Pro Tip: Identifying the starting structure of a protein is helpful in product formulation and ingredient design.

Starting in the 1970s, researchers established that a protein’s structure dictates its function, which has led to a field of research studying the structure-function relationships of protein. In nature, proteins are created by plants and animals to serve specific tasks, such as growing new muscle tissue or serving as nutritional reserves in seeds for sprouting new plants.

While these structures help sustain life in plants and animals, the native structure is a useful starting point to understand how the protein will function in food systems. This is true even though the structure is changed during processing and isolation.

Figure 1: A-Depiction of protein primary structure, colored based on secondary structure types; B-Soy protein native structure highlighting secondary structure elements. Green spirals are α-helices, gold arrows are β-strands, red lines are random coils; C-Soy protein native structure with the organization of β-strands highlighted in gold. This is an example of a common tertiary structure fold in plant-based protein; D-Native quaternary structure of protein. Each chain of the protein is depicted in a different color, showing how the protein forms a trimer, made up of three individual protein chains.

 

All proteins are comprised of four structures. The primary structure is the sequence of amino acids. This is often represented by a string of letters. The primary structure influences many important properties, such as surface hydrophobicity and electrostatic interactions, both of which inform solubility and emulsification of protein.

The secondary structures are α-helices, β-strands and random coils. These structures can be experimentally determined through Fourier Transform Infrared Spectroscopy (FTIR), circular dichroism and RAMAN spectroscopy since they are largely stabilized through hydrogen bonding.

The tertiary structure of the protein is the way secondary structures come together to form specific folds and is stabilized by disulfide bonds, hydrogen bonds, hydrophobic interactions and electrostatic interactions.

Last, the quaternary structure is the way different protein chains come together to form larger protein complexes. In many plant-based proteins, these are dimers, trimers and hexamers. By analyzing these structures through computational and experimental means, it is possible to gain insights that relate to how the protein will function in food, and even manipulate the structure to achieve new functionalities.

All proteins have unique structural features that define how the protein will behave in food. For example, the soy protein depicted in the figure above has a relatively high temperature of denaturation, which is partially caused by the β-sheet structures adding a large amount of stability, along with several disulfide bonds; this also influences how gels form from soy protein.

In future Pro Tips, I will look at a variety of plant and animal protein native structures and point out the structures that help explain why these proteins change the properties of baked goods like cookies, cakes and bread.

Harrison Helmick is a PhD candidate at Purdue University. Connect on LinkedIn and see his other baking tips at BakeSci.com.

His research is conducted with the support of Jozef Kokini, Andrea Liceaga, and Arun Bhunia.