 Phosphatidylinositol 4-kinases are phosphoinositide kinases that form phosphatidylinositol 4-phosphate (PI4P).  These enzymes can be further classified into type II PI4Ps and type III PI4Ps.  Type II enzymes are localized in the membrane and include the isoforms PI4K2A and PI4K2B.  Type III enzymes include PI4KA and PI4KB.  

 Phosphatidylinositol 4-Kinases (PI4Ks)

Phosphoinositide kinases are a type of lipid kinases that form phosphoinositides.  Because their products regulate cell life, development, and death, their activity is closely monitored by regulatory proteins, and they can be translocated to produce specific phosphoinositides.  Dysregulation of these enzymes are associated with many human diseases, including cancers, developmental disorders, and immune deficiencies.  Types of phosphoinositide kinases include:

- PI3Ks/type III PI4Ks
- type II PI4Ks
- PIP kinases

Michigan State University

Phosphoinositides are a class of phospholipids that are formed from phosphatidylinositol.  Seven types of phosphoinositides exist:

- phosphatidylinositol 3-phosphate (PI3P)
- phosphatidylinositol 4-phosphate (PI4P)
- phosphatidylinositol 5-phosphate (PI5P)
- phosphatidylinositol 4,5-bisphosphate (PI4,5P2)
- phosphatidylinositol 3,5-bisphosphate (PI3,5P2)
- phosphatidylinositol 3,4-bisphosphate (PI3,4P2)
- phosphatidylinositol 3,4,5-trisphosphate (PIP3)

These molecules collectively make up less than 1% of all lipids, but they function as important mediators of cell growth and development as well as membrane signaling and trafficking.  

Phosphoinositides

Enzymes are proteins that function as biological catalysts, meaning they can increase the rate of a reaction without being consumed.  

Enzymes

Burke, J. E. (2018). Structural basis for regulation of phosphoinositide kinases and their involvement in human disease. Molecular Cell, 71(5), 653-673.
https://www.sciencedirect.com/science/article/pii/S1097276518306397
doi: 10.1016/j.molcel.2018.08.005

Structural Basis for Regulation
of Phosphoinositide Kinases
and Their Involvement in Human Disease

Phosphoinositide Kinases

Serine proteases are a type of enzyme that are diversely spread throughout all kingdoms of life.  Their function is to cleave a bond in a peptide via hydrolysis.  Typically, they are endoproteases, meaning they work in the middle of a peptide chain.  Additionally, they are allosteric enzymes.  This means that their function and activity can be controlled by an effector molecule that induces a conformational shape change in the enzyme.  Their name is derived from the serine amino acid and its side chain that plays an important role in its function that resides in the enzyme active site.

Serine Proteases

RNA polymerase builds mRNA molecules through base pairing. If the DNA nucleotide is a C, the polymerase will add a G to the RNA (and similarly, a A to a T). RNA polymerase always builds RNA from the 5' to 3' position.

RNA Polymerase

A Disintegrin And Metalloproteinases (ADAMs) are proteinases that facilitate a process known as proteolytic ectodomain, or shedding.  These enzymes can remove integral membrane proteins, including type I and type II transmembrane proteins and GPI-anchored proteins.  ADAM-mediated shedding is involved in processes including cell fate determination, cell proliferation, immunity, and angiogenesis.  These enzymes are hallmarked by a conserved structure and target some of the same transmembrane proteins as substrates.  

A Disintegrin And Metalloproteinases (ADAMs)

Cysteine cathespin proteases are a family of proteases that primarily function as endopeptidases.  They are notably found in lysosomes where they work to process and degrade biological molecules.  Their function is relevant to many physiological processes, including:

- degrading proteins in the lysosomes
- cell death (inclduing autophagy)
- the post-translational modifications of proteins
- extracellular matrix (ECM) remodeling
- immune signaling

Members of this family include Cathepsin L (L1), B, C, F, H, K, O, V (L2), X, S, and W.  

Cysteine Cathespin Proteases

Serine Protease Inhibitors

This is the main protease in coronaviruses. It cleaves proteins in coronavirues at conserved places. It is a cysteine protease that cleaves a peptide bond between the glutamine and a serine, alanine or glycine. It processes the coronavirus replicase polyprotein. 

3C-like Protease

- Beta-lactamase enzymes are enzymes that cleave beta-lactam antibiotics. 
- They are enzymes responsible for beta-lactam antibiotic resistance.
- Their mechanism of action against antibiotics is cleaving the beta-lactam ring in beta-lactam antibiotics which prevents it being effective. 

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