Protein structures refer to the three-dimensional arrangement of atoms in a protein molecule. There are three levels of protein structure: primary, secondary, and tertiary.

The primary structure of a protein is its linear sequence of amino acids. This sequence is held together by peptide bonds.

The secondary structure of a protein is the way in which the protein molecule folds into a specific shape. This is determined by the hydrogen bonding between the peptide bonds in the protein. The two common secondary structures are the alpha helix and beta pleated sheet.

Tertiary protein structure is the three-dimensional arrangement of the entire polypeptide chain of a protein, including all its secondary structures, in space. It is the final stage of protein folding and is determined by the interactions between the R-groups (side chains) of the amino acids that make up the protein. These interactions can be covalent, ionic, hydrogen bonds, van der Waals forces, and hydrophobic interactions.

Tertiary protein structure is critical to the function of proteins as it determines their overall shape, which affects their activity, specificity, and stability. Proteins with the same primary sequence can fold into different tertiary structures, resulting in different functions. The tertiary structure is also responsible for the binding of proteins to ligands, such as substrates, cofactors, and other molecules.
Examples of proteins with complex tertiary structures include enzymes, antibodies, and hemoglobin. Enzymes have active sites that are often located in deep pockets formed by the three-dimensional folding of the protein. Antibodies, which are proteins that recognize and neutralize foreign substances in the body, have a Y-shaped tertiary structure that enables them to bind specifically to their target antigens. Hemoglobin, which carries oxygen in the blood, has a complex tertiary structure that allows it to undergo conformational changes upon oxygen binding and release.

In summary, tertiary protein structure is the final level of protein folding and is crucial to protein function and activity. It is determined by the interactions between the R-groups of amino acids and gives rise to the three-dimensional shape of the protein.

The quaternary structure of a protein refers to the arrangement of multiple protein subunits in a single protein complex. This structure is determined by the interactions between the subunits and can be affected by the presence of other molecules, such as ligands or cofactors.

Quaternary structure is called so because it refers to the fourth level of protein structure that describes the arrangement of multiple polypeptide chains in a protein complex.

In other words, quaternary structure describes how individual polypeptide chains assemble and interact with each other to form a larger, functional protein complex. This level of structure is only observed in proteins that have multiple polypeptide subunits, such as enzymes, antibodies, and hemoglobin.

Quaternary structure is critical to the function of such proteins, as it determines how the subunits come together to form an active protein complex, which may exhibit properties and functions not found in the individual subunits. For example, hemoglobin is a tetrameric protein made up of four polypeptide subunits, and the interactions between these subunits are crucial for its oxygen-binding and transport function.

A characteristic of protein quaternary structure is the arrangement of multiple polypeptide chains, or subunits, to form a larger, functional protein complex.

In quaternary protein structure, each subunit may have its own tertiary structure, and the subunits interact with each other through non-covalent bonds such as hydrogen bonds, hydrophobic interactions, and electrostatic interactions to form a stable protein complex. The resulting complex may exhibit properties and functions not found in the individual subunits, allowing for greater versatility and specialization in protein function.

The number and arrangement of subunits in a protein complex can vary widely, from dimers (2 subunits) to tetramers (4 subunits) to much larger complexes with many subunits. Examples of proteins with quaternary structure include hemoglobin, which is a tetramer made up of two alpha and two beta subunits, and lactose dehydrogenase, which is a tetramer made up of two different types of subunits.

Overall, the quaternary structure of protein is an essential aspect of its structure and function, as it determines how multiple subunits come together to form a functional protein complex. The resulting complex may exhibit properties and functions that are not present in the individual subunits, allowing for greater versatility and specialization in protein function.

Tertiary and quaternary structure are not the same, but they are related levels of protein structure.
Tertiary structure refers to the three-dimensional arrangement of the entire polypeptide chain of a protein, including all its secondary structures, in space. It is determined by the interactions between the R-groups of the amino acids that make up the protein, and it gives rise to the overall three-dimensional shape of the protein. Tertiary structure is critical to the function of proteins, as it determines their activity, specificity, and stability.

Quaternary protein structure, on the other hand, refers to the arrangement of multiple polypeptide chains, or subunits, in a protein complex. In quaternary structure, each subunit may have its own tertiary structure, and the subunits interact with each other through non-covalent bonds to form a stable protein complex. The resulting complex may exhibit properties and functions not found in the individual subunits.

Thus, while tertiary structure refers to the three-dimensional structure of an individual polypeptide chain, quaternary structure refers to the arrangement of multiple polypeptide chains in a larger protein complex. They are related because the tertiary structure of each subunit contributes to the overall quaternary structure of the protein complex.