G protein

Not to be confused with Protein G.

Phosducin- transducin beta-gamma complex. Beta and gamma subunits of G-protein are shown by blue and red, respectively.

Guanosine diphosphate

Guanosine triphosphate

G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell to its interior. Their activity is regulated by factors that control their ability to bind to and hydrolyze guanosine triphosphate (GTP) to guanosine diphosphate (GDP). When they are bound to GTP, they are 'on', and, when they are bound to GDP, they are 'off'. G proteins belong to the larger group of enzymes called GTPases.

There are two classes of G proteins. The first function as monomeric small GTPases (small G-proteins), while the second function as heterotrimeric G protein complexes. The latter class of complexes is made up of alpha (α), beta (β) and gamma (γ) subunits.^[1] In addition, the beta and gamma subunits can form a stable dimeric complex referred to as the beta-gamma complex
^[2].

G proteins located within the cell are activated by G protein-coupled receptors (GPCRs) that span the cell membrane.^[3] Signaling molecules bind to a domain of the GPCR located outside the cell, and an intracellular GPCR domain then in turn activates a particular G protein. Some inactive-state GPCRs have also been shown to be "pre-coupled" with G proteins.^[4] The G protein activates a cascade of further signaling events that finally results in a change in cell function. G protein-coupled receptor and G proteins working together transmit signals from many hormones, neurotransmitters, and other signaling factors.^[5] G proteins regulate metabolic enzymes, ion channels, transporter proteins, and other parts of the cell machinery, controlling transcription, motility, contractility, and secretion, which in turn regulate diverse systemic functions such as embryonic development, learning and memory, and homeostasis.^[6]

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Contents

• 
  1 History
  


• 
  2 Function
  


• 
  3 Diversity
  


• 
  4 Signaling
  
  
  
  • 
    4.1 Heterotrimeric
    
    
    
    • 
      4.1.1 Common mechanism
      
      
      
      • 
        4.1.1.1 Activation
        
      
      
      • 
        4.1.1.2 Termination
        
      
      
      
      
    
    
    • 
      4.1.2 Specific mechanisms
      
      
      
      • 
        4.1.2.1 G_αs
        
      
      
      • 
        4.1.2.2 G_αi
        
      
      
      • 
        4.1.2.3 G_αq/11
        
      
      
      • 
        4.1.2.4 G_α12/13
        
      
      
      • 
        4.1.2.5 G_β
        
      
      
      
      
    
    
    
    
  
  
  • 
    4.2 Small GTPases
    
  
  
  
  


• 
  5 Lipidation
  


• 
  6 References
  


• 
  7 External links
  

History

G proteins were discovered when Alfred G. Gilman and Martin Rodbell investigated stimulation of cells by adrenaline. They found that when adrenaline binds to a receptor, the receptor does not stimulate enzymes (inside the cell) directly. Instead, the receptor stimulates a G protein, which then stimulates an enzyme. An example is adenylate cyclase, which produces the second messenger cyclic AMP.^[7] For this discovery, they won the 1994 Nobel Prize in Physiology or Medicine.^[8]

Nobel prizes have been awarded for many aspects of signaling by G proteins and GPCRs. These include receptor antagonists, neurotransmitters, neurotransmitter reuptake, G protein-coupled receptors, G proteins, second messengers, the enzymes that trigger protein phosphorylation in response to cAMP, and consequent metabolic processes such as glycogenolysis.

Prominent examples include (in chronological order of awarding):

• The 1947 Nobel Prize in Physiology or Medicine to Carl Cori, Gerty Cori and Bernardo Houssay, for their discovery of how glycogen is broken down to glucose and resynthesized in the body, for use as a store and source of energy. Glycogenolysis is stimulated by numerous hormones and neurotransmitters including adrenaline.


• The 1970 Nobel Prize in Physiology or Medicine to Julius Axelrod, Bernard Katz and Ulf von Euler for their work on the release and reuptake of neurotransmitters.


• The 1971 Nobel Prize in Physiology or Medicine to Earl Sutherland for discovering the key role of adenylate cyclase, which produces the second messenger cyclic AMP.^[7]
  


• The 1988 Nobel Prize in Physiology or Medicine to George H. Hitchings, Sir James Black and Gertrude Elion "for their discoveries of important principles for drug treatment" targeting GPCRs.


