DYNAMIC OF BIOMOLECULES AND CELLS 2014
Unfolding the elastomeric protein titin using atomic force microscopy
Why atomic force microscopy is most suited to this task
An essay on the role atomic force microscopy plays in the unfolding of titin and why atomic force microscopy is suited to such experiments
Proteins are a group of molecules, present in the human body (and other living organisms), that have varied functions. They are made up of chains of smaller molecules called amino acids. The amino acids are arranged in long strings, these strings are then folded into shapes to create a functional component.
Proteins have lots of different functions, such as bio-regulation (in the case of hormones). There exist transport proteins that for example move minerals through the body, structural proteins make up the skin, bones and some proteins are catalytic (enzymes).
This essay focusses on an elastomeric protein. An elastomer is an elastic polymer; elastomeric proteins are multi-unit proteins that display elasticity.
Elasticity is ability of a solid material to return to the original form after deformation. The physical origin for elasticity varies per material. In the case of rubber and other elastic polymers, the elasticity arises from the stretching of the polymer chain (when force is applied to it) that the elastic polymer is made of.
To study the elasticity of proteins, a tool has to be used that can measure the force required to make a certain extension of the polymer. One such tool is the atomic force microscope (AFM).
An AFM uses a sharp tip on a cantilever that will deflect when it is brought in close proximity to a surface. The deflection of the cantilever can be measured and the topography of the sample can be measured (See Figure 1).
Another application of AFM, besides imaging, is ‘force spectroscopy’. If the spring constant of the cantilever is known, then the previously mentioned deflection measurement could be used to calculate the force applied to the cantilever. By keeping track of the gap between sample and tip (by using a piezoelectric displacement stage), and combining this with the calculated force, a force-distance curve can be produced. Force spectroscopy can then be used to, for example, measure mechanical properties of living materials. To describe a material one can use its functional attributes like stiffness (determined the modulus of elasticity), strength (stress when material fractures), toughness (energy needed to break the material), extensibility (stress at failure) and spring capacity (capacity of the material to store energy)1.
The AFM is an ideal tool to perform mechanical experiments of elastomeric proteins, because it can be used to measure the forces for single molecule stretching and/or rupturing.
Titin is one such an elastomeric protein. Titin provides elasticity and stability to striated muscle tissue (present in skeletal and cardiac muscle). It is located in the sarcomere, which lie...