Protein Structure and Properties: A Comprehensive Look
Primary Structure of Proteins
The primary structure is the most basic form of protein organization. It is determined by the amino acid sequence of the protein chain, i.e., the number of amino acids and the order in which they are linked by peptide bonds. The amino acid side chains extend from a main chain. By convention, the order of writing is always from the amino-terminal to the carboxy-terminus. A characteristic zigzag in their disposal is due to the planarity of the peptide bond rotation that causes the amino acid to balance forces of attraction.
Secondary Structure of Proteins
The secondary structure of protein folding is the local, regular arrangement between nearby amino acid residues of the polypeptide chain. It is adopted through the formation of hydrogen bonds between side chains (radicals) of amino acids near the chain.
- Alpha Helix: In this structure, the polypeptide chain is coiled upon itself due to twisting about the alpha carbon produced by each amino acid. This structure is maintained by intrachain hydrogen bonds formed between the NH group of a peptide bond and the -C=O group of the fourth amino acid that follows.
- Collagen Helix: A particular variety of secondary structure characteristic of collagen protein found in tendons and connective tissue, is a particularly rigid structure.
- Beta Sheets or Folded Sheets: Some regions of protein adopt a zigzag structure, establishing links among themselves through interchain hydrogen bonds. All peptide bonds are involved in these cross-links, thus conferring greater stability to the structure. The beta form is a simple shape formed by two or more polypeptide chains, either parallel (running in the same direction) or antiparallel (running in opposite directions), and are attached tightly by hydrogen bonds and various arrangements between free radicals of amino acids. This arrangement has a lamellar structure and is folded in the manner of an accordion.
Tertiary Structure of Proteins
Tertiary structure refers to how the protein is folded in space. This structure is stable thanks to the associations that occur between the radical-R of the amino acids. These bonds can be hydrogen bonds, electrostatic attraction, Van der Waals forces, or disulfide bonds. This domain structure is composed of between 50 and 300 amino acids linked by a polypeptide backbone. These domains are very stable so that they can appear in different proteins.
Quaternary Structure of Proteins
Quaternary structure is the structure formed by proteins possessing two or more polypeptide chains (protomers) that may be similar or different. For the protein to be functional, the protomers should be united by hydrogen bonds or Van der Waals forces or, as in the case of immunoglobulin, disulfide bonds. Collagen polypeptide = 3 chains. Hemoglobin polypeptide = 4 chains.
Properties of Proteins
Solubility: It always stays as long as strong and weak links are present. Increasing the temperature and pH, the solubility is lost.
Electrolytic Capacity: It is determined through electrophoresis, an analytical technique in which if the proteins are transferred to the positive pole, it is because the molecule is negatively charged and vice versa.
Specificity: Each protein has a specific function that is determined by its primary structure. Function resides in the position of some amino acids in the linear sequence of species. Proteins with similar composition and structure.
pH Buffer (called buffer effect): They act as buffers of pH due to their amphoteric character, i.e., they can behave as acids (electron accepting) or bases (electron donating).
Holoproteins
Composed exclusively of amino acids. They are divided into:
Fibrous: Have simple structures, mainly comprising polypeptides that are ordered or rolled along parallel beams. They are insoluble in water and have important structural and protective functions. This part includes collagen, myosin, keratin, fibrin, and elastin.
Globular: Are more complex than fibrous. The chains that form are folded to form a compact structure. They are soluble in water or polar solvents and are responsible for the biological activities of the cells. Belong to this group are actin, albumin, globulins, histones, and protamines.
Heteroproteins
Composed of a protein group and a prosthetic group. They are divided into:
Chromoproteins: Proteins whose prosthetic group is a pigment. They can be porphyrin or non-porphyrin.
Nucleoproteins: Proteins whose prosthetic group is a nucleic acid.
Glycoproteins: Proteins whose prosthetic group is a carbohydrate covalently bound to the polypeptide chain.
Phosphoproteins: Proteins whose prosthetic group is phosphoric acid.
Lipoproteins: Proteins whose prosthetic group is a lipid. They may be low-density (LDL) or high-density (HDL).