• The 1992 Nobel Prize in Physiology or Medicine to Edwin G. Krebs and Edmond H. Fischer for describing how reversible phosphorylation works as a switch to activate proteins, and to regulate various cellular processes including glycogenolysis.^[9]
  


• The 1994 Nobel Prize in Physiology or Medicine to Alfred G. Gilman and Martin Rodbell for their discovery of "G-proteins and the role of these proteins in signal transduction in cells".^[10]
  


• The 2000 Nobel Prize in Physiology or Medicine to Eric Kandel, Arvid Carlsson and Paul Greengard, for research on neurotransmitters such as dopamine, which act via GPCRs.


• The 2004 Nobel Prize in Physiology or Medicine to Richard Axel and Linda B. Buck for their work on G protein-coupled olfactory receptors.^[11]
  


• The 2012 Nobel Prize in Chemistry to Brian Kobilka and Robert Lefkowitz for their work on GPCR function.^[12]
  

Function

G proteins are important signal transducing molecules in cells. "Malfunction of GPCR [G Protein-Coupled Receptor] signaling pathways are involved in many diseases, such as diabetes, blindness, allergies, depression, cardiovascular defects, and certain forms of cancer. It is estimated that about 30% of the modern drugs' cellular targets are GPCRs." ^[13] The human genome encodes roughly 800 ^[14]G protein-coupled receptors, which detect photons of light, hormones, growth factors, drugs, and other endogenous ligands. Approximately 150 of the GPCRs found in the human genome have still-unknown functions.

Whereas G proteins are activated by G protein-coupled receptors, they are inactivated by RGS proteins (for "Regulator of G protein signalling"). Receptors stimulate GTP binding (turning the G protein on). RGS proteins stimulate GTP hydrolysis (creating GDP, thus turning the G protein off).

Diversity

Sequence relationship among the 18 human G_α proteins.^[15]

All eukaryotes use G proteins for signaling and has evolved a large diversity of G proteins. For instance, humans encode 18 different G_α proteins, 5 G_β proteins, and 12 G_γ proteins.^[15]

Signaling

G protein can refer to two distinct families of proteins. Heterotrimeric G proteins, sometimes referred to as the "large" G proteins, are activated by G protein-coupled receptors and are made up of alpha (α), beta (β), and gamma (γ) subunits. "Small" G proteins (20-25kDa) belong to the Ras superfamily of small GTPases. These proteins are homologous to the alpha (α) subunit found in heterotrimers, but are in fact monomeric, consisting of only a single unit. However, like their larger relatives, they also bind GTP and GDP and are involved in signal transduction.

Heterotrimeric

Main article: Heterotrimeric G proteins

Different types of heterotrimeric G proteins share a common mechanism. They are activated in response to a conformational change in the GPCR, exchanging GDP for GTP, and dissociating in order to activate other proteins in a particular signal transduction pathway. The specific mechanisms, however, differ between protein types.

Common mechanism

Activation cycle of G-proteins (purple) by a G-protein-coupled receptor (GPCR, light blue) receiving a ligand (red). Ligand binding to GPCRs (2) induces a conformation change that facilitates the exchange of GDP for GTP on the α subunit of the heterotrimeric complex (3-4). Both GTP-bound Gα in the active form and the released Gβγ dimer can then go on to stimulate a number of downstream effectors (5). When the GTP on Gα is hydrolyzed to GDP (6) the original receptor is restored (1).^[16]

Receptor-activated G proteins are bound to the inner surface of the cell membrane. They consist of the G_α and the tightly associated G_βγ subunits. There are many classes of G_α subunits: G_sα (G stimulatory), G_iα (G inhibitory), G_oα (G other), G_q/11α, and G_12/13α are some examples. They behave differently in the recognition of the effector molecule, but share a similar mechanism of activation.

