First messenger

FIGURE 1-35 Model of how G proteins switch effectors on and off. (A) In their resting state, G proteins that consist of a, /3, and y-subunits are bound by the nucleotide guanosine diphosphate (GDP) and have no contact with receptors. (B) When a hormone or other first messenger binds to a receptor, the receptor causes the G protein to exchange GDP for the nucleotide guanosine triphosphate (GTP), which activates the G protein. (C) The G protein then dissociates, after which the GTP-bound a-subunit diffuses along the membrane and binds to an effector, activating it. The switch is "on." (D) After a few seconds, the a-subunit converts GTP to GDP, thereby inactivating itself. The a-subunit will then reassociate with the fi-y complex. [Modified with permission from the article by Linder, M. E., Gilman, A. G. (1992). G proteins. Sri. Am. July, p. 59.]

its cognate hormone. These ligand domains display little amino acid sequence homology with the ligand binding of the cognate receptors. Nevertheless, the PTP ligand-binding domain also displays a high affinity for its ligand: usually the Kd for the PTP ligand is 10-100 X lower than the Kd of the hormone's cognate receptor.

Since the hormone does not form a covalent linkage with the PTP, the interaction of the hormone (H) with PTP is governed by mass action considerations:

and the formation, k+l, and dissociation, k-lr steps for H • PTP are readily reversible. The equation for Kd is

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