Active site in the context of "Protein quaternary structure"

An enzyme is a biological macromolecule, usually a protein, that acts as a biological catalyst, accelerating chemical reactions without being consumed in the process. The molecules on which enzymes act are called substrates, which are converted into products. Nearly all metabolic processes within a cell depend on enzyme catalysis to occur at biologically relevant rates. Metabolic pathways are typically composed of a series of enzyme-catalyzed steps. The study of enzymes is known as enzymology, and a related field focuses on pseudoenzymes—proteins that have lost catalytic activity but may retain regulatory or scaffolding functions, often indicated by alterations in their amino acid sequences or unusual 'pseudocatalytic' behavior.

Enzymes are known to catalyze over 5,000 types of biochemical reactions. Other biological catalysts include catalytic RNA molecules, or ribozymes, which are sometimes classified as enzymes despite being composed of RNA rather than protein. More recently, biomolecular condensates have been recognized as a third category of biocatalysts, capable of catalyzing reactions by creating interfaces and gradients—such as ionic gradients—that drive biochemical processes, even when their component proteins are not intrinsically catalytic.

Proteasomes are essential protein complexes responsible for the degradation of proteins by proteolysis, a chemical reaction that breaks peptide bonds. Enzymes that help such reactions are called proteases. Proteasomes are found inside all eukaryotes and archaea, and in some bacteria.In eukaryotes, proteasomes are located both in the nucleus and in the cytoplasm. The proteasomal degradation pathway is essential for many cellular processes, including the cell cycle, the regulation of gene expression, and responses to oxidative stress. The importance of proteolytic degradation inside cells and the role of ubiquitin in proteolytic pathways was acknowledged in the award of the 2004 Nobel Prize in Chemistry to Aaron Ciechanover, Avram Hershko and Irwin Rose.

The core 20S proteasome (blue in the adjacent figure) is a cylindrical, compartmental protein complex of four stacked rings forming a central pore. Each ring is composed of seven individual proteins. The inner two rings are made of seven β subunits that contain three to seven protease active sites, within the central chamber of the complex. Access to these proteases is gated on the top of the 20S, and access is regulated by several large protein complexes, including the 19S Regulatory Particle forming the 26S Proteasome. In eukaryotes, proteins that are tagged with Ubiquitin are targeted to the 26S proteasome and is the penultimate step of the Ubiquitin Proteasome System (UPS). Proteasomes are part of a major mechanism by which cells regulate the concentration of particular proteins and degrade misfolded proteins.

Enzyme catalysis is the increase in the rate of a process by an "enzyme", a biological molecule. Most enzymes are proteins, and most such processes are chemical reactions. Within the enzyme, generally catalysis occurs at a localized site, called the active site.

Most enzymes are made predominantly of proteins, either a single protein chain or many such chains in a multi-subunit complex. Enzymes often also incorporate non-protein components, such as metal ions or specialized organic molecules known as cofactor (e.g. adenosine triphosphate). Many cofactors are vitamins, and their role as vitamins is directly linked to their use in the catalysis of biological process within metabolism. Catalysis of biochemical reactions in the cell is vital since many but not all metabolically essential reactions have very low rates when uncatalysed. One driver of protein evolution is the optimization of such catalytic activities, although only the most crucial enzymes operate near catalytic efficiency limits, and many enzymes are far from optimal. Important factors in enzyme catalysis include general acid and base catalysis, orbital steering, entropic restriction, orientation effects (i.e. lock and key catalysis), as well as motional effects involving protein dynamics

A protein complex or multiprotein complex is a group of two or more associated polypeptide chains. Protein complexes are distinct from multidomain enzymes, in which multiple catalytic domains are found in a single polypeptide chain.

Protein complexes are a form of quaternary structure. Proteins in a protein complex are linked by non-covalent protein–protein interactions. These complexes are a cornerstone of many (if not most) biological processes. The cell is seen to be composed of modular supramolecular complexes, each of which performs an independent, discrete biological function.

The carbonic anhydrases (or carbonate dehydratases) (EC 4.2.1.1) form a family of enzymes that catalyze the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid (i.e. bicarbonate and hydrogen ions). The active site of most carbonic anhydrases contains a zinc ion. They are therefore classified as metalloenzymes. The enzyme maintains acid-base balance and helps transport carbon dioxide.

Carbonic anhydrase helps maintain acid–base homeostasis, regulate pH, and fluid balance. Depending on its location, the role of the enzyme changes slightly. For example, carbonic anhydrase produces acid in the stomach lining. In the kidney, the control of bicarbonate ions influences the water content of the cell. The control of bicarbonate ions also influences the water content in the eyes. Inhibitors of carbonic anhydrase are used to treat glaucoma, the excessive build-up of water in the eyes. Blocking this enzyme shifts the fluid balance in the eyes to reduce fluid build-up thereby relieving pressure.