Download 1 - PBL Group 14

1. Describe the major types of target proteins on which drugs act. (Refer to
Fig 3.1 Rang & Dale’s Pharmacology, p.25) kgkgkjg
The majority of protein targets for drug action on mammalian cells can be broadly
divided into 4 catagories:
Receptors
Receptors are the sensing elements in the system of chemical communications that
coordinates the function of all the different cells in the body, the chemical messengers
being the various hormones, transmitters and other mediators. Many therapeutically
useful drugs act, either as agonists or antagonists, on receptors for known endogenous
mediators. Drugs act on receptors in two main ways:
 Agonist
 Antagonist
Ion channels
Some ion channels (known as ligand-gated ion channels) incorporate a receptor and
open only when the receptor is occupied by an agonist; others are gated by different
mechanisms, voltage-gated ion channels being particularly important. In general,
drugs can affect ion channel function by interacting either with the receptor site of
ligand-gated channels, or with other parts of the channel molecule thus making them a
protein target separate from their own receptors. The interaction can be indirect,
involving a G-protein and other intermediaries, or direct, where the drug itself binds
to the channel protein and alters its function. In the simplest case, exemplified by the
action of local anaesthetics on the voltage-gated sodium channel the drug molecule
plugs the channel physically blocking ion permeation. Drugs affect ion channels in
two main ways:
 Blockers
 Modulators
Enzymes
Many drugs are targeted on enzymes. Often, the drug molecule is a substrate analogue
that acts as a competitive inhibitor of the enzyme; in other cases, the binding is
irreversible and non-competitive (e.g. aspirin , acting on cyclo-oxygenase). Drugs
may also act as false substrates, where the drug molecule undergoes chemical
transformation to form an abnormal product that subverts the normal metabolic
pathway. It should also be mentioned that drugs may require enzymic degradation to
convert them from an inactive form, the prodrug, to an active form. Furthermore, drug
toxicity often results from the enzymic conversion of the drug molecule to a reactive
metabolite. As far as the primary action of the drug is concerned, this is an unwanted
side reaction, but it is of major practical importance.
 Inhibitor
 False Substrate
 Pro-drug
Carrier molecules
The transport of ions and small organic molecules across cell membranes generally
requires a carrier protein, because the permeating molecules are often too polar (i.e.
insufficiently lipid-soluble) to penetrate lipid membranes on their own. There are
many examples of such carriers, including those responsible for the transport of
glucose and amino acids into cells, the transport of ions and many organic molecules
by the renal tubule, the transport of Na+ and Ca2+ out of cells, and the uptake of
neurotransmitter precursors (such as choline) or of neurotransmitters themselves (such
as noradrenaline, 5-hydroxytryptamine [5-HT], glutamate, and peptides) by nerve
terminals. The amine transporters belong to a well-defined structural family, distinct
from the corresponding receptors. In most cases, the transport of organic molecules is
coupled to the transport of ions (usually Na+), either in the same direction (symport)
or in the opposite direction (antiport). The carrier proteins embody a recognition site
that makes them specific for a particular permeating species, and these recognition
sites can also be targets for drugs whose effect is to block the transport system.
 Normal Transport (business as usual)
 Inhibitor
 False Substrate
2. Describe the Structure and Activity of Receptors and Ion Channels
Receptors
Receptors elicit many different types of cellular effect. Some of them are very rapid,
such as those involved in synaptic transmission, operating within milliseconds,
whereas other receptor-mediated effects, such as those produced by thyroid hormone
or various steroid hormones, occur over hours or days. Based on molecular structure
and the nature of this linkage (the transduction mechanism), we can distinguish four
receptor types:
Type 1: ligand-gated ion channels
Type 2: G-protein-coupled receptors
Type 3: kinase-linked and related receptors
Type 4: nuclear receptors.
(these receptors are discussed below under receptor-effector likages)
Ion Channels
We have discussed ligand-gated ion channels as one of the four main types of drug
receptor. There are many other types of ion channel that represent important drug
targets, even though they are not generally classified as 'receptors' because they are
not the immediate targets of fast neurotransmitters.
Ions are unable to penetrate the lipid bilayer of the cell membrane, and can get across
only with the help of membrane-spanning proteins in the form of channels or
transporters. Ion channels consist of protein molecules designed to form water-filled
pores that span the membrane to avoid the polarity of the lipid bi-layer, and can
switch between open and closed states. The rate and direction of ion movement
through the pore is governed by the electrochemical gradient for the ion in question,
which is a function of its concentration on either side of the membrane, and of the
membrane potential. Ion channels are characterised by:
 their selectivity for particular ion species, determined by the size of the pore
and the nature of its lining
 their gating properties (i.e. the nature of the stimulus that controls the
transition between open and closed states of the channel)
 their molecular architecture.
SELECTIVITY
Channels are generally either cation-selective or anion-selective. Cation-selective
channels may be selective for Na+, Ca2+ or K+, or non-selective and permeable to all
three. Anion channels are mainly permeable to Cl-, although other types also occur.
GATING
Voltage-gated channels
These channels open when the cell membrane is depolarised. They form a very
important group because they underlie the mechanism of membrane excitability. The
most important channels in this group are selective sodium, potassium or calcium
channels.
Ligand-gated channels
These are activated by binding of a chemical ligand to a site on the channel molecule.
Fast neurotransmitters, such as glutamate, acetylcholine, GABA and ATP act in this
way, binding to sites on the outside of the membrane. The vanilloid receptor TRPV1
mediates the pain-producing effect of capsaicin on sensory nerves.
Some ligand-gated channels in the plasma membrane respond to intracellular rather
than extracellular signals, the most important being the following.
 Calcium-activated potassium channels, which occur in most cells, and open,
thus hyperpolarising the cell, when [Ca2+]i increases.
 ATP-sensitive potassium channels, which open when the intracellular ATP
concentration falls because the cell is short of nutrients. These channels, which
are quite distinct from those mediating the excitatory effects of extracellular
ATP, occur in many nerve and muscle cells, and also in insulin-secreting cells.
Calcium release channels
These are present on the endoplasmic or sarcoplasmic reticulum rather than the
plasma membrane. The main ones, IP3 and ryanodine receptors are a special class of
ligand-gated calcium channels that control the release of Ca2+ from intracellular
stores.
Store-operated calcium channels
When the intracellular Ca2+ stores are depleted, channels in the plasma membrane
open to allow Ca2+ entry. The mechanism by which this linkage occurs is poorly
understood, but store-operated calcium channels (SOCs) are important in the
mechanism of action of many GPCRs that elicit Ca2+ release. The opening of SOCs
allows [Ca2+]i to remain elevated even when the stores are running low, and also
provides a route through which the stores can be replenished
3. Describe the various forms of receptor-effector linkages
p.31

