**Core Concept**
Protein binding and aggregation are crucial processes in various physiological and pathological conditions. Protein aggregation can lead to the formation of amyloid fibrils, which are associated with several neurodegenerative diseases, such as Alzheimer's and Parkinson's. The process of protein aggregation is influenced by the presence of various factors, including temperature, pH, and the presence of specific enzymes.

**Why the Correct Answer is Right**
Proteins are typically held together by covalent disulfide bonds between cysteine residues. This process is facilitated by the enzyme protein disulfide isomerase (PDI), which catalyzes the formation of disulfide bonds between cysteine residues. The formation of disulfide bonds is essential for the proper folding and stability of proteins. In the absence of disulfide bonds, proteins can aggregate and form insoluble fibrils.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because this option does not specify the type of bond that holds proteins together.
**Option B:** Incorrect because this option refers to a process that occurs during protein synthesis, not protein binding or aggregation.
**Option C:** Incorrect because this option is a type of bond that occurs during protein synthesis, but it is not the primary mechanism of protein binding or aggregation.

**Clinical Pearl / High-Yield Fact**
The formation of disulfide bonds is essential for the proper folding and stability of proteins. Disulfide bonds can be disrupted by the presence of reducing agents, such as beta-mercaptoethanol (BME) or dithiothreitol (DTT), which can prevent protein aggregation.

**Correct Answer:** D. Disulfide bonds.