Activation

When a ligand activates the G protein-coupled receptor, it induces a conformational change in the receptor that allows the receptor to function as a guanine nucleotide exchange factor (GEF) that exchanges GDP for GTP – thus turning the GPCR "on". The GTP (or GDP) is bound to the G_α subunit in the traditional view of heterotrimeric GPCR activation. This exchange triggers the dissociation of the G_α subunit (which is bound to GTP) from the G_βγ dimer and the receptor as a whole. However, models which suggest molecular rearrangement, reorganization, and pre-complexing of effector molecules are beginning to be accepted.^[4][17][18] Both G_α-GTP and G_βγ can then activate different signaling cascades (or second messenger pathways) and effector proteins, while the receptor is able to activate the next G protein.^[19]

Termination

The G_α subunit will eventually hydrolyze the attached GTP to GDP by its inherent enzymatic activity, allowing it to re-associate with G_βγ and starting a new cycle. A group of proteins called Regulator of G protein signalling (RGSs), act as GTPase-activating proteins (GAPs), are specific for G_α subunits. These proteins accelerate the hydrolysis of GTP to GDP, thus terminating the transduced signal. In some cases, the effector itself may possess intrinsic GAP activity, which then can help deactivate the pathway. This is true in the case of phospholipase C-beta, which possesses GAP activity within its C-terminal region. This is an alternate form of regulation for the G_α subunit. Such G_α GAPs do not have catalytic residues (specific amino acid sequences) to activate the G_α protein. They work instead by lowering the required activation energy for the reaction to take place.^[20]

Specific mechanisms

G_αs

G_αs activates the cAMP-dependent pathway by stimulating the production of cyclic AMP (cAMP) from ATP. This is accomplished by direct stimulation of the membrane-associated enzyme adenylate cyclase. cAMP can then act as a second messenger that goes on to interact with and activate protein kinase A (PKA). PKA can phosphorylate a myriad downstream targets.

The cAMP-dependent pathway is used as a signal transduction pathway for many hormones including:

• 
  ADH – Promotes water retention by the kidneys (created by the V2 cells of the posterior pituitary)


• 
  GHRH – Stimulates the synthesis and release of GH (somatotroph cells of the anterior pituitary)


• 
  GHIH – Inhibits the synthesis and release of GH (somatotroph cells of anterior pituitary)


• 
  CRH – Stimulates the synthesis and release of ACTH (anterior pituitary)


• 
  ACTH – Stimulates the synthesis and release of cortisol (zona fasiculata of the adrenal cortex in the adrenal glands)


• 
  TSH – Stimulates the synthesis and release of a majority of T4 (thyroid gland)


• 
  LH – Stimulates follicular maturation and ovulation in women; or testosterone production and spermatogenesis in men


• 
  FSH – Stimulates follicular development in women; or spermatogenesis in men


• 
  PTH – Increases blood calcium levels. This is accomplished via the Parathyroid hormone 1 receptor (PTH1) in the kidneys and bones, or via the Parathyroid hormone 2 receptor (PTH2) in the central nervous system and brain, as well as the bones and kidneys.


• 
  Calcitonin – Decreases blood calcium levels (via the calcitonin receptor in the intestines, bones, kidneys, and brain)


• 
  Glucagon – Stimulates glycogen breakdown in the liver


• 
  hCG – Promotes cellular differentiation, and is potentially involved in apoptosis.^{[citation needed]}
  


• 
  Epinephrine – released by the adrenal medulla during the fasting state, when body is under metabolic duress. It stimulates glycogenolysis, in addition to the actions of glucagon.

G_αi

G_αi inhibits the production of cAMP from ATP.

G_αq/11

G_αq/11 stimulates the membrane-bound phospholipase C beta, which then cleaves PIP₂ (a minor membrane phosphoinositol) into two second messengers, IP3 and diacylglycerol (DAG).
The Inositol Phospholipid Dependent Pathway is used as a signal transduction pathway for many hormones including:

• ADH (Vasopressin/AVP) – Induces the synthesis and release of glucocorticoids (Zona fasciculata of adrenal cortex in kidney); Induces vasoconstriction (V1 Cells of Posterior pituitary)


• TRH – Induces the synthesis and release of TSH (Anterior pituitary)


• TSH – Induces the synthesis and release of a small amount of T4 (Thyroid Gland)


• Angiotensin II – Induces Aldosterone synthesis and release (zona glomerulosa of adrenal cortex in kidney)


• GnRH – Induces the synthesis and release of FSH and LH (Anterior Pituitary)

G_α12/13

• 
  G_α12/13 are involved in Rho family GTPase signaling (see Rho family of GTPases). This is through the RhoGEF superfamily involving the RhoGEF domain of the proteins' structures). These are involved in control of cell cytoskeleton remodeling, and thus in regulating cell migration.

