Hierarchy of Protein Structure

We now begin to build our way up to the three dimensional (3-D) structure of a protein. The 3-D structure of any protein is complicated, so to help us understand we have developed a convenient way to organise, model, and 'picture' this structure.

In particular, Linderstrom-Lang (1952) first proposed that there was a hierarchy of protein structure with four levels: primary, secondary, tertiary, and quaternary. You are probably familiar with this hierarchy as this structural classification is the most convenient starting point for teaching basic protein structure. To revise:

The implications of this hierarchy are that:

However, subsequent work has revealed that this four-level hierarchy of protein structure is a bit too simple and the three dimensional structure of proteins can now be conveniently described by a six level structural hierarchy, shown in (Figure 1), which includes:

Figure 1. Protein Hierarchy

Hierarchy

The definitions of primary, secondary, tertiary, and quaternary structure are as above and the definitions of the other levels of structure are:

Supersecondary structure is the next level up from secondary structure and involves the association of secondary structures. Also known as structural motifs1.

Domains are larger associations of two or more secondary structures, two or more supersecondary elements, or mixtures of two or more secondary and supersecondary structures. They can also be known as 'folds', and 'modules'. As we will see later in Protein Tertiary Structure domains are independently folding units of tertiary structure and contain between 35 and 200 amino acids. Note that some other authors may present slightly different definitions of the terms above and we will look at some definitions later in Protein Tertiary Structure.

Motif. Please note that the term 'motif' is often used in other contexts. For example, a motif can be either a structural or a sequence motif:

 
A structural or sequence motif may or may not have a particular function associated with it.


Introduction | Protein Hierarchy | Secondary Structure | Helices | Sheets | Loops | SuperSecondary Structure | Tertiary Structure | All alpha structure | All beta structure | Mixed alpha/beta structure | Mixed alpha+beta structure | Other Tertiary Structure
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