Classification and structure
The boundary between a regular polypeptide and a full-fledged protein is conventionally drawn based on molecular weight. If the mass exceeds 10,000, which corresponds to roughly 90 or more amino acid residues in the chain, the molecule is classified as a protein.
Based on their chemical composition, all protein molecules are divided into two broad groups:
- Simple proteins: composed exclusively of an amino acid chain.
- Conjugated proteins: contain an additional non-protein moiety. Depending on the attached component, various classes of compounds are formed. Association with lipids yields lipoproteins, with polysaccharides — glycoproteins, and with polynucleotides — nucleoproteins. Metal ions can also serve as the non-protein component.
Substances with a mass of less than 10,000 (containing fewer than 90 residues) are classified as low-molecular-weight polypeptides. They are no less important for the organism. A prime example is insulin, which regulates blood glucose concentration.
Biological Functions
Protein molecules act as key "working" and building elements of living systems. Each species exhibits strict specificity, possessing its own unique proteins that differ in amino acid composition.
The main areas of their activity include:
- Structural function: forming the basis of skin, tissues, and cell membranes (e.g., collagen and membrane proteins).
- Contractile function: enabling muscle movement (actin and myosin).
- Catalytic function: accelerating biochemical reactions through enzymatic activity.
- Transport function: carrying essential compounds (hemoglobin transports oxygen).
- Protective function: neutralizing foreign agents and antigens using immunoglobulins.
- Receptor and signaling functions: facilitating mutual cell recognition, for instance, with the participation of selectins.
Biosynthesis, Metabolism, and Production
Proteins differ radically from most other substances in their unique method of molecular formation. In living cells, synthesis occurs on ribosomes via the polycondensation of amino acids on RNA templates. This process is strikingly efficient: the precise reproduction of hundreds of amino acid residues in a strictly predetermined sequence takes mere seconds.
Protein metabolism in nature depends on the type of organism. Plants are capable of independently synthesizing the amino acids required to build their molecules. Animals, on the other hand, obtain them from food.
The metabolic pathway in animals is as follows:
- Hydrolysis (digestion): dietary proteins are broken down by digestive enzymes with the addition of water molecules into free amino acids.
- Synthesis: the organism's own specific proteins are assembled from the resulting amino acids with the participation of RNA and enzymes.
Since synthesizing a protein of a strictly predetermined structure using traditional chemical methods is virtually impossible, biotechnology and the microbiological industry employ specially selected strains of microorganisms to produce the required compounds, after which individual proteins are isolated from biological mixtures.
Calculation of the Number of Amino Acid Residues
To roughly estimate the length of a protein molecule, a simple mathematical algorithm is used. The average molecular weight of a single amino acid residue, approximately 112, is taken as the baseline.
Let us examine this calculation using the example of human insulin polypeptide. Its total molecular weight is 5734. Dividing the molecular mass by the mass of a single residue ($5734 / 112$), we obtain a value close to 51. Thus, we can conclude that the insulin molecule consists of 51 amino acid residues.
Briefly: answers to topical questions
What types of chemical bonds form the primary, secondary, and tertiary structures of a protein?
The primary, secondary, and tertiary structures of a protein are formed by covalent and non-covalent chemical bonds.
- Primary structure — stabilized by strong covalent peptide bonds between the $\alpha$-amino group and the $\alpha$-carboxyl group of amino acids.
- Secondary structure — maintained by hydrogen bonds between the oxygen and hydrogen atoms of the peptide backbone.
- Tertiary structure — formed through interactions between amino acid side chains (radicals). It includes covalent disulfide bonds, as well as non-covalent interactions: hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals forces.
Which specific enzymes carry out protein hydrolysis in the human digestive tract?
Protein hydrolysis in the digestive tract is carried out by specialized proteolytic enzymes (proteases). The process occurs under the action of two main groups of enzymes:
- Pancreatic enzymes — trypsin, chymotrypsin, elastase, as well as carboxypeptidases A and B.
- Small intestinal epithelial enzymes — aminopeptidases, dipeptidases, and tripeptidases.
Other proteolytic enzymes capable of breaking down polypeptide structures are also present in the oropharynx, stomach, and intestines.
Which amino acids are proteinogenic, and how many types of them participate in protein synthesis?
Protein synthesis involves 20 standard $\alpha$-amino acids, known as proteinogenic amino acids. Depending on whether they can be synthesized by the organism, they are divided into groups:
- Non-essential — alanine, asparagine, aspartic acid, glutamine, glutamic acid, glycine, proline, serine.
- Essential — valine, isoleucine, leucine, lysine, methionine, threonine, tryptophan, phenylalanine (must be obtained from the diet).
- Conditionally essential — arginine, histidine, tyrosine, cysteine.
All of them belong to the L-series and exhibit amphoteric properties.
What is the isoelectric point of a protein, and what does it depend on?
The isoelectric point (pI) is the pH of the medium at which a protein is in an isoelectric state, meaning its net electrical charge is zero. The pI value depends on the ratio of ionized anionic ($-COO^-$) and cationic ($-NH_3^+$) groups of amino acids within a specific protein. The net charge of the molecule is determined by the concentration of $H^+$ and $OH^-$ ions in the solution: in an acidic medium, the dissociation of carboxyl groups is suppressed, whereas in an alkaline medium, protons of amino groups are bound. At the isoelectric point, the number of positively and negatively charged groups is equal, causing the protein to lose its net charge, exhibit minimal solubility, and minimal hydration.
What does the process of protein denaturation entail, and what physicochemical factors cause it?
Denaturation involves the disruption of the native spatial conformation of a protein (secondary, tertiary, and quaternary structures) due to the rupture of weak bonds, without breaking the strong peptide bonds of the primary structure. This process leads to the loss of biological activity, altered solubility, and destruction of the active site. Denaturation is caused by the following factors:
- Physical factors — high temperature, ultraviolet and microwave radiation, ionizing particles, mechanical stress.
- Chemical factors — acids, alkalis, heavy metal salts, organic solvents (alcohol, phenol), alkaloids, and toxins (urea).
Class and exam questions
What is the difference between a polypeptide and a protein?
A protein is a high-molecular-weight polypeptide. The boundary is drawn at a molecular weight exceeding 10,000 and the presence of approximately 90 or more amino acid residues in the chain.
Can a protein be chemically synthesized in a laboratory?
It is virtually impossible to obtain a protein of a strictly predetermined structure using traditional chemical methods. Biotechnological methods utilizing microorganisms are employed for their artificial production.
Where do amino acids for human protein synthesis come from?
Humans and animals obtain foreign proteins from their diet. In the digestive tract, these are hydrolyzed by enzymes into amino acids, from which the organism's own proteins are then assembled on ribosomes.
What else to study on this topic
- Translation Mechanisms and the Role of RNA in Biosynthesis
- Industrial Methods of Microbiological Synthesis
- Protein Specificity Within a Single Species
- Mechanism of Enzymatic Hydrolysis in the Gastrointestinal Tract
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