G_β

• The G_βγ complexes sometimes also have active functions. Examples include coupling to and activating G protein-coupled inwardly-rectifying potassium channels.

Small GTPases

Main article: Small GTPases

Small GTPases, also known as small G-proteins, bind GTP and GDP likewise, and are involved in signal transduction. These proteins are homologous to the alpha (α) subunit found in heterotrimers, but exist as monomers. They are small (20-kDa to 25-kDa) proteins that bind to guanosine triphosphate (GTP). This family of proteins is homologous to the Ras GTPases and is also called the Ras superfamily GTPases.

Lipidation

In order to associate with the inner leaflet^{[clarification needed]} of the plasma membrane, many G proteins and small GTPases are lipidated, that is, covalently modified with lipid extensions. They may be myristolated, palmitoylated or prenylated.

References

1. 
   ^ Hurowitz EH, Melnyk JM, Chen YJ, Kouros-Mehr H, Simon MI, Shizuya H (April 2000). "Genomic characterization of the human heterotrimeric G protein alpha, beta, and gamma subunit genes". DNA Research. 7 (2): 111–20. doi:10.1093/dnares/7.2.111. PMID 10819326..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"""""""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
   


2. 
   ^ Clapham DE, Neer EJ (1997). "G protein beta gamma subunits". Annual Review of Pharmacology and Toxicology. 37: 167–203. doi:10.1146/annurev.pharmtox.37.1.167. PMID 9131251.
   


3. 
   ^ "Seven Transmembrane Receptors: Robert Lefkowitz". 2012-09-09. Retrieved 2016-07-11.
   


4. 
   ^ ^ab Qin K, Dong C, Wu G, Lambert NA (August 2011). "Inactive-state preassembly of G(q)-coupled receptors and G(q) heterotrimers". Nature Chemical Biology. 7 (10): 740–7. doi:10.1038/nchembio.642. PMC 3177959. PMID 21873996.
   


5. 
   ^ Reece J, C N (2002). Biology. San Francisco: Benjamin Cummings. ISBN 0-8053-6624-5.
   


6. 
   ^ Neves SR, Ram PT, Iyengar R (May 2002). "G protein pathways". Science. 296 (5573): 1636–9. Bibcode:2002Sci...296.1636N. doi:10.1126/science.1071550. PMID 12040175.
   


7. 
   ^ ^ab The Nobel Prize in Physiology or Medicine 1994, Illustrated Lecture.
   


8. 
   ^ Press Release: The Nobel Assembly at the Karolinska Institute decided to award the Nobel Prize in Physiology or Medicine for 1994 jointly to Alfred G. Gilman and Martin Rodbell for their discovery of "G-proteins and the role of these proteins in signal transduction in cells". 10 October 1994
   


9. 
   ^ "The Nobel Prize in Physiology or Medicine 1992 Press Release". Nobel Assembly at Karolinska Institutet. Retrieved 21 August 2013.
   


10. 
    ^ Press Release
    


11. 
    ^ "Press Release: The 2004 Nobel Prize in Physiology or Medicine". Nobelprize.org. Retrieved 8 November 2012.
    


12. 
    ^ Royal Swedish Academy of Sciences (10 October 2012). "The Nobel Prize in Chemistry 2012 Robert J. Lefkowitz, Brian K. Kobilka". Retrieved 10 October 2012.
    


13. 
    ^ Bosch DE, Siderovski DP (March 2013). "G protein signaling in the parasite Entamoeba histolytica". Experimental & Molecular Medicine. 45 (1038): e15. doi:10.1038/emm.2013.30. PMC 3641396. PMID 23519208.
    


14. 
    ^ Baltoumas FA, Theodoropoulou MC, Hamodrakas SJ (June 2013). "Interactions of the α-subunits of heterotrimeric G-proteins with GPCRs, effectors and RGS proteins: a critical review and analysis of interacting surfaces, conformational shifts, structural diversity and electrostatic potentials". Journal of Structural Biology. 182 (3): 209–18. doi:10.1016/j.jsb.2013.03.004. PMID 23523730.
    


15. 
    ^ ^ab Syrovatkina V, Alegre KO, Dey R, Huang XY (September 2016). "Regulation, Signaling, and Physiological Functions of G-Proteins". Journal of Molecular Biology. 428 (19): 3850–68. doi:10.1016/j.jmb.2016.08.002. PMC 5023507. PMID 27515397.
    


