The folding of proteins into their tertiary structure is influenced by various intramolecular and extramolecular factors. Intramolecular factors include the nature of the side chains of the amino acids (hydrophobic, hydrophilic, positively charged, negatively charged), which determine the types of interactions that can occur within the protein. Hydrophobic interactions play a significant role, as hydrophobic side chains tend to cluster away from the aqueous environment, influencing the folding pattern of the protein. Additionally, hydrogen bonds, ionic bonds, and van der Waals forces between side chains contribute to the tertiary structure. Disulfide bridges between cysteine residues can also lock parts of the protein in place, providing further stability. Externally, factors such as pH, temperature, and the presence of metal ions or cofactors can affect protein folding by altering the chemical properties of the amino acid side chains or by stabilising specific folded states. Chaperone proteins are another extramolecular factor; they assist in correctly folding proteins, preventing improper interactions that could lead to dysfunctional proteins.

Protein misfolding and aggregation are at the heart of numerous diseases, known collectively as protein misfolding disorders or amyloid diseases. Misfolding occurs when proteins fail to fold into their normal functional three-dimensional structures, often resulting in non-functional proteins that can aggregate to form insoluble fibrils or plaques. These aggregates can disrupt cell function in several ways: they can physically interfere with cellular components, induce inflammation, or cause cellular stress by overwhelming the protein degradation machinery. Diseases associated with protein misfolding and aggregation include Alzheimer's disease, where misfolded amyloid-β peptides aggregate to form plaques; Parkinson's disease, associated with the aggregation of α-synuclein protein; and Huntington's disease, which involves the aggregation of huntingtin protein with an expanded polyglutamine tract. The exact mechanisms by which these aggregates cause disease are complex and vary between conditions but often involve the loss of function of the misfolded proteins and toxic gain of function due to the aggregates.

Chaperone proteins are essential in the cell because they facilitate the correct folding of nascent or misfolded polypeptides and prevent their aggregation. The folding of proteins is a highly complex process, with many newly synthesised polypeptides requiring assistance to reach their functional conformation. Without chaperones, proteins might misfold, leading to loss of function and potentially forming toxic aggregates that can damage cells. Chaperones act by binding to hydrophobic regions of the polypeptide chain that are typically buried in the interior of the properly folded protein, shielding them from aggregation with other hydrophobic regions. They may also provide a secluded environment in which a polypeptide can fold without the risk of forming incorrect interactions. Furthermore, chaperones can help refold partially denatured proteins that have been unfolded due to stress conditions like heat shock. Thus, chaperone proteins are crucial for maintaining cellular protein homeostasis and preventing diseases associated with protein misfolding and aggregation.

The cellular environment plays a crucial role in influencing protein structure and function, affecting aspects such as pH, ion concentration, redox potential, and the presence of various small molecules and cofactors. The pH of the cellular environment can affect the ionisation state of amino acid side chains, altering their interactions and, consequently, the protein's structure and function. For example, enzymes have optimal pH levels at which they function most effectively, and deviations from this pH can lead to denaturation or reduced activity. Ion concentrations, particularly of divalent cations like Ca²⁺ and Mg²⁺, can impact protein stability and activity, as these ions can form stabilising interactions with specific sites on the protein. The redox potential influences the formation of disulfide bonds, which are crucial for the stability of many extracellular proteins. Additionally, the cellular milieu is rich in small molecules and cofactors that can bind to proteins, inducing conformational changes that regulate their activity. This complex interplay ensures that proteins fold correctly and perform their functions efficiently within the dynamic cellular environment.

Post-translational modifications (PTMs) are chemical modifications that occur to proteins after their synthesis and are significant in modulating protein structure, function, and interactions. Common PTMs include phosphorylation, glycosylation, ubiquitination, acetylation, and methylation. These modifications can have various effects on proteins: they can alter the protein's charge, hydrophobicity, or conformation, which can affect the protein's activity, interaction with other molecules, stability, and location within the cell. For example, phosphorylation, the addition of a phosphate group, often regulates enzyme activity and protein-protein interactions by inducing conformational changes. Glycosylation, the addition of sugar moieties, is crucial for protein folding, stability, and cell-cell recognition processes. Ubiquitination, the attachment of ubiquitin molecules, usually targets a protein for degradation by the proteasome, regulating protein levels and functions within the cell. These modifications allow for a dynamic response to cellular signals and conditions, enabling proteins to perform a vast array of functions and play integral roles in cellular processes and signalling pathways.