Five Basic Steps in Signal Transduction

When an extracellular signaling molecule activates a membrane receptor, intracellular molecules are altered, which creates a response called signal transduction. Signaling transduction involves the binding of extracellular molecules and ligands to cell receptors that trigger events inside the cell. Extracellular molecules are integral trans-membrane proteins that make up most of the receptors. This process includes receptors passing messages to cells and those cells being amplified to cause responses.

Initial Contact

• Receptors on the cell surface begin the process by signaling other receptors to change shape, thus changing the receptor to its active messaging form. These receptors trigger events that begin to take place within the cell. This activates certain proteins that release other chemicals, which then combine with molecules. The protein is released from the receptor and splits into two parts: one part is activated and continues to signal and the other part leaves the receptor and reverts to being inactive.

Chemical Activation

• The protein activates the enzyme called adenylyl cyclase, which is part of the protein signaling process that transmits chemical signals from outside of the cell across the membrane to the inside of the cell. After activation, adenylyl cyclase converts large numbers of ATP molecules (intracellular energy transfer) into signaling molecules. These signaling molecules carry messages of the first initial contact into the cell.

Chemical Subunits

• The protein soon inactivates itself. With the protein no longer attached, the adenylyl cyclase shuts down and can no longer make energy conversions. Certain molecules produced by adenylyl cyclase continue the signaling process by binding to another type of protein enzyme. This binding action triggers the protein to separate into subunits, two of which are catalytically active. The active protein subunits create chemical reactions in which they add phosphate groups to another type of enzyme, phosphate kinase.

Enzyme Breakdowns

• Phosphate kinase then phosphorylates another enzyme called glycogen phosphorylase.
  Phosphorylation is the process of adding a phosphate group to an organic molecule. Once this process is complete, the enzyme becomes activated. In this state of activation, the glycogen produces cellular responses to the first initial contact. Glycogen breaks down other glycogen into component molecules. Throughout this process, the enzyme adds a phosphate group to each of the glucose subunits.

Cellular Response

• Another enzyme removes the phosphate groups from the molecules, so without the phosphate groups, the glucose molecules are transported across the plasma membrane of the cell. Once this is outside of the cell, the glucose enters the bloodstream and is swallowed by other cells and is then used as fuel to initiate a response to the first moment of contact that triggered the signal transduction process. Periods of amplification happen throughout the transduction process, and that causes the different chemicals and enzymes to be released. Short periods of activation are vital and must be regulated when in the process of reaction, which the body terminates accordingly.