Kinases are enzymes that phosphorylate the substrates they bind to, meaning that the kinase transfers a phosphate group to the substrate. ATP donates the phosphate group used in this process. Phosphorylation by kinases can lead to a variety of effects including: activation, inhibition, stabilization, destabilization, and localization. Kinases are very important in cellular activities and metabolic processes. 

Receptors are proteins embedded in the plasma membrane that receive and transmit signals from extracellular and intracellular sources. Receptors can also be embedded in the plasma membrane of the nucleus. Receptors bind to ligands, which can elicit a variety of responses including: activation, partial activation, and inhibition.

Ligands are molecules that form complexes with other molecules, proteins, or genetic material. The binding of a ligand causes a conformational change to the complex that leads to a response. This response by associated molecules is not limited to activation, but also includes inhibition, partial activation, and inhibition of constitutive activity. Ligands can be either selective or non-selective, meaning that they can bind to only specific molecules or to many receptors, respectively.

Second messenger systems begin with an extracellular ligand that binds to a receptor on the cell surface. The receptor then activates intracellular primary effectors (proteins that transduce the signal from the plasma membrane to the cytosol). In the cytosol, effectors activate second messenger molecules, which regulate intracellular process including transcription, neurotransmitter release, and enzyme activation. Second messengers have several common characteristics: they are localized, are easily synthesized and degraded, and are intracellular. These systems are responsible for diverse cellular processes and are able to amplify signals through kinase cascades. Common second messenger systems are the cAMP system and the tyrosine kinase system.

The cAMP second messenger system is involved in many signaling pathways, such as the regulation of glycogen, growth hormone, and lipid metabolism. In the cAMP system, ligand binding activates a G protein-coupled receptor and the associated intracellular G protein. The activated G protein then stimulates the enzyme adenylate cyclase to produce cAMP second messenger molecules from ATP. The cAMP molecules activate protein kinases that, in turn, activate a variety of target proteins through phosphorylation. Cyclic AMP or cAMP systems are capable of transducing a variety of signals. 

The MAP kinase signaling system is a common method of cellular signaling that regulates transcription of genes within a cell. The pathway begins when a ligand binds to a membrane receptor. The activated receptor activates an associated Ras protein, which is a GTPase that is activated when bound to GTP. The active Ras protein then donates a phosphate group to a MAP kinase protein and activates it. This begins a MAP kinase phosphorylation cascade. MAP kinases phosphorylate other MAP kinases in a serial fashion, which allows for signal amplification. After a series of MAP kinase phosphorylation events, specific MAP kinases phosphorylate transcription factors, regulating their activity and gene expression. Thus, the pathway ends with transcription factor regulation.

The Golgi apparatus is an organelle that modifies and packages proteins for export. The Golgi receives proteins in vesicles from the endoplasmic reticulum on the cis face. Protein modification by enzymatic activity occurs within the Golgi. The modified proteins are then packaged into vesicles on the trans face, some of which are destined for extracellular transport via exocytosis. 

Prokaryotes, like eukaryotes, rely on cell signaling to coordinate metabolic activities and transduction pathways in response to other cells and the environment. Prokaryotes lack membrane-bound organelles; therefore, membrane trafficking is not the same as in eukaryotes. Prokaryotic membrane trafficking involves outer membrane vesicles, which hare nanoscale expansions of the periplasm that pinch off from bacterial cells and fuse to target cells. Cell signaling in prokaryotes also takes place at the host-pathogen interface, where host cells can recognize prokaryotic factors displayed on the plasma membrane. Additionally, prokaryotes take part in complex signaling pathways during quorum sensing to ascertain population density. 

Recognition of pathogens is an example of cell signaling and very important to the immune system’s ability to defend the cell from disease. Host cells have specific receptors that recognize pathogens; pattern recognition receptors bind pathogen-associated molecular patterns, B cell receptors bind epitopes, and cytokine receptors bind to cytokines. In this way, pathogens are recognized by host cells and begin signaling pathways to defend against them.

When a protein is active in it’s native state, it is said to be constitutively active. In the case of protein receptors, a receptor is constitutively active when it is active without binding to a ligand.