16. 
    ^ Stewart, Adele; Fisher, Rory A. (2015). Progress in Molecular Biology and Translational Science. Elsevier. pp. 1–11. doi:10.1016/bs.pmbts.2015.03.002. ISBN 9780128029381.
    


17. 
    ^ Digby GJ, Lober RM, Sethi PR, Lambert NA (November 2006). "Some G protein heterotrimers physically dissociate in living cells". Proceedings of the National Academy of Sciences of the United States of America. 103 (47): 17789–94. Bibcode:2006PNAS..10317789D. doi:10.1073/pnas.0607116103. PMC 1693825. PMID 17095603.
    


18. 
    ^ Khafizov K, Lattanzi G, Carloni P (June 2009). "G protein inactive and active forms investigated by simulation methods". Proteins. 75 (4): 919–30. doi:10.1002/prot.22303. PMID 19089952.
    


19. 
    ^ Yuen JW, Poon LS, Chan AS, Yu FW, Lo RK, Wong YH (June 2010). "Activation of STAT3 by specific Galpha subunits and multiple Gbetagamma dimers". The International Journal of Biochemistry & Cell Biology. 42 (6): 1052–9. doi:10.1016/j.biocel.2010.03.017. PMID 20348012.
    


20. 
    ^ Sprang SR, Chen Z, Du X (2007). "Structural basis of effector regulation and signal termination in heterotrimeric Galpha proteins". Advances in Protein Chemistry. Advances in Protein Chemistry. 74: 1–65. doi:10.1016/S0065-3233(07)74001-9. ISBN 978-0-12-034288-4. PMID 17854654.
    

External links

Wikimedia Commons has media related to G proteins.

• 
  GTP-Binding Proteins at the US National Library of Medicine Medical Subject Headings (MeSH)

v t e Proteins: carrier proteins
Fatty acid	FABP1 FABP2 FABP3 FABP4 FABP5 FABP6 FABP7
Hormone	peptide hormone: Follistatin Growth hormone binding protein Insulin-like growth factor binding protein IGFBP1 IGFBP2 IGFBP3 IGFBP4 IGFBP5 IGFBP6 IGFBP7 Neurophysins Neurophysin I II steroid hormone: Sex hormone binding globulin/Androgen binding protein Transcortin Thyroxine-binding globulin Transthyretin
Metal/element	calcium Calcium-binding protein Calmodulin-binding proteins copper Ceruloplasmin iron Iron-binding proteins Transferrin receptor
Vitamin	Retinol binding protein 1 2 3 4 Transcobalamin
Pigment	Plant Light-Harvesting Complex Orange Carotenoid Protein Phycobiliprotein Photosynthetic Reaction Centers Phytochrome Rhodopsin
Other	Acyl carrier protein Adaptor protein Cholesterylester transfer protein F-box protein GTP-binding protein Latent TGF-beta binding protein Major urinary proteins Membrane transport protein Odorant binding protein

