What is the effect of additive fixatives on proteins?

• A. They preserve native three-dimensional structure
• B. They alter their three-dimensional shape by changing electrical charges at the site of attachment
• C. They completely degrade proteins
• D. They have no effect on proteins

Answer: (B)

Additive fixatives work by chemically binding to proteins and forming crosslinks. This attachment changes the electrical charges at reactive sites on the proteins (for example, aldehyde fixatives react with amino groups), which constrains how the protein chains interact and fold. The result is a stabilized, fixed structure that is no longer in its native, fully flexible form—the 3D shape is altered because the charges at attachment sites influence how the proteins pack and crosslink. This makes the structure rigid enough for microscopy, but it can impact antigenicity. So the correct idea is that these fixatives alter the protein’s three-dimensional shape by changing charges at the attachment sites.