Proteins and Structure
Polymers of Alpha-Amino Acids and their organization.
By chemca Team • Updated Jan 2026
Proteins are the most abundant biomolecules. They are polymers of $\alpha$-amino acids connected by peptide bonds.
1. Amino Acids
Building Blocks
Contain amino ($-NH_2$) and carboxyl ($-COOH$) groups. Depending on the relative position, they are $\alpha, \beta, \gamma$, etc. Proteins yield only $\alpha$-amino acids.
Zwitterion: In aqueous solution, the carboxyl group loses a proton and the amino group accepts a proton, forming a dipolar ion.
$$ R-CH(NH_2)-COOH \rightleftharpoons R-CH(NH_3^+)-COO^- $$
Classification:
- Essential: Cannot be synthesized by body (e.g., Valine, Leucine).
- Non-Essential: Synthesized by body (e.g., Glycine, Alanine).
2. Peptide Bond
Amide Linkage
Proteins are connected by peptide linkages ($-CO-NH-$) formed between the $-COOH$ of one amino acid and the $-NH_2$ of another with the loss of water.
$$ H_2N-CH(R)-COOH + H_2N-CH(R')-COOH \xrightarrow{-H_2O} H_2N-CH(R)-CO-NH-CH(R')-COOH $$
3. Structure of Proteins
Levels of Organization
Primary Structure: The specific sequence of amino acids in the polypeptide chain. Any change creates a different protein.
Secondary Structure: The shape the chain takes due to Hydrogen bonding between $-CO-$ and $-NH-$ groups.
- $\alpha$-Helix: Spiral structure (e.g., Keratin).
- $\beta$-Pleated Sheet: Sheets laid side-by-side (e.g., Silk Fibroin).
Tertiary Structure: Overall 3D folding of the polypeptide chain. Stabilized by H-bonds, Disulphide links, Van der Waals, and Ionic bonds. (Fibrous vs Globular).
Quaternary Structure: Spatial arrangement of multiple polypeptide subunits with respect to each other (e.g., Hemoglobin).
4. Denaturation of Proteins
Loss of Activity
When a protein in its native form is subjected to physical change (heat) or chemical change (pH change), the Hydrogen bonds are disturbed.
Key Point: Globules unfold and helix get uncoiled. Secondary and Tertiary structures are destroyed, but Primary structure remains intact. The protein loses its biological activity (e.g., Coagulation of egg white).
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