v t e Intracellular signaling peptides and proteins
MAP	see MAP kinase pathway
Calcium	Intracellular calcium-sensing proteins Calcineurin Ca2+/calmodulin-dependent protein kinase
G protein	Heterotrimeric cAMP: Heterotrimeric G protein Gs/Gi Adenylate cyclase cAMP 3',5'-cyclic-AMP phosphodiesterase Protein kinase A cGMP: Transducin Gustducin Guanylate cyclase cGMP 3',5'-cyclic-GMP phosphodiesterase Protein kinase G G alpha subunit Gα GNAO1 GNAI1 GNAI2 GNAI3 GNAT1 GNAT2 GNAT3 GNAZ GNAS GNAL GNAQ GNA11 GNA12 GNA13 GNA14 GNA15/GNA16 G beta-gamma complex Gβ GNB1 GNB2 GNB3 GNB4 GNB5 Gγ GNGT1 GNGT2 GNG2 GNG3 GNG4 GNG5 GNG7 GNG8 GNG10 GNG11 GNG12 GNG13 BSCL2 G protein-coupled receptor kinase AMP-activated protein kinase Monomeric ARFs Rabs Ras HRAS KRAS NRAS Rhos Arfs Ran Rhebs Raps RGKs
Cyclin	Cyclin-dependent kinase inhibitor protein Cyclin-dependent kinase Cyclin
Lipid	Phosphoinositide phospholipase C Phospholipase C
Other protein kinase	Serine/threonine: Casein kinase 1 2 eIF-2 kinase EIF2AK3 Glycogen synthase kinase GSK1 GSK2 GSK-3 GSK3A GSK3B IκB kinase CHUK IKK2 IKBKG Interleukin-1 receptor associated kinase IRAK1 IRAK2 IRAK3 IRAK4 Lim kinase LIMK1 LIMK2 p21-activated kinases PAK1 PAK2 PAK3 PAK4 Rho-associated protein kinase ROCK1 ROCK2 Ribosomal s6 kinase RPS6KA1 Tyrosine: ZAP70 Focal adhesion protein-tyrosine kinase PTK2 PTK2B BTK both Dual-specificity kinase
Other protein phosphatase	Serine/threonine: Protein phosphatase 2 Tyrosine: protein tyrosine phosphatase: Receptor-like protein tyrosine phosphatase Sh2 domain-containing protein tyrosine phosphatase both: Dual-specificity phosphatase
Apoptosis	see apoptosis signaling pathway
GTP-binding protein regulators	see GTP-binding protein regulators
Other	Activating transcription factor 6 Signal transducing adaptor protein I-kappa B protein Mucin-4 Olfactory marker protein Phosphatidylethanolamine binding protein EDARADD PRKCSH
see also deficiencies of intracellular signaling peptides and proteins

v t e Hydrolases: acid anhydride hydrolases (EC 3.6)
3.6.1	Pyrophosphatase Inorganic Thiamine Apyrase Thiamine-triphosphatase
3.6.2	Adenylylsulfatase Phosphoadenylylsulfatase
3.6.3-4: ATPase	3.6.3 Cu++ (3.6.3.4) Menkes/ATP7A Wilson/ATP7B Ca+ (3.6.3.8) SERCA ATP2A1 ATP2A2 ATP2A3 Plasma membrane ATP2B1 ATP2B2 ATP2B3 ATP2B4 SPCA ATP2C1 ATP2C2 Na+/K+ (3.6.3.9) ATP1A1 ATP1A2 ATP1A3 ATP1A4 ATP1B1 ATP1B2 ATP1B3 ATP1B4 H+/K+ (3.6.3.10) ATP4A Other P-type ATPase ATP8B1 ATP10A ATP11B ATP12A ATP13A2 ATP13A3 3.6.4 Dynein Kinesin Myosin Katanin
3.6.5: GTPase	3.6.5.1: Heterotrimeric G protein Gαs Gαi GNAI1 GNAI2 GNAI3 Gαq/11 GNAQ GNA11 Gα12/13 GNA12 GNA13 Transducin GNAT1 GNAT2 Gustducin GNAT3 3.6.5.2: Small GTPase > Ras superfamily Rho family of GTPases: Cdc42 CDC42 TC10 TCL RhoUV RhoU RhoV Rac Rac1 2 3 RhoG RhoBTB 1 2 RhoH Rho A B C Rnd 1 2 3 RhoDF RhoF RhoD other: Ras HRAS KRAS NRAS Rab RAB23 RAB27 Arf ARF6 SAR1B ARL13B ARL6 Ran Rheb Rap RGK 3.6.5.3: Protein-synthesizing GTPase Prokaryotic IF-2 EF-Tu EF-G Eukaryotic 3.6.5.5-6: Polymerization motors Dynamin Tubulin

v t e Enzymes
Activity	Active site Binding site Catalytic triad Oxyanion hole Enzyme promiscuity Catalytically perfect enzyme Coenzyme Cofactor Enzyme catalysis
Regulation	Allosteric regulation Cooperativity Enzyme inhibitor Enzyme activator
Classification	EC number Enzyme superfamily Enzyme family List of enzymes
Kinetics	Enzyme kinetics Eadie–Hofstee diagram Hanes–Woolf plot Lineweaver–Burk plot Michaelis–Menten kinetics
Types	EC1 Oxidoreductases (list) EC2 Transferases (list) EC3 Hydrolases (list) EC4 Lyases (list) EC5 Isomerases (list) EC6 Ligases (list)

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