Question

Sedimentation coefficients (in Svedberg units) are often non-additive for macromolecular complexes. For example, the assembled ribosome and proteosome each have lower total sedimentation coefficients than one might expect given the constituents. How might a macromolecular assembly have a higher sedimentation coefficient than the sum of its subunits?

Short answer

Compactness and streamlined shape can increase sedimentation coefficients.

Step-by-step solution

Understanding the Sedimentation Coefficient

The sedimentation coefficient, measured in units called Svedbergs (S), reflects how quickly a particle sediments when subjected to a centrifugal force. It is influenced by the mass, shape, density of the particle and the solvent. It is not simply additive because larger, assembled complexes may have a different shape and density compared to their individual subunits.

Examining Shape and Compactness

When subunits of a macromolecular complex come together, they can achieve a more compact and streamlined shape. A more compact shape that is elongated or streamlined can lead to less frictional resistance in a centrifuge, which can result in a higher sedimentation rate.

Considering Mass and Volume

Although the mass of a macromolecular assembly may appear to sum up to the mass of the individual parts, the overall volume of the assembled complex could be smaller due to inter-subunit interactions that decrease the internal voids, thus leading to a higher sedimentation coefficient.

Role of Hydration and Solvent Interaction

The assembly of macromolecules may alter how the complex interacts with the surrounding solvent, possibly by reducing the layer of hydration or making the assembly denser, which would result in a faster sedimentation rate.

Summary of the Mechanisms

A macromolecular assembly can have a higher sedimentation coefficient than the sum of its subunits if the assembled complex forms a more compact, streamlined shape, minimizes internal voids, and has altered solvent interaction, effectively reducing friction and displacement.

Key concepts

Macromolecular Complexes

Macromolecular complexes involve the assembly of multiple biomolecules, such as proteins or nucleic acids, that operate together as a single entity. These complexes can behave very differently from their individual subunits. For instance, when subunits assemble, they may form a new shape, and this shape affects how the complex moves in a solution, especially under centrifugal force.
One reason assembled complexes may have higher sedimentation coefficients than their separate parts is their shape transformation. Once the subunits join, the complex becomes more compact, reducing internal voids and offering less resistance to movement in a centrifuge. This can lead to surprisingly efficient sedimentation that belies the expectations based on simple arithmetic addition of the subunits' sedimentation coefficients.
Additionally, interactions among the subunits can change properties like surface charge and hydrodynamic volume, affecting the density and buoyancy and thus contributing to the overall sedimentation behavior.

Ribosome and Proteosome

The ribosome and the proteosome are quintessential examples of macromolecular complexes that illustrate non-additive sedimentation coefficients. These complexes are crucial for cellular processes, with ribosomes synthesizing proteins and proteosomes degrading unneeded or damaged proteins.
Both these complexes are composed of numerous subunits. When these come together, they create a highly organized and compact structure. Despite being large, they sediment faster than expected due to their efficient packaging and interaction with the surrounding medium. The ribosome, for example, is made up of RNA and proteins that interact closely to minimize resistance and void spaces.

• The ribosome achieves its functionality through a highly interactive and compact arrangement of its subunits.
• The proteosome's configuration is equally optimized for its function, aiding in rapid sedimentation.

This means the total sedimentation behavior isn't just a summation of the individual subunits but is an emergent property of the full structure.

Centrifugal Force

Centrifugal force plays a key role in understanding why some complexes may have higher sedimentation coefficients. This force is utilized in centrifuges to separate particles based on their size, shape, and density.
When a complex is subjected to centrifugal force, its movement depends on factors like its shape and compactness as well as the medium it's in. A streamlined shape will reduce friction and thus sediment faster. Complexes that change shape upon assembly tend to show altered sedimentation coefficients because they create less resistance as they spin down.
Due to their shape and interaction with the solvent, assembled macromolecular complexes tend not to act like just a collection of their parts. Instead, they take advantage of their combined structure to enhance their sedimentation behavior, revealing a higher sedimentation coefficient than one would expect by simply adding the values of individual components.

Hydration and Solvent Interaction

Hydration and solvent interactions have a subtle but significant effect on the sedimentation coefficients of macromolecular complexes. As subunits assemble, the complex might change how it interacts with water and other molecules around it.
In some cases, macromolecular assemblies become more compact, which reduces the hydration shell—the layer of water molecules that often surrounds proteins and other macromolecules. This change can densify the complex, helping it to settle more rapidly in the centrifugal field.
Additionally, a reduced hydration layer might mean there is less drag on the complex, aiding in quicker sedimentation. In a state of enhanced solvent interaction, proteins and complexes can become more dense, essentially increasing their rate of movement in a centrifuge. These alterations in hydration and solvent interaction are crucial for understanding non-additive sedimentation coefficients. As the solvent's role is modified, even small changes can significantly impact sedimentation outcomes.