Type 1: ligand-gated ion channels (Milliseconds). These are membrane
proteins with a similar structure to other ion channels, and incorporate a
ligand-binding (receptor) site, usually in the extracellular domain. Receptors
of this type control the fastest synaptic events in the nervous system, in which
a neurotransmitter acts on the postsynaptic membrane of a nerve or muscle



cell and transiently increases its permeability to particular ions. Most
excitatory neurotransmitters, such as acetylcholine at the neuromuscular
junction cause an increase in Na+ and K+ permeability. This results in a net
inward current carried mainly by Na+, which depolarizes the cell and
increases the probability that it will generate an action potential.
Type 2: G-protein-coupled receptors (Seconds) (GPCRs). They are
membrane receptors that are coupled to intracellular effector systems via a Gprotein. They constitute the largest family, and include receptors for many
hormones and slow transmitters. GPCRs function through a mechanism
involving a G-protein. The most obvious difference is that a photon, rather
than an agonist molecule, produces the response. The protein has its own
inbuilt agonist molecule which isomerises from the trans (inactive) to the cis
(active) form when it absorbs a photon.
Type 3: kinase-linked and related receptors (Hours) This is a large and
heterogeneous group of membrane receptors responding mainly to protein
mediators. They comprise an extracellular ligand-binding domain linked to an
intracellular domain by a single transmembrane helix. In many cases, the
intracellular domain is enzymic in nature (with protein kinase or guanylyl
cyclase activity). They play a major role in cell division, growth,
inflammation, tissue repair, apoptosis and immune responses.
Type 4: nuclear receptors (Hours) It is convenient to regard the entire
nuclear receptor family as ligand-activated transcription factors that transduce
signals by modifying gene transcription. They differ from other receptors
because they are not embedded in membranes but are present in the soluble
pahse of the cell. The term nuclear receptors is something of a misnomer,
because some are actually located in the cytosol and migrate to the nuclear
compartment when a ligand is present.
Analogue: A compound that resembles another in structure but is not necessarily an
isomer; analogues are often used to block enzymatic reactions by combining with
enzymes.
Enzymes: Enzymes are proteins, Enzymes are produced by living cells, Enzymes
control all chemical processes of living systems. Enzymes are unique to one substrate,
Enzymes combine with the substrate to form an enzyme substrate molecule.
Substrate: The substance acted on and changed by an enzyme; the reactant
considered to be attacked in a chemical reaction.