page_title: Cytosol
section_title: Protein complexes
hierarchical_section_title: Cytosol / Organization / Protein complexes
caption_attribution_description: English: Legend: (left) Purified carboxysomes (material courtesy of S. Heinhorst and G. Cannon) as visualized by cryo-electron microscopy (courtesy of M.J. Yeager and K.A. Dryden, University of Virginia). (right) Models for the structure of the carboxysome. Current data suggest that the shell is composed of several hundred hexameric protein building blocks and 12 pentameric building blocks. The three-dimensional atomic structures of the shell proteins have been determined by X-ray crystallography. RuBisCO, the main interior enzyme is shown packed inside in a regular arrangement for simplicity, though the actual organization of the enzymes is not understood yet. The other key enzyme, carbonic anhydrase, which is present in lesser amounts, is not illustrated (image by T.O. Yeates, UCLA).
context_page_description: The cytosol, also known as intracellular fluid or cytoplasmic matrix, or groundplasm, is the liquid found inside cells. It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into many compartments.
  In the eukaryotic cell, the cytosol is surrounded by the cell membrane and is part of the cytoplasm, which also comprises the mitochondria, plastids, and other organelles; the cell nucleus is separate. The cytosol is thus a liquid matrix around the organelles. In prokaryotes, most of the chemical reactions of metabolism take place in the cytosol, while a few take place in membranes or in the periplasmic space. In eukaryotes, while many metabolic pathways still occur in the cytosol, others take place within organelles.
  The cytosol is a complex mixture of substances dissolved in water. Although water forms the large majority of the cytosol, its structure and properties within cells is not well understood.
context_section_description: Proteins can associate to form protein complexes, these often contain a set of proteins with similar functions, such as enzymes that carry out several steps in the same metabolic pathway. This organization can allow substrate channeling, which is when the product of one enzyme is passed directly to the next enzyme in a pathway without being released into solution. Channeling can make a pathway more rapid and efficient than it would be if the enzymes were randomly distributed in the cytosol, and can also prevent the release of unstable reaction intermediates. Although a wide variety of metabolic pathways involve enzymes that are tightly bound to each other, others may involve more loosely associated complexes that are very difficult to study outside the cell. Consequently, the importance of these complexes for metabolism in general remains unclear.
caption: Carboxysomes are protein-enclosed bacterial microcompartments within the cytosol. On the left is an electron microscope image of carboxysomes, and on the right a model of their structure.
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