BIOCHEMICAL PATHWAYS

==Metabolism== Metabolism is all the chemical reactions that are needed for an organism to stay alive. Virtually all of these reactions are enzyme catalysed. The role of enzymes in our metabolism can be divided into: [image:http://i.imgur.com/Rwr7W19.png?1] ''Anabolic enzymes'' catalyse chemical reactions in which bonds are formed. The enzyme temporarily binds to the substrate molecules and brings them into an orientation that enables a bond to form between them. These reactions are needed to build larger molecules and chemical structures. [image:http://i.imgur.com/BwydY1i.png?2] ''Catabolic enzymes'' catalyse chemical reactions in which bonds are broken. The enzyme temporarily binds to a substrate molecule and increases the stress on an existing bond until it breaks. These reactions are needed to break larger molecules into their chemical components. Digestive enzymes are catabolic, they break down larger chemical substances into smaller soluble molecules that can be used as building blocks for growth and repair. ==Biochemical Pathway== A biochemical pathway (also called a '''metabolic pathway''') is a series of enzyme-mediated reactions where the product of one reaction is used as the substrate in the next. [image:http://i.imgur.com/1BgC9Jm.png?1] Each enzymes is coded by a different gene. For instance, lets assume '''enzyme A''' is coded for by '''gene A'''. Similarly '''enzyme B''' is coded for by the '''gene B'''. [image:http://i.imgur.com/nQaXpXq.png?11] In this example let's assume that enzyme A converts a white compound (chemical 1) into a brown compound (chemical two). Enzyme B then uses the brown compound as a substrate and converts it into a black compound (chemical 3). For a mouse to have the black phenotype it must have both the ''A'' and ''B'' genes. If the mouse only has the A gene then the brown compound will accumulate and it will have the brown phenotype. If the mouse only has the B gene the white compound accumulates and the effect of the B gene is masked (although enzyme B is present it has no substrate to act on). A ''mutation'' within a gene will often result in a non-functional enzyme. This itself wouldn't usually affect the biochemical pathway as most mutations are recessive and the normal (dominant) allele would still produce a functional enzyme. [image:http://i.imgur.com/dQs3Dtg.png] Provided the mouse has at least one functional ''A'' and ''B'' gene the final (black) product is produced. If the mouse inherits two mutant ''b'' alleles (non-functional) the brown compound accumulates. If the mouse inherits two mutant ''a'' alleles (non-functional) the white compound accumulates. This can be summarised as follows (left). ==Feedback Inhibition== [image:http://i.imgur.com/kZMmUH9.png?1] In many biochemical pathways the final product acts as an ''enzyme inhibitor'', inhibiting the action of the first enzyme. This is known as ''feedback inhibition'' and acts as a mechanism to prevent excessive accumulation of the final product. Often, the initial substrate can also be used in other biochemical pathways and feedback inhibition also ensures it isn't wasted. ==Biochemical Pathways in Humans== The biochemical pathways in human cells are quite complex with many many intersecting pathways. Our phenotype is ultimately the result of the complex interactions of the proteins encoded by our genes. Below is a chart illustrating many of the known biochemical pathways in humans. [image:http://i.imgur.com/eKTfhPu.png?1] [http://www.cc.gatech.edu/~turk/bio_sim/articles/metabolic_pathways.png see the full size version here]