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.