Understanding Amino Acids and Protein Structure
Properties of Amino Acids
Stereoisomerism occurs because the alpha carbon is asymmetric. Stereoisomers are considered two different optical activities. Amino acids can be isomers L, while D isomers are also present. These isomers can influence antibiotic-bacterial properties. The existence of polar groups in amino acids enables hydrogen bonding, which affects their melting points, boiling temperatures, and solubility.
Peptide Bonds
A peptide bond is the union between the carboxyl group of one amino acid and the amino group of another, releasing a molecule of water. The compound obtained is called a dipeptide. The amide bond can be hydrolyzed, separating the two amino acids. This bond is rigid with specific angles and distances, stabilizing the carbon and oxygen atoms that share electrons. A dipeptide can join another amino acid, releasing another water molecule and forming a tripeptide, and so on.
Primary Structure
The primary structure is the sequence of amino acids in the protein chain. The stability of this structure is provided by the peptide bond, which is responsible for the functionality of the protein.
Secondary Structure
The secondary structure refers to the first folds in the space of a primary structure. There are two types:
- Alpha Helix: This structure is found in keratin (nails, hair, etc.). It is a string of amino acids wound in space, stabilized by hydrogen bonds and parallel planes occurring around every 3 to 6 amino acids.
- Beta Sheet: This structure resembles an accordion, with the R groups perpendicular to the plane. Stability is also provided by hydrogen bonds, which can be intra- or inter-chain. This structure is typical of fibrous proteins, such as spider silk.
Tertiary Structure
The tertiary structure consists of loops in the space of molecules with secondary structures. The stability of this structure depends on the functionality and is maintained by four types of bonds:
- Disulfide Bonds: Strong covalent bonds between two cysteine amino acids.
- Hydrogen Bonds: Established between nonionic polar groups with partial charges on side chains.
- Electrostatic Forces: Occur between negatively charged R groups and positively charged amino acids.
- Van der Waals Forces: Weaker interactions among nonpolar amino acids.
There are two types of tertiary structures:
- Globular: These proteins have a high degree of folding and lead to spherical shapes, making them soluble.
- Fibrillary: These proteins have less folding, resulting in elongated shapes that are poorly soluble and highly resistant.
Quaternary Structure
The quaternary structure refers to the relative arrangement of protein subunits with each other. The union between them is maintained by the same types of bonds that stabilize the tertiary structure. Examples include hemoglobin, muscle proteins, multienzyme complexes, and immunoglobulins.
Functions of Proteins
- Structural: Proteins form structures such as membranes, organelles, and connective tissues.
- Hormonal: Proteins are involved in gene regulation and activation processes related to genetic information.
- Catalytic: Proteins act as enzymes that facilitate chemical reactions in living organisms.
- Immune: Proteins provide molecular identity to living organisms (antigens) and help in the response to foreign substances (antibodies).
- Transport: Proteins transport molecules through permeases.