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the sequence of folding is different, this also means that the refolding a denatured polypeptide often differs significantly from the original folding.)

Many transcription factors consist of two domains—one that binds DNA and another that binds the signal molecule. When the signal molecule is bound, it changes the shape of its own domain (Fig. 7.16). The change in conformation is then transmitted to the DNA-binding domain, which also changes shape. Thus, although they fold separately, domains do interact physically.

Many proteins consist of subunits, usually an even number.

Quaternary Structure of Proteins

Many proteins consist of several individual polypeptide chains. This is especially true of proteins whose total molecular weight is much greater than 50,000 daltons (i.e. around 400 amino acids). [Although occasional polypeptide chains are found with a 1,000 or more amino acids, they are relatively rare.] The assembly of these multiple subunits yields the quaternary structure (Fig. 7.17). (Proteins with only one polypeptide chain have no quaternary structure.) The subunits, or protomers, are usually present as an even number, most often two or four. The terms dimer, trimer, tetramer, oligomer and multimer refer to structures with two, three, four, few/several and multiple subunits, respectively. Less than 10 percent of multimeric proteins have an odd number of subunits. The subunits may be all identical or all different or several each of two (or more) different types. For example, the lactose repressor consists of four identical subunits, whereas hemoglobin has two a-subunits and two b-subunits. The multimeric Formed of multiple subunits protomer A single polymer chain that is itself a subunit for a higher level of assembly

FIGURE 7.16 Interactions between Protein Domains to Activate a DNA-Binding Site

A) The unfolded protein shows two domains (I and II) and a linker region. B) When folded, both domain I and domain II form binding sites. C) A signal molecule binds to domain I and changes its conformation. The interaction between the two domains triggers domain II to change shape so opening up its binding site.

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