Properties of a Ligand

Ligands show the following properties:

• Affinity—strength of binding with the receptor

• Competition—ability of different ligands to bind to the same receptor

• Agonist activity—ability of a ligand to trigger the cell response

• Antagonist activity—ability of a ligand to bind to a receptor without triggering the cell response, i.e. to block the receptor

• Half life—time taken for ligand to be metabolized to half its concentration

• Lipid solubility—lipid-insoluble ligands activate receptors at the membrane surface. Lipid soluble ligands activate intracellular or intranuclear receptors.

Receptors

"Receptor' refers to the region of a protein molecule that provides a binding site for a ligand. Receptors are usually situated on integral membrane proteins, and are activated by lipid-insoluble messengers at the membrane surface. Lipid-soluble messengers may cross the membrane to activate intracellular receptors. Receptors are not fixed components of the membrane, but are free to change position and to alter in population density.

Receptor Properties

The properties of receptors are:

• Specificity—selectivity of a receptor determining how specifically it bind to a single ligand

• Saturation—percentage of receptors already occupied by ligand

• Down regulation—a decrease in the number of receptors available for a given ligand

• Up regulation—an increase in the number of available receptors for a given ligand

• Sensitivity—responsiveness of a target cell to a given ligand dependent on the density of receptors

TYPES OF FIRST CHEMICAL MESSENGER AND THEIR TARGET CELLS

Messenger

Origin

Route

Target cell

Neurotransmitters: noradrenaline acetylcholine serotonin

Neurone

Synapse

Adjacent neurone

Hormones: thyroxine insulin Cortisol

Gland

Circulation

Multiple tissues

Neurohormones:

vasopressin oxytocin

ACTH

TSH

Neurone

Circulation

Multiple tissues Endocrine glands

Neuromuscular transmitter: acetylcholine

Neurone

Circulation

Muscle cell

Paracrine agents:

eicosanoids cytokines

Local cell

Extracellular fluid

Neighbouring cells

Autocrine agent:

eicosanoids cytokines

Local cell

Extracellular fluid

Cell of origin

Figure PG.24

Figure PG.24

• Supersensitivity—increased sensitivity of a cell as a result of up regulation Binding Site Modulation

The first stage in transduction is the production of a change in shape or modulation of the binding site, when the ligand binds to the receptor. There are two main mechanisms by which this occurs, allosteric modulation and covalent modulation.

Allosteric Modulation

In this case, the receptor possesses two binding sites one site, the functional site for the ligand and the other (regulatory site) for a modulator molecule. Binding of the modulator molecule enables ligand binding to occur at the functional site (Figure PG.25).

Covalent Modulation

This form of modulation requires the attachment of a phosphate group to the receptor (phosphorylation) to enable the functional site to bind the ligand (Figure PG.25).

Membrane Signal Transduction

When a ligand binds to a receptor, modulation sets off a sequence of events into which can be thought of as transduction of a chemical signal received by the cell. Signal transduction ultimately results in alterations in cell function. These changes can affect:

• Membrane permeability

• Membrane potential

• Membrane transport

• Contractile activity

Covalent Modulation
Figure PG.25 Allosteric and covalent modulation

• Secretory activity

• Protein synthesis

Different mechanisms are activated following modulation of the receptor protein. Membrane Signal Transduction Mechanisms

Receptors act as ion channels that are opened or closed by ligand binding (Figure PG.26).

Receptors function as protein kinases that are activated on ligand binding (Figure PG.26).

Receptors activate G proteins, which then mediate further actions. These include gating ion channels or releasing second messengers intracellularly (Figure PG.27).

Roeeplor ion d'-onnel exfitiie'lu or- fly (J

cell me.Tibione

-v recefjfor-

cyloplosnn J

Figure PG.26 Signal transduction mechanisms

Vintiie oil^li/

Figure PG.26 Signal transduction mechanisms

Ion enamel

FiriraielMor Hj H

itKLiiitor oO motion

G protein dill ■lioi'ibii.inc v Cytoplasm

G proti'n octj^tion of □denylyl cydpii

Exlmcellulor fluid

G proti'n octj^tion of □denylyl cydpii

Exlmcellulor fluid

Cjc

nyly □in

O-l nieir-brori

|

Figure PG.27 G protein signal transduction

CAMP

Figure PG.27 G protein signal transduction

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