1. "Receptor-ligand interactions cell signaling and Enzyme inhibitionBY
DR. GHULAM ABBAS

2. No cell lives in isolation
Survival depends on an elaborate intercellular communication network that coordinates growth, differentiation and metabolism.
Cells adjacent to one another frequently communicate through cell-cell contact.
Other forms of communication cover larger distances = extracellular signaling molecules.

3. Extracellular Signalling
Signaling molecules are released by signaling cells.
The signal is called the ligand.
The ligand binds to its specific receptor on a target cell. This ligand-receptor interaction induces a conformational or shape-change in the receptor.
Produces a specific response - called the cellular response can include a vast array of compounds
e.g. small amino acid derivatives, small peptides, proteins.

4. Cell-to-cell communication by extracellular signaling usually involves six steps
(1) Synthesis of the signaling molecule by the signaling cell
(2) Release of the signaling molecule by the signaling cell
(3) Transport of the signal to the target cell
(4) Detection of the signal by a specific receptor protein – receptor-ligand specificity
(5) A change in cellular metabolism, function, or development = cellular response
triggered by the receptor-ligand complex – specific to the ligand-receptor complex
(6) Removal of the signal, which usually terminates the cellular response – degredation of ligand

5. Circulating & Local Hormones
Circulating hormones
act on distant targets
travel in blood
endocrine hormones
Local hormones
Paracrine (cell-cell communication) hormones & autocrine (a cell secretes a hormone or chemical messenger) hormones

6. Hormones
two types
lipid soluble
water soluble

7. Lipid-Soluble Hormones
Lipid-soluble hormones can easily enter a cell by diffusing through the plasma membrane.
-PROBLEM: How do they travel in the water-based blood??
-SOLUTION: They are carried by carrier-proteins
-these hormones then enter their target cell where they result in a specific cellular effect or response

8. Water-soluble Hormones
Water soluble hormones can easily travel within the blood
-PROBLEM: How do they enter a cell and result in a cellular response??
-SOLUTION: binding to specific cell-surface receptors
-this binding activates the receptor and results in a series of cellular events called the second messenger system

9. Lipid-soluble Hormones
Steroids
lipids derived from cholesterol.
different functional groups attached to core of structure provide uniqueness.
interact with specific intracellular receptors (within the cell) to turn specific genes on or off
effective for hours or days
Thyroid hormones
tyrosine ring plus attached iodines are lipid-soluble
activate enzymes involved in the catabolism of fats and glucose
help set our basal metabolic rate
Retinoids
vitamin A derivatives
have dramatic effects on proliferation and differentiation plus cellular death (i.e. apoptosis)

10. Water-soluble Hormones
Amino acid derivatives, small peptides and protein hormones
modified amino acids or amino acids put together
serotonin, melatonin, histamine, epinephrine
larger peptide hormones
insulin and glucagon
Eicosanoids
derived from arachidonic acid (fatty acid)
prostaglandins or leukotrienes
prostaglandins despite being lipidphilic – bind to cell surface receptors

11. Signal Trans. Components: 2nd Messengers
While there are a large number of extracellular receptor ligands ("first messengers"), there are relatively few small molecules used in intracellular signal transduction ("second messengers").
Among second messengers cAMP, cGMP, 1,2-diacylglycerol (DAG), and calcium are inculded. Second messengers are small molecules that diffuse rapidly through the cytoplasm to their protein targets. Another advantage of second messengers is that they facilitate amplification of an extracellular signal.

12. Ligand Agonists & Antagonists in Medicine
Synthetic analogs of receptor ligands are widely used in medicine.
Compounds called agonists mimic the function of the natural ligand by binding to the receptor and inducing the normal response.
Antagonists bind to the receptor but induce no response. Instead, they typically block binding and signaling. Examples of an epinephrine agonist (isoproterenol) and antagonist (alprenolol) are shown.
Isoproterenol binds to bronchial smooth muscle cell epinephrine receptors with 10-fold higher affinity than epinephrine, and is used to treat asthma, etc.
Alprenolol is a beta-blocker that binds to cardiac muscle cell epinephrine receptors, blocking epinephrine action and slowing heart contractions. It therefore helps treat cardiac arrhythmias and angina.

