Levels of chain structural organization
The basis of the structure of any protein molecule is the primary structure. It represents a strict sequence of amino acid residues connected to each other by strong peptide bonds. This baseline level dictates all further geometry of the molecule.
The next level — secondary structure — describes the spatial configuration of the chain itself. The main factor in its stabilization is hydrogen bonds. They occur between the oxygen atom of one peptide group and the hydrogen atom of another. Depending on thermodynamic advantage, two types of folding are distinguished:
- Helical coiling: typical for globular proteins.
- Zigzag folding: characteristic of fibrous proteins that form fibers.
The standard for helical folding is the $\alpha$-helix. Its structure is strictly geometrical: there are 3.6 amino acid residues per turn (the pitch of the helix is 0.54 nm), and the total diameter is 0.5 nm. The backbone atoms are aligned strictly along the line of the helix, while the amino acid side chains face outward from its axis.
Tertiary structure and globule formation
The protein helix is a flexible structure capable of folding into a compact ball called a globule (from Lat. globulus). This spatial configuration of the helix represents the tertiary structure.
The globule maintains its shape due to additional chemical bonds between regions of the chain that may be far apart in the primary sequence:
- Disulfide bridges: covalent bonds formed upon the oxidation of cysteine residues.
- Ester bridges: formed between the carboxyl group of one amino acid (e.g., glutamic acid) and the hydroxyl group of another (e.g., serine).
- Salt (ionic) bridges: the result of electrostatic interaction between oppositely charged groups (e.g., negatively charged glutamic acid and positively charged lysine).
A classic example of a protein with a helical structure is myoglobin. It gives muscles their red color and serves as a reservoir for oxygen. The myoglobin molecule comprises 153 amino acid residues and forms about 30 turns of the helix interspersed with non-helical fragments.
Quaternary structure and complex assemblies
Some proteins can only perform their functions when multiple polypeptide chains (subunits) assemble into a single molecule. This combination and mutual arrangement of subunits is called the quaternary structure.
A prime example is hemoglobin, the oxygen-carrying protein. It consists of four separate chains, each folded into a globule and bound to a special cyclic structure containing an iron atom (heme).
Even relatively small molecules can have a complex organization. For instance, the hormone insulin consists of two chains: a short one (21 residues) and a long one (30 residues). They are firmly bound together by two interchain disulfide bridges. The third disulfide bridge is located inside the short chain, forming a small loop.
Larger proteins, such as immunoglobulins, consist of heavy and light chains. Here, disulfide bridges perform large-scale structural work: they stitch heavy chains to light chains, heavy chains to each other, and also participate in the formation of internal domains.
Structural research methods
Understanding protein structure has become possible thanks to the development of analytical methods:
- Elemental analysis gave scientists initial insights into the chemical composition of molecules.
- Amino acid analysis, based on the gradual chemical hydrolysis of the protein, made it possible to decipher the primary structure.
- X-ray crystallography became the pinnacle of method evolution. If a protein is capable of forming crystals, this method makes it possible to calculate the exact coordinates of each atom and reconstruct an absolute three-dimensional model of the molecule.
Briefly: answers to topical questions
What types of chemical bonds and interactions stabilize the tertiary structure of a protein?
The tertiary structure of a protein is stabilized by strong covalent and weak polar or nonpolar bonds between amino acid side chains. The following types of interactions are distinguished:
- Disulfide bonds — covalent bonds between the SH groups of cysteine residues.
- Ionic bonds — electrostatic attraction between oppositely charged groups.
- Hydrogen bonds — occur between polar uncharged and other hydrophilic groups.
- Hydrophobic interactions — association of nonpolar side chains inside the globule.
- Van der Waals forces — intermolecular interactions in the hydrophobic core.
- Isopeptide bonds — between amino groups and COOH groups.
- Ester bonds — between a COOH group and an OH group.
Which amino acids can participate in the formation of ester bridges during tertiary structure formation?
Amino acids containing specific functional groups in their side chains participate in the formation of ester (carboxyl-ester) bridges during the formation of the protein tertiary structure. A covalent ester bond arises between the following classes of amino acids:
- Dicarboxylic amino acids — provide a carboxyl group (COOH group).
- Hydroxy amino acids — provide a hydroxyl group (OH group).
What laboratory methods are used to decipher the primary structure of a protein?
Methods for analyzing the linear sequence of amino acids are used to decipher the primary structure of a protein. The main laboratory methods include:
- Sanger method — determination of the N-terminal amino acid using dinitrofluorobenzene.
- Edman degradation — sequential cleavage of amino acids from the N-terminus using phenylisothiocyanate.
- Enzymatic hydrolysis — cleavage of the protein into peptides by trypsin and chymotrypsin followed by sequencing.
- Mass spectrometry — a modern high-precision method for structural analysis.
- Hydrolysis and amino acid separation — basic steps of composition analysis.
What types of protein secondary structure are distinguished in biochemistry?
Biochemistry distinguishes two main types of secondary structure, which represent local ordered folding of the polypeptide chain.
- Alpha-helix (α-helix) — a right-handed helix characteristic of globular proteins. It is held by hydrogen bonds within a single chain, with side chains directed outward.
- Beta-structure (β-pleated sheet) — chain segments arranged in a parallel or antiparallel manner. They are held by hydrogen bonds between different chains, characteristic of fibrous proteins.
What amino acids are capable of forming salt (ionic) bridges in a protein molecule?
Salt (ionic) bridges in a protein molecule are formed due to electrostatic attraction between oppositely charged side chains of acidic and basic amino acids. The following participate in this interaction:
- Aspartic acid (Asp) — a negative (anionic) amino acid.
- Glutamic acid (Glu) — a negative (anionic) amino acid.
- Lysine (Lys) — a positive (cationic) amino acid.
- Arginine (Arg) — a positive (cationic) amino acid.
- Histidine (His) is a positive (cationic) amino acid.
Levels and their bonds: Primary — Peptide; Secondary — Hydrogen; Tertiary — Three types of bridges (disulfide, ester, salt); Quaternary — Parts (subunits) assemble together.
Class and exam questions
What is the main difference between the bonds in secondary and tertiary structures?
Secondary structure is stabilized exclusively by hydrogen bonds. Tertiary structure is supported by stronger interactions (disulfide, salt, and ester bridges) between distant regions of the chain.
Which proteins are characterized by a zigzag molecular shape?
The zigzag folding of the chain is typical for fibrous proteins, which form various fibers. Globular proteins, conversely, most often have a helical coiling.
What is required for the successful X-ray diffraction analysis of a protein?
The key condition for applying this method is the ability of the studied protein to form a crystal lattice.
How do disulfide bonds participate in the structure of immunoglobulins?
In immunoglobulins, S—S bridges link heavy and light chains, fasten heavy chains to each other, and also form structural loops (domains) within the chains themselves.
What else to study on this topic
- Mechanisms of chemical protein hydrolysis in amino acid analysis
- Comparative characteristics of globular and fibrous proteins
- Biochemical structure of heme and its role in oxygen binding
- Structure and functions of immunoglobulin domains
- Chemical properties of cysteine and the mechanism of disulfide bridge formation
Master the entire topic in the "Chemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on the topic «Protein Structure» and 3,009 more pages for the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 95 video lessons for the subject
Other topics in "Chapter 18. Major Classes of Bioorganic Compounds"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.