Formation of a protein involves four distinct levels of structure. The tertiary structure is the third level of protein formation, and occurs when the side chains of the individual amino acids interact. These side chains can attract one another to form hydrogen bonds or disulfide bonds, or they can repel each other and contribute ot hydrophobic interactions. The result is a three-dimensional shape. This is the final level of structure to create a function protein subunit.

The primary structure of the protein is derived from the chain of amino acids synthesized during translation; this amino acid sequence is the primary structure. Secondary structure is generated from the interactions between amino and carboxyl groups in the polypeptide backbone. These groups can form hydrogen bonds to generate alpha-helices and beat-pleated sheets. Tertiary structure, as described above, results in a functional protein subunit. For some protiens, tertiary structure is the final step in folding. For other proetins, multiple subunits can be bound together to generate a quaternary structure.

There are four essential levels of protein structure. The fourth and final level is called quaternary structure. This level of structure is only present in proteins with multiple subunits. Since hemoglobin has four subunits, we know that the question is talking about the quaternary structure of the protein.

Primary structure is simply the amino acid sequence generated during translation. Soon after translation, the carboxyl and amino groups present in the polypeptide backbone begin to form hydrogen bonds. The result is the protein's secondary structure, frequently made of alpha-helices and beta-pleated sheets. Tertiary structure is the three-dimensional shape of the final polypeptide, and is derived from hydrophobic interactions, hydrogen bonds, and disulfide bonds related to the amino acid side chains (R groups). When multiple polypeptides (subunits) join together, they generate a quaternary structure.

Protein quaternary structure involves interactions between different subunits. Each subunit will be created by folding an independent polypeptide chain into a 3-dimensional tertiary structure. The joining of these independent subunits results in quaternary structure. In order for a protein to have quaterary structure, it must have multiple subunits; this means it must consists of at least two polypeptide chains.

The primary structure of a protein is the sequence of amino acids, which determines the unique shape of the protein.  The secondary structure consists of the coiled and folded patterns that contribute to the protein’s overall shape (alpha helix or beta pleated sheet respectively).  The tertiary structure is the overall shape of the polypeptide that results from interactions and hydrogen bonding between the side chains, or R groups, of the various amino acids present. It is during this stage of protein formation that disulfide bridges and hydrophobic interactions are first seen.  Last,the quaternary structure is the overall protein structure resulting from the aggregation of at least two polypeptide units. 

The protein does possess primary, secondary, and tertiary structure but since the protein has three distinct subunits, the entire molecule is exhibiting a higher order quaternary structure. 

Primary structure refers to a linear sequence of amino acids in the polypeptide chain, such as the example in the question stem. Secondary structure has two main types, the alpha helix and the beta strand (or beta sheets). The alpha helix or beta sheets are folded into a compact globular structure to form the tertiary structure. Quaternary structure is a three-dimensional structure of a multi-subunit protein and how the subunits fit together. There is no such thing as auxiliary protein structure.

A peptide bond is a covalent chemical bond formed between two amino acids. The process is known as a dehydration synthesis reaction (or a condensation reaction). It is named so because the synthesis of the peptide bond occurs when the carboxyl group of one amino acid reacts with the amino group of another amino acid, which releases one molecule of water.

Peptide bonds are responsible for the primary structure of a protein (The primary structure of a protein is the linear sequence of its amino acids.) Further folding through hydrogen bonds and other forces are responsible for secondary, tertiary, and if applicable, quaternary structure.

Water is known as the “universal solvent.” Life could not exist on earth without water. Our bodies are mostly water; therefore, the environment of our cells is aqueous as well. Hydrophobic (“water fearing") amino acids would condense to "hide" from an aqueous environment. Polar and/or hydrophilic (“water loving”) amino acids would be found on the exterior of globular proteins near the aqueous environment. Hydrophobicity and hydrophilicity are major forces that drive the formation of the tertiary or three-dimensional shape of a protein post translation.

Primary structure of protein is the sequence of amino acids. Secondary structure is formed by hydrogen bonds between the peptide backbone which forms either alpha helices or beta sheets. Tertiary structure is dependent on side chains and the environment in which the protein is. Quaternary structure is conferred once bonds between two or more polypeptide chains are formed.