13. ENZYME INHIBITION

14. Inhibitors
Inhibitors are chemicals that reduce the rate of enzymic reactions.
The are usually specific and they work at low concentrations.
They block the enzyme but they do not usually destroy it.
Many drugs and poisons are inhibitors of enzymes in the nervous system.

15. The effect of enzyme inhibition
Irreversible inhibitors: Combine with the functional groups of the amino acids in the active site, irreversibly.
Examples: nerve gases and pesticides, containing organophosphorus, combine with serine residues in the enzyme acetylcholine esterase.
Reversible inhibitors: These can be washed out of the solution of enzyme by dialysis.
There are two categories

16. The effect of enzyme inhibition
Enzyme inhibitor complex
Reversible reaction
E + I EI
Competitive: These compete with the substrate molecules for the active site
The inhibitor’s action is proportional to its concentration
Resembles the substrate’s structure closely

17. Succinate dehydrogenase
Continue…
Fumarate + 2H++ 2e-
Succinate
Succinate dehydrogenase
CH2COOH
COOH
CH2
Malonate
CHCOOH

18. Continue…
Non-competitive: These are not influenced by the concentration of the substrate. It inhibits by binding irreversibly to the enzyme but not at the active site.
Examples
Cyanide combines with the Iron in the enzymes cytochrome oxidase
Heavy metals, Ag or Hg, combine with –SH groups.
These can be removed by using a chelating agent such as EDTA.

19. Enzyme pathways A B C D E F
Cell processes (e.g. respiration or photosynthesis) consist of series of pathways controlled by enzymes.
A B C D E F eF eD eC eA eB
Each step is controlled by a different enzyme (eA, eB, eC etc) . This is possible because of enzyme specificity.

20. Competitive inhibitors

21. The switch: Allosteric inhibition
Allosteric means “other site”
Active site E
Allosteric site

22. Switching off These enzymes have two receptor sites
Substrate
cannot fit into the active site
Inhibitor molecule
Inhibitor fits into allosteric site
These enzymes have two receptor sites
One site fits the substrate like other enzymes.
The other site fits an inhibitor molecule

23. The allosteric site the enzyme “on-off” switch
Active site E
Allosteric site empty E
Conformational change
Substrate
fits into the active site
Inhibitor molecule is present
Substrate
cannot fit into the active site
The inhibitor molecule is absent
Inhibitor fits into allosteric site

24. The enzyme’s molecular shape changes
A change in shape
When the inhibitor is present it fits into its site and there is a conformational change in the enzyme molecule
The enzyme’s molecular shape changes
The active site of the substrate changes
The substrate cannot bind with the substrate.

26. Enzyme Inhibition
Since most clinical drug therapy is based on inhibiting the activity of enzymes, analysis of enzyme inhibition kinetics is fundamental to the modern design of pharmaceuticals
Examples: The use of methotrexate in cancer chemotherapy to semi-selectively inhibit DNA synthesis of malignant cells
The use of aspirin to inhibit the synthesis of prostaglandins which are at least partly responsible for the aches and pains of arthritis
the use of sulfa drugs to inhibit the folic acid synthesis that is essential for the metabolism and growth of disease-causing bacteria.
In addition, many poisons (such as cyanide, carbon monoxide and polychlorinated biphenols (PCBs)) produce their life- threatening effects by means of enzyme inhibition

27. Examples of irreversible non competitive inhibitors
1-Alkylating agents like iodoacetamide (bind to - SH’s).
2-heavy metals (silver & mercury) bind to -SH’s.
3-cyanide inhibit mitochondrial cytochrome oxidase.
4-Fluoride inhibit enolase enzyme
5- Penicillin is an antibiotic , inhibits bacterial transpeptidase.
6-Nerve gas and organo- phosphorus on cholinesterase.
7-Aspirin as anti-platelet aggregator on cyclo-oxygenase
Inhibit prostaglandins and thromboxane synthesis.
Antimetabolites
Block one or more of the metabolic pathway involved in DNA synthesis
Used in treatment of cancer.