10.0 Alpha + Beta Tertiary Structure

This general domain tends to have the alpha helix and beta sheet segregated in different parts of the polypeptide chain.

Alpha + beta tertiary structural domains occur in proteins with one domain (eg. carbonic anhydrase, cytochrome b5, lysozyme, pancreatic trypsin inhibitor, and ribonuclease), and occurs at least once in proteins with two domains (eg. thermolysin, T4-lysozyme, and lactate dehydrogenase), or three domains (eg. glutathione reductase).

• The beta strands tend to be all antiparallel

• There are also no supersecondary 'beta alpha beta' motifs.

• There are presently (as of version 1.39) about 113 subclasses listed in SCOP for this domain

• The beta sheets and alpha helices can take on many permutations throughout the domain and make it difficult to recognise analogies to particular structures. However, the domain can take on similar analogous structures seen previously for alpha/beta domains and examples of these are given below.

10.1 Alpha + beta

Simply a protein with an alpha + beta arrangement and where no specific 'structural analogy' can be made. This is seen, for example, in lyozyme and FK506 binding protein and shown in Figure 10.1 and Figure 10.2 respectively.

10.2 Alpha + beta barrels

In this barrel the beta sheets tend to surround an alpha helix core seen in green fluorescent protein and shown in Figure 10.3

10.2 Alpha + beta n-layer sandwich

This domain describes the structure where the beta sheet and alpha helices are arranged in 'layers' similar to a multi-layered sandwich.

The number 'n' indicates the layers of either beta sheets or alpha helices and generally ranges from 2 to 4.

An examples of a 3-layer sandwich is seen in serum response factor and shown in Figure 10.4.