Fundamentals of Molecular Biology. Part 1: Molecular Biology of the Cell - A. N. Ogurtsov 2011

Molecular Foundations of Protein Function
Protein Assemblies

Multimeric protein associates consist of two or more Polypeptides or subunits. The fourth level of structural Organization in Proteins—The quaternary Structure—defines the number (stoichiometry) and relative spatial arrangement of subunits within a multimeric protein. For example, hemagglutinin is a trimer composed of three identical subunits held together by non-covalent bonds (Figure 116(6)). Other multimeric proteins are likewise composed of several identical or different subunits.

The multimeric nature of many proteins is crucial for The regulation of their Functions.

Enzymes belonging to the same metabolic pathway are often associated as subunits of a single large protein, thereby enhancing the operational efficiency of that pathway.

The highest level of Cell/13.html">Protein Structure involves the assembly of proteins into macromolecular aggregates. Typically, these are massive structures with a mass on the order of 1 megadalton (1 MDa), a size of 30–300 nm, and containing dozens (or even hundreds) of polypeptide chains, as well as Nucleic Acids in some cases.

Such macromolecular aggregates include the viral capsid (virus shell) and cytoskeletal filament bundles that support The Plasma Membrane and give it its specific shape. Other macromolecular aggregates function as molecular machines, driving most complex cellular processes by integrating individual Protein Functions into a single coordinated operation.

For instance, the METABOLISM/31.html">Transcription machinery that initiates Messenger RNA (mRNA) synthesis consists of RNA polymerase (itself a multimeric protein) and about 50 additional components, including transcription factors, promoter-binding proteins, helicases, and other Protein Complexes (Figure 118).

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Figure 118 – Molecular complex of mRNA Translation initiation

Transcription factors and promoter-binding proteins accurately position RNA polymerase on the promoter, a DNA region that marks the start site for transcription of a specific Gene. Once a helicase unwinds the DNA double helix, RNA polymerase simultaneously moves along the DNA (translocation) and synthesizes mRNA (transcription). We will examine Introduction/24.html">DNA Transcription in detail In the second part of the book.