Biochemistry: The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Enzymes: The Protein Catalysts of Cells
Regulation of Enzymatic Activity
Spatial Compartmentalization: Isolated Compartments and Organized Assemblies

Important aspects of Metabolic control are closely linked to the Spatial Organization of The Cell. In Bacteria, the periplasmic space (Chapter 5, Section D) is isolated from the Cytosol, and Enzymes localized within this space do not mix with other cellular enzymes. A number of enzymes are membrane-bound or attached to the membrane. Eukaryotic Cells possess a greater number of isolated compartments than bacterial cells; these include The Nucleus, Mitochondria (comprising their internal matrix and intermembrane space), Lysosomes, Microbodies, and vacuoles. Another membrane-bounded compartment consists of the cytosolic tubules and vesicles of The Endoplasmic reticulum.

The rate of metabolite Transport Across the membranes separating different cellular compartments is limited and tightly regulated. Furthermore, membrane surfaces harbor receptors that serve as targets for the majority of Hormones and Neurotransmitters, and these receptors are subject to chemical modification.

Alongside enzymes dissolved in the cytosol that do not form long-lived complexes with other Proteins, there are enzymes (catalyzing a series of sequential reactions) attached to the membrane to form an organized assembly. Such properties appear to be characteristic of mitochondrial oxidative enzymes (Chapter 10, Section D). Certain enzymes associate to form high-molecular-weight complexes. Examples of such complexes include keto acid dehydrogenases (Chapter 8, Section K) and cytoplasmic fatty acid synthetases (Chapter 11, Section D). In both cases, the product of the reaction catalyzed by the first enzyme is covalently attached to a carrier molecule, in which state it undergoes consecutive transformations mediated by the other Enzymes of the complex¹⁾.