Last updated on January 5, 2019 at 18:36
Proteins and peptides
Proteins and peptides are used as signalling molecules in many systems in the body:
- Nervous system (neuropeptides, like neuropeptide Y, serotonin, MSH, ACTH)
- Endocrine system (like insulin, glucagon and somatostatin)
- Immune system (cytokines and chemo are small proteins)
- GI tract (cholecystokinin, gastrin)
- Heart (atrial natriuretic peptide)
- Adipose tissue (leptin)
Peptides and proteins vary in size from 3 to around 200 amino acid residues. We call them peptides if they’re around 50 amino acids or less, and proteins if they are longer than 50.
Endogenous peptides and proteins act on receptors, either G-protein coupled receptors or tyrosine-kinase receptors.
Peptide hormones are usually synthesized like this: from prepro-hormone -> pro-hormone -> hormone. Preprohormones are long peptides with an N-terminal signal peptide that can contain many smaller peptide hormones, or many copies of one smaller peptide hormone. When the signal peptide is cleaved off does it become a prohormone. The prohormone is then further cleaved at many places to release the smaller peptide hormones. You might know this process from how POMC works.
Synthesis of many hormones from the preprohormone pre-POMC (the one on the top, with the signal peptide).
Peptides don’t work well as drugs, for the following reasons:
- They can’t be given orally because they are hydrolysed in the gut and poorly absorbed
- They are difficult to manufacture
- They are quickly hydrolysed in the plasma (have short half-life)
- They don’t penetrate the blood-brain barrier
- They may be immunogenic (act as antigens)
The kinin system
The kinin system has three major components:
Kininogens are protein precursors that are cleaved by enzymes called kallikreins into hormones called kinins, which are biologically active and act on receptors.
There are two kininogens: High-molecular-weight kininogen (HMW kininogen) and low-molecular-weight kininogen (LMW kininogen). HMW is cleaved by kallikreins into the most famous kinin, bradykinin. LMW kininogen is cleaved into another kinin, kallidin.
The most important part of the kinin system.
To make things more complicated must kallikreins themselves be activated by another enzyme which should be familiar, factor XII, a part of the coagulation cascade. As you may or may not recall from physiology is factor XIIa activated by the intrinsic pathway of coagulation, which is activated by tissue injury.
From this can we understand that the kinin system is activated by tissue injury, and that the end result is that kinins like bradykinin and kallidin are formed. They act on receptors called B1 and B2 receptors, to elicit the following effects:
- Vasodilation in arteries, vasoconstriction in veins and large arteries, causing hyperaemia
- Increased capillary permeability
- Release histamine
- Smooth muscle
- Nervous system
- Activate nociceptors to produce pain in the skin and viscera
- Prostaglandins increase this pain-eliciting effect
- Increase renal blood flow
- Immune system
- Induce production of inflammatory cytokines by macrophages
From these effects can we understand that kinins are important in eliciting all effects of inflammation (hyperaemia, swelling, pain and so on).
There are some drugs that act on the kinin system, namely icatibant and aprotinin, but I don’t think they’re important to know.
Neuropeptides are endogenous peptides produced by sensory neurons. They exist in very low concentration in the brain.
Certain neuropeptides are also neurotransmitters (like tachykinins and endogenous opioids), although they act slightly differently than conventional neurotransmitters (like GABA, glutamate, dopamine and so on). Peptide neurotransmitters aren’t produced as quickly as conventional neurotransmitters, so they don’t function as “fast” neurotransmitters between synapses but instead as “neuro-modulators”, where neurons are more “permanently” regulated.
Tachykinins are pro-inflammatory neuropeptides. The most famous of them is substance P. Tachykinins have similar effects as bradykinin, and act on receptors called NK1, NK2 and NK3. A drug called aprepitant is an NK1-receptor antagonist and has anti-emetic effect, although I don’t think it’s very important to know.
CGRP is another pro-inflammatory neuropeptide. It’s a potent vasodilator and is involved in nociception. It binds to CGRP1 and CGRP2 receptor. No drugs targeting these receptors are in the market now, but some are researched.
