH groups (the first and fifth units). Example: globin.
4. Beta-structure (pleated sheet): hydrogen bonds fasten parallel sections of the chain. Example: keratin.
5. Triple helix: three chains intertwine together. Example: collagen.
6. Tertiary structure. The polypeptide chain compactly folds into a coil, or globule. The shape is maintained by a whole complex of interactions: hydrogen and ionic bonds, hydrophobic interactions, and strong covalent disulfide bridges (-S-S-). Enzymes, antibodies, and hormones exist in this form.
7. Quaternary structure. Not characteristic of all macromolecules. It represents the combination of several finished globules (subunits) into a single working complex. A classic example is hemoglobin, consisting of four chains, each of which holds an iron-containing heme for oxygen transport.

Molecular shape and properties

Depending on the final shape, two types of proteins are distinguished:

• Fibrous: have the appearance of long threads, poorly soluble in water. They mainly perform skeletal functions (bone collagen, hair keratin, muscle myosin, blood fibrinogen).
• Globular: folded into spherical structures and highly soluble. These include enzymes and blood plasma proteins.

Under the influence of aggressive external factors (high temperature, radiation, exposure to acids or alkalis), a protein loses its natural spatial shape. This process is called denaturation. If only the higher levels of organization (quaternary, tertiary, secondary) are destroyed, while the primary peptide chain is preserved, the process can be reversible—after the harmful factor disappears, the molecule is able to recover (renaturation). However, if the primary structure is destroyed, denaturation becomes irreversible.

Diversity of functions

Due to their complex organization, proteins take on the vast majority of working processes in living systems:

• Structural: act as building material for cell membranes and organelles.
• Enzymatic (catalytic): absolutely all biochemical reactions are accelerated by enzymes, which are proteinaceous in nature.
• Protective: immunoglobulins, antibodies, and interferon provide the immune response.
• Contractile: actin and myosin ensure muscle function, as well as the movement of cellular structures (flagella, cilia).
• Regulatory: many hormones (e.g., insulin) control physiological processes.
• Transport: hemoglobin transports gases through the blood, and membrane proteins transport substances inside the cell.
• Energy: breakdown releases energy (17.6 kJ/g).
• Receptor: perception of signals; for example, rhodopsin in the retina responds to light.
• Storage: accumulation of nutrients (casein, egg yolk proteins).
• Toxic: poisons of insects, snakes, and spiders.
• Trophic: nutrition of the developing embryo (seed endosperm proteins).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Translation mechanisms: how the ribosome assembles the polypeptide chain
• Structure of amino acids and the role of their side chains in the formation of tertiary structure
• Hemoglobin: cooperative oxygen binding by subunits
• Enzyme operating principles and enzyme kinetics

Other topics in the section "Chemical composition of the cell"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.