Chemistry 106: Drugs in Society Lecture 16: An Introduction to the Modern View of Drug Effect 5/04/18 By the end of this session, you should be able to 1. Develop a sense of scale between the individual and the cell surface receptor 2. Define receptor 3. Define endogenous ligand 4. Know the general means by which endogenous ligands (chemical messengers) interact with a cell to communicate a message 5. Define agonist and antagonist and know how these influence the communication of the endogenous ligand with its receptor 6. Know what an enzyme is, and the role of enzymes in the body 7. Know the basic role the nicotinic acetylcholine receptor plays in voluntary muscular contraction 8. Know the basic role the enzyme acetylcholine esterase plays in voluntary muscular contraction 9. Appreciate how the structure of the nicotinic acetylcholine receptor allows its use in the rapid response necessary for voluntary muscular contraction, compared to the structure and slower responses governed by the muscarinic acetylcholine receptor 10. Know that the accessible binding site on the nicotinic acetylcholine receptor allows compounds of highly varying structure - as long as they have a positively charged nitrogen - to bind to it. This is unusual amongst receptors but understandable given the nicotinic acetylcholine receptor s location and the role it must play. 11. Appreciate the fact that the same site used to kill humans and insects can be used to improve the lives of individuals suffering a debilitating auto-immune disorder 1
Before we get started, let s develop a sense of scale between the cell surface receptor and the individual Typical Human Hair Thickness 2
An Introduction to Receptors and Enzymes as Drug Targets: Neurotransmission, Acetylcholine & the Acetylcholine Receptor or Myesthenia Gravis and the Case of the Missing Receptors The neuromuscular blocking effects of curare extracts were first reported as early as 1510, when explorers of the Amazon River found natives using these plant extracts as arrow poison Crum-Brown and Frasier essentially undertake the first attempt at SAR or structure-activity relationships, by converting of a number of divergent compounds - both with regard to size and initial biological effect - to muscle relaxants when a basic nitrogen is converted into a permanently [positively] charged species. 3
Considering the above examples, we now know Morphine completely changes which receptor it preferentially binds to, from opioid receptor(s) to nicotinic acetylcholine receptor Nicotine is converted from an agonist at the nicotinic acetylcholine receptor to an antagonist Atropine is converted from an antagonist of the muscarinic acetylcholine receptor to an antagonist at the nicotinic acetylcholine receptor As we take a closer look at the neuromuscular junction, ask yourself the following questions How is the nicotinic receptor recognizing its endogenous ligand acetylcholine, and how does it convert recognition into regulation of muscle tone? How are antagonists interrupting the normal physiological process of neural transmission to muscle cells? What role does acetylcholinesterase play in the cycle of nerve signaling at acetylcholine receptors, and how does inhibition of this enzyme compare to inhibition of the nicotinic acetylcholine receptor? 4
Let s begin with a brief overview of neuromuscular transmission, appreciating the difference in distribution of various neurotransmitter receptors ACh N = nicotinic acetylcholine receptor ACh M = muscarinic acetylcholine receptor NE = alpha adrenergic receptor (NE = norepinephrine, like epinephrine or adrenaline) NE = beta adrenergic receptor D = dopamine receptor For the purposes of this discussion, notice voluntary muscle contraction is governed by binding of acetylcholine to nicotinic acetylcholine receptors 5
It is interesting to note that a number of chemical warfare agents from the biological world have helped elucidate the neuromuscular junction 6
Clearly, activation of the acetylcholine receptor is crucial to eliciting a response. Just as importantly, termination of the response must occur. Notice the arrow leading acetylcholine from the receptor on the muscle cell to the tethered acetylcholinesterase molecules capable of cleaving it to choline and acetate 7
One of the interesting findings of Crum-Brown and Frasier was the changing of atropine from a blocker of the muscarinic acetylcholine receptor to a blocker of the nicotinic acetylcholine receptor. Let s investigate First, why are these receptors named as they are? OH O O N + Acetylcholine O N + Muscarine N Nicotine N The first 2 structures should dispel the notion that molecules possessing a nitrogen with 4 bonds and a positive charge will make them antagonists of nicotinic acetylcholine receptors since they both have them and all 3 are agonists; that is, they are receptor activators. Acetylcholine activates any acetylcholine receptor Muscarine activates the muscarinic acetylcholine receptor subtype (machr) Nicotine activates the nicotinic acetylcholine receptor subtype (nachr) Clearly, we see an example of one communicating molecule acting as the endogenous ligand for more than one type of receptor. Let s look at the two acetylcholine receptors and see if differences in structure may be related to differences in function 8
