Sometimes
controversial level of organisation of
protein structure. Also know as a
metabolon by those who believe in it. Since each level of the structure hierarchy is less stable than the one below (1° > 2° > 3° etc) it is especially difficult to isolate these high order
complexes.
It therefore refers to transient associations between monomers and multimers (dimers, trimers and so on). The primosome, where the elements of transcription assemble in a multimeric complex is one example. This sort of structure is not in dispute - it is the more permanent complexes. Multienzyme systems like the citrate synthase complex are perhaps quaternary; but association of other TCA cycle enzymes would make a 5° situation.
See: