"Receptor-ligand interactions cell signaling and Enzyme inhibition BY DR. GHULAM ABBAS

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.

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.

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

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

Hormones two types lipid soluble water soluble

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

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

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)

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

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.

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.

ENZYME INHIBITION

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.

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

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

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

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.

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.

Competitive inhibitors

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

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

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

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.

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

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.