The nicotinic acetylcholine receptor Key features of the nachr: Binding of 2 agonist molecules generates a conformational change which allows selective passage of sodium ions directly The binding domain for acetylcholine is on the top of the molecule. How does this fact help rationalize the binding of a variety of antagonist molecules? 9
The muscarinic acetylcholine receptor Key features of the machr: Binding of an agonist molecule generates a conformational change which allows an associated GTP binding protein (G-protein) to bind GTP, dissociate into subunits, and ultimately influence Ca +2 distribution in muscle cells The binding domain for acetylcholine is buried in the molecule o The machr mediates slower smooth muscle contraction, consistent with a harder to reach binding domain Thus acetylcholine receptor structure relates nicely to receptor function readily accessible acetylcholine binding sites lead to rapid channel opening within the receptor, influx of sodium ions and muscle contraction. If you would like further insight into muscular contraction, see the addendum at the end of these notes 10
Acetylcholinesterase Shown below is a depiction of the first transition state, which has been stabilized by the imidazole side-chain of the amino acid histidine residue (thus lowering the energy barrier to transition and speeding up the reaction) The transition state intermediate shown above provides an example of what is termed general acid catalysis; the imidazole ring of histidine donates a proton, making the enzyme behave as if an acid. The second step hydrolysis of the intermediate is critical, since it has been an area extensively exploited for human use and abuse. Namely, by slowing the hydrolysis rate by providing stable intermediates, we can turn on neural pathways governed by acetylcholine. This can be done in reversible and irreversible fashion Alternatively stated the reaction is a 2 step process Free Enzyme + acetylcholine Acetylated enzyme + choline Acetylated enzyme + H2O Free Enzyme + acetate So if acetyl group is replaced by something more stable, the enzyme is blocked for a longer period of time, raising acetylcholine levels 11
Reversible acetylcholinesterase inhibitors tend to be carbamate esters. Examples are the therapeutic agent physostigmine and the insecticide carbaryl. Irreversible acetylcholinesterase inhibitors are typified by the organophosphates, which are powerful insecticides and warfare agents. 12
For activity, those compounds which contain P=S must first be converted to P=O. The dogma is insects will make this conversion but humans don t. 13
Finally, neostigmine is a therapeutic agent commonly used to treat acute attacks of myasthenia gravis, which is a disorder brought about by the autoimmune destruction of the nicotinic acetylcholine receptor. The key features of myasthenia gravis are shown by the panels below Question: What is the rationale for the use of neostigmine to treat MG? 14
Addendum: Voluntary Muscular Contraction So the nicotinic acetylcholine receptor controls muscular contraction by allowing sodium ions to pass into the muscle cell. Then what happens? First a bit o anatomy 15
The nerve terminal motor end plate communicates with the outer wrapping of each muscle cell (fiber), the sarcolemma. It does so in coordinated fashion as motor units. 16
The sarcolemma communicates very rapidly with the sarcoplasmic reticulum where calcium is stored. Depolarizing the sarcolemma allowing sodium through depolarizes the sarcoplasmic reticulum which causes it to release calcium ions Notice how extensively the myofibrils are covered with Ca +2 containing sarcoplasmic reticulum release of calcium causes muscles to contract. Here s how: There are fixed filaments in the myofibril, the heavy filament principally myosin proteins bonded together and thin filaments actin, troponin, and tropomyosin proteins bonded together. Notice how at the base plate of each they are the same type of filaments pointed in the opposite direction. Importantly, this makes the filaments free of one another until myosin can find its binding site on actin. This is normally hidden by tropomyosin, until you guessed it Ca +2 ions bind to troponin, which changes the shape of troponin and pulls the tropomyosin out of the way so myosin can bind 17
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In the very cool muscular contraction portion of our show, the myosin heads grab and release actin binding sites as a function of ATP 1 hydrolysis, allowing the myosin to walk along the actin strands, effectively ratcheting the base plates together. Since there are many myosin heads which are offset in their grab-rotate-release cycle, there is no chance of the myosin sliding backwards. There is a pump associated with the sarcoplasmic reticulum, so when the nicotinic acetylcholine receptor closes, Ca +2 is removed, the tropomyosin blocks the myosin binding site once again and muscular contraction ceases We are always generating ATP, which is why we are always taking in oxygen (food + O2 = ATP). No more oxygen, no more ATP. No more ATP and the cycle ends with actin and myosin bound together rigor mortis (bottom figure) 1 ATP is the principal energy currency of the cell and is principally what metabolizing food is all about. If you are trying to burn up some additional energy [calories] what might you do? Work your muscles perhaps? 19