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What Controls a Cell's Biochemical Pathways?

A biochemical pathway is a specific sequence of chemical reactions in which the product of the first reaction becomes the substrate for the succeeding one. These elaborate chain reactions occur within an organism's cells until the desired final products are produced. A number of factors serve to control a biochemical pathway, including thermodynamic conditions within the cell, availability of the principal molecules and concentrations of reaction intermediates and by-products.

Enzymes
- Enzymes play a key role in controlling biochemical pathways. They can trigger reactions, act as substrates and even bridge two or more independent pathways. One notable example is the enzyme SHB17, which links the digestion of glucose and other sugars directly to the synthesis of ribose, which in turn plays a critical role in DNA and RNA production and repair. Other essential biochemical pathways regulated by enzymes include glycolysis, glucogenesis, the citric acid cycle, the metabolism of glycogen and fatty acids, and the pentose phosphate pathway.
Growth Factors
- Growth factors are proteins or steroid hormones which trigger the pathways responsible for cellular growth and differentiation. One example, the growth factor IGF1, attaches to a receptor on the cell membrane, kicking off a pathway which ultimately gives the green light to mitosis. IGF1 activates a series of phosphorylation reactions involving enzymes called kinases. These kinases take phosphate groups from ATP molecules to other substrates in the cell, until finally phosphorylated AKT kinase signals the nucleus to start the cell cycle.
Regulatory Proteins
- While growth factors stimulate cell division, other regulatory proteins stop the process or instruct the cell to undergo apoptosis or programmed cell death. One example of a regulatory protein, PTEN, stops mitosis by carrying away the phosphate group bound to the phosphoinositide 3-kinase (PI¬3-kinase). When this happens, the kinase can no longer act as substrate for the succeeding reaction, leading to a dead end for the pathway. Regulatory proteins also schedule apoptosis when no growth factor is available, or when the cell gets damaged beyond repair.
Mutations in Principal Molecules
- Mutations in principal molecules aren't so much a control as they are an interruption or interference to a normal pathway. Minor mutations in enzymes or proteins can have dramatic effects, as is the case with cancer. In this modification to the normal growth pathway, a mutated PI3-kinase molecule self-activates the growth cycle, leading to runaway mitosis. This, however, can be stopped by the regulatory protein PTEN, which dephosphorylates the mutated PI3-kinase. Cancer will result if both the PI3-kinase and PTEN molecules become mutated, as there will then be no braking mechanism for the pathway.

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