Mechanical Protein Functions

Proteins convert chemical energy to mechanical work through conformational changes in their structure. Simple chemical changes, like ATP or GTP hydrolysis, can trigger small shifts in protein domains that amplify into much larger movements. For example, myosin uses ATP hydrolysis to act as a lever, pulling actin filaments and causing muscle contraction. These conformational changes allow proteins to perform mechanical functions like cell movement and molecular transport.

GTP hydrolysis powers the elongation factor EF-Tu, which transfers tRNA molecules to the ribosome during protein synthesis. When GTP is hydrolyzed to GDP, the released phosphate group causes a small conformational change in the nucleotide-binding site. This shift moves an alpha helix at the interface of protein domains, causing them to swing open and release the bound tRNA so it can add an amino acid to the growing protein strand.

Helicases use ATP hydrolysis to break hydrogen bonds between DNA strands and move along the DNA molecule. The energy released from ATP hydrolysis powers conformational changes in the helicase protein, enabling it to separate the double helix during DNA replication and transcription. This mechanical function demonstrates how chemical energy drives the unwinding of genetic material.

Mechanical protein functions fall into two categories: proteins that generate mechanical forces and proteins subjected to mechanical forces. Proteins like myosin and ion pumps generate force for cell movement and molecular transport. Proteins like keratin provide structural support and are subjected to mechanical stress. Some proteins, such as actin, perform multiple roles, acting as both a track for motor proteins and a structural component of the cytoskeleton.

Actin filaments exert pressure on the cell membrane, causing the cell to form filopodia and lamellipodia, which are membrane extensions that allow cells to migrate. Myosin motor proteins walk along actin filaments, pulling them and generating the mechanical forces needed for cell movement. The cytoskeleton formed by actin filaments provides both structural support and the dynamic mechanical capability for cells to change shape and move.

EF-Tu contains three distinct domains, with one binding GTP. When GTP hydrolyzes to GDP, the inorganic phosphate release triggers a conformational change in the nucleotide-binding site. This causes an alpha helix at the domain interface to shift position, making the three domains swing open relative to each other. This domain movement releases the tRNA held at their interface, allowing it to enter the ribosome.

Scientists use optical tweezers, a specialized technique that can measure the force actin produces when deforming the cell membrane. This tool allows researchers to quantify the mechanical work performed by actin filaments as they generate cellular movements and structural changes. Optical tweezers provide direct measurement of the mechanical forces underlying cell migration and shape changes.