Most enzymes discussed in the preceding chapters consist of single proteins that catalyze single biochemical reactions. Many of them contain one type of polypeptide chain, although most exist as oligomers of a polypeptide, and some consist of different polypeptides that cooperate to catalyze one reaction. Increasing attention is being focused on enzymes that catalyze more complex processes and are composed of more than one enzyme or enzymatic domain, each of which catalyzes or facilitates a specific biochemical process. These complex enzymes are the subjects of this chapter. Complex enzymes are so numerous and the processes they catalyze so complex that a complete discussion would fill a book. We therefore limit this discussion to a few examples. The first complex enzymes to be discovered were the multienzyme complexes. They included the four terminal electron transport complexes of the respiratory chain: complex I, known as NADH dehydrogenase (formerly DPNH dehydrogenase); complex II, known as succinate dehydrogenase; complex III, known as cytochrome c reductase; and complex IV, known as cytochrome c oxidase. Other multienzyme complexes discovered at about the same time were the pyruvate dehydrogenase and α-ketoglutarate dehydrogenase complexes, the fatty acid synthase complexes, and the glycine reductase complex and the anthranilate synthase complex. Later, the multimodular polyketide synthases and nonribosomal polypeptide synthetases were characterized. The ATP synthases are multiprotein complexes that function as molecular motors in catalyzing a complex reaction, the condensation of ADP with Pi driven by proton translocation to form ATP. The ribosome catalyzes the polymerization of amino acids in defined sequences specified by the nucleotide sequences in species of mRNA, and nitrogenase catalyzes the ATP-dependent reduction of molecular nitrogen to ammonia. Some of the actions of complex enzymes link together common biochemical reactions of the types discussed in preceding chapters. Others catalyze difficult reactions through mechanistic coupling to energy-producing processes that provide driving force for otherwise unfavorable transformations. We present examples of each type. Catalysis by an α-ketoacid dehydrogenase complex is carried out by three physically associated enzymes, a TPP-dependent α-ketoacid dehydrogenase (E1), a dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3).
Memory and Imprinting Effects in Multienzyme Complexes. I. Isolation, Dissociation, and Reassociation of a Phosphoribulokinase-Glyceraldehyde-3-Phosphate Dehydrogenase Complex from Chlamydomonas Reinhardtii Chloroplasts