Somatostatin doesn’t just exist in the GI tract but also as an anti-inflammatory neuropeptide produced by the hypothalamus. It inhibits growth hormone and TSH secretion. Somatostatin analogues are used to treat cancers that secrete any of the hormones that are inhibited by somatostatin like VIP, gastrin, glucagon, insulin and so on. They’re also used to treat acromegaly, which occurs due to too much growth hormone.
Endogenous opioids like endorphin, enkephalin and dynorphins play an important role in motivation, emotion, and stress and pain control, among other things.
Vasopressin and oxytocin regulated water reabsorption in the kidney and contract the uterus, respectively.
In case you’ve forgotten what purines are – they, together with pyrimidines, are the bases that makes up the nucleotides that make up DNA, RNA, AMP, GMP and other stuff.
However, they don’t just severe as components of other molecules, but they serve as signalling molecules as well. Adenosine, which is an adenine with a ribose attached to it, binds to its own special receptor called adenosine receptor, also known as receptor P1.
|Adenosine receptor 1 (A1)||Gi (cAMP ↓)||
|Adenosine receptor 2A (A2A)||Gs (cAMP ↑)||
|Adenosine receptor 2B (A2B)||Gs (cAMP ↑)||Mast cell activation||Adenosine||Theophylline|
The CNS stimulation you get from caffeine occurs because caffeine is an adenosine antagonist, and adenosine makes you feel tired.
Theophylline is used to induce bronchorelaxation in COPD and asthma patients. It has stimulatory cardiac side-effects.
There are some receptors that bind ADP and ATP as well:
|P2Y family||Mainly Gq (activates PLC)||ADP and ATP|
|P2Y12||Gi (cAMP ↓)||Platelet coagulation||ADP||Clopidogrel, prasugrel, ticlopidine|
|P2X family||Receptor-gated cation channels||
Platelets have secretory vesicles that contain ATP and ADP. When the platelets are activates is ATP and ADP released, which then binds to P2Y12 receptors, enhancing platelet aggregation. Clopidogrel, prasugrel and ticlopidine are P2Y12 antagonists that have antiaggregating effects.
Gasotransmitters are signalling molecules that are small gases, like NO, CO and H2S. They all diffuse freely across membranes, so they don’t have membrane receptors. Nitric oxide is the most important.
Nitric oxide, NO, is produced from arginine by the enzyme nitric oxide synthase, NO synthase or simply NOS. As you probably don’t remember from biochemistry do we have three types of NO synthase:
- Endothelial NO synthase, eNOS, which produces NO for use as a vasodilator
- Neuronal NO synthase, nNOS, which produces NO for use as a neurotransmitter
- Inducible NO synthase, iNOS, which is found in immune cells and produces NO in response to inflammation
NO activates soluble guanylyl cyclase inside cells, which produces cGMP, and important second messenger in nerves, smooth muscle, monocytes and platelets. Increased cGMP causes smooth muscle to relax, which is why it has vasodilatory effects.
The effects of NO can be summed up in a table:
|Organ/cell||Physiological role||Pathological role|
|Too much||Too little|
|Vascular smooth muscle, endothelium||Control of blood pressure||Hypotension (septic shock)||Atherosclerosis, thrombosis|
|Immune system||Defence against microbes||–||–|
|CNS||Neurotransmission, memory, appetite||Excitotoxicity||–|
NO is important therapeutically as well:
- Nitroprusside is a drug that is broken down into NO. It’s used to treat hypertension.
- Nitrates are also broken down into NO. They can be administered as soft capsules or as a sublingual spray. They quickly vasodilate the coronaries and are used to treat angina and acute myocardial infarction.
- Sildenafil, which you probably know as Viagra, increases the effect of NO. I don’t need to explain what it’s used for, do I?
- Inhaled NO is used for adult and neonatal respiratory distress syndromes.
19. Adrenergic receptor antagonists
21. Local anesthetics