kvaternäärirakenne

Kvaternäärirakenne, or quaternary structure, refers to the highest level of protein folding, where two or more protein subunits assemble into a larger functional complex. It is distinct from the primary, secondary, and tertiary structures that describe the amino acid sequence, local folding into alpha helices and beta sheets, and the overall 3D shape of a single polypeptide chain, respectively. The interactions stabilizing a quaternary state include non-covalent bonds such as hydrogen bonds, hydrophobic interactions, ionic interactions, and van der Waals forces, as well as covalent crosslinks such as disulfide bridges between cysteine residues of different subunits.

Quaternary arrangements are commonly classified by the number of subunits: dimers, trimers, tetramers, and higher-order oligomers. Symmetry plays an important role; symmetrical complexes may exhibit rotational or translational symmetry, whereas asymmetric complexes lack such regularity. Functionally, quaternary structure enables cooperative binding, allosteric regulation, and efficient catalysis. Hemoglobin serves as a classic example, wherein four polypeptide chains (two alpha, two beta) form a tetramer that displays oxygen-binding cooperativity. Myoglobin, the oxygen storage protein, is a monomer and therefore lacks quaternary organization.

Biological quaternary assemblies can be transient or stable. Enzymes such as trypsinogen convert to a single active chain upon cleavage, illustrating that quaternary organization can be modulated during activation. In contrast, structural complexes like the ribosome, composed of thousands of proteins and RNAs, form enormous, highly organized quaternary structures essential for cellular function. The study of quaternary structure relies on techniques such as X‑ray crystallography, cryo-electron microscopy, analytical ultracentrifugation, and native mass spectrometry, which together reveal details about subunit stoichiometry, interfaces, and dynamics.