All About Insulin

Insulin is an essential hormone for all vertebrates, as it modulates the concentration of glucose in the blood after food intake. It exists naturally in the body, but humans have managed to develop artificial variants.

Written and verified by el biólogo Samuel Antonio Sánchez Amador.

Last update: 10 June, 2023

Insulin is a hormone produced by the pancreas that is responsible for regulating the amount of glucose present in the blood. It’s interesting to find out that it’s a polypeptide —a molecule made up of more than 10 amino acids— that is highly conserved among vertebrates, since according to scientific sources, human and porcine insulin only differ by one amino acid.

Diabetes —a pathology characterized by excessive blood sugar levels— is increasingly the order of the day, as the World Health Organization (WHO) estimates that 1 in 11 adults suffer from it. This disease, created by the lack of insulin production or its misuse, has almost doubled its prevalence in the last 35 years.

Insulin structure

As we’ve already said, insulin is defined as a polypeptide of a hormonal nature made up of 51 amino acids, which are the basic organic molecules of proteins. In its active form, it consists of two chains. These are the following:

1. Chain A: Made up of 20 amino acids
2. Chain B: Made up of 31 amino acids

It should be noted that these chains are linked to each other by disulfide bridges (covalent bonds between two sulfur atoms). In addition, depending on the situation, insulin can have various three-dimensional shapes, as its conformation changes over time.

For example, when it’s stored in beta cells for later use, it’s in the form of a hexamer made up of six basic insulin molecules. On the other hand, its active form consists of the monomeric hormone, that is, the double chain that we have described earlier.

Insulin types

The Diabetes Education Online Foundation informs us that there are several types of insulin. The categories respond to the following classification criteria:

• Onset: How long they take to act
• Peak: When they produce the maximum impact on the body
• Duration: Temporary space of action
• Concentration
• Route of administration: If injected under the skin or given intravenously

Under normal physiological conditions, insulin is secreted in two different patterns. One occurs continuously and is known as a basal pattern. The objective of this mechanism is to maintain blood glucose concentrations in a fasting state, and accounts for 50% of the production of the hormone throughout the day.

On the other hand, we have the prandial pattern, which activates the secretion of insulin after food intake. We can distinguish two large groups of insulin according to the pattern that has led to its synthesis. Below we’ll describe these two categories in a general way.

Basal insulins

Basal insulins are considered to be those that cover the hormonal requirements between meals. In this category, we find the insulins detemir, glargine, glargine U-300, degludec, and other biosimilar analogs.

They differ among themselves in the way they’re obtained —by recombinant DNA techniques they can be synthesized from bacteria or yeast, for example—, the onset of action, maximum peak, and maximum duration. Although the ideal situation is that they have a flat and sustained action over time.

Prandial insulins

Prandial insulins are short-acting and aim to replicate the normal physiological pattern after eating foods rich in carbohydrates. For example, within this category we can find rapid insulin, which begins to act inside 30 minutes after its administration, peaking between 2 and 4 hours and with a maximum duration of 6 hours.

In another subcategory are ultrafast analogs, where we can list the variations lispro, apart, and glulisine. These molecules have been modified as far as the order of amino acids in the chains is concerned, since a greater alacrity in physiological processes is sought.

In these cases, the action begins after 5-15 minutes, its maximum peak occurs between 30 and 90 minutes and its effect doesn’t last beyond 4 hours. Therefore, these analogs to human insulin have a faster and shorter pattern of action.

It’s necessary to emphasize that there’s also a third group: premixed insulins. They provide both a basal and a prandial component in a fixed combination.

The role of insulin on glucose

Glucose is a monosaccharide made up of six carbon atoms and is the form of sugar found freely in fruits and honey. Its yield is 3.75 kilocalories per gram under normal conditions.

It should be noted that this is a carbohydrate of essential importance, as it’s the most abundant organic compound in nature in its combined form. It’s the main component of complex polymers such as starch, a carbohydrate found in grains, potatoes, dairy products, and corn.

Therefore, it isn’t surprising that glucose is directly the primary fuel for all human body tissues. The brain uses 25% of the total glucose ingested, but, because it isn’t capable of storing it efficiently, there must always be a constant and controlled supply.

This is where insulin comes into play, the anabolic hormone par excellence, which allows cells to have the necessary supply of glucose.

Mechanism of action

Insulin is synthesized in the beta cells of the pancreas —located in the islets of Langerhans— and its release depends on various factors, both exogenous and endogenous. Some of them can be the intake of proteins or carbohydrates or the concentration of growth hormones.

In these beta cells, insulin is synthesized from proinsulin, a protein chain made up of 81 amino acids. Various enzymes are responsible for cutting peptide C, a chain of 30 amino acids that separates chains A and B. This chemical reaction gives rise to the active insulin molecule, which is ready to act.

Explained in a simple and quick way, this hormone acts as a key, since by adhering to specific cell receptors it allows glucose to enter the cell by opening transporter channels.

Insulin functions

As we have been able to observe, the main function of insulin is to allow cells to uptake glucose. Through glycolysis and cellular respiration, tissues obtain energy in the form of ATP to perform their relevant functions.

We can summarize the functions of insulin in the following points:

• Stimulates gluconeogenesis: That is, it promotes the anabolic pathway that gives rise to glycogen from glucose. This is carried out in the liver, since excess glucose ingested in the diet is stored in the form of glycogen here.
• Inhibits glycogenolysis: Limits the process contrary to the previously described, that is, the catabolic pathway that breaks down glycogen into glucose. This is explained when there’s abundant glucose in the blood and no more is needed.
• Promotes glycolysis: Favors the metabolic process carried out in the cell cytoplasm, through which energy is obtained from the oxidation of glucose.
• Increases glucose transport: In skeletal muscle and adipose tissue.
• Other functions: Increases sodium retention in the kidneys, favors the synthesis of triglycerides, and stimulates protein synthesis.

The terms insulin and cellular metabolism are correlated. The glycogen reserves in the liver are of essential importance for the physiological functioning of the human being, because when we haven’t ingested food in a while, this compound gives us the necessary energy for vital processes.

Diabetes and insulin

We can’t finish this article without making a special mention of diabetes, a chronic disease that appears when the pancreas doesn’t produce enough insulin or when the body doesn’t effectively use the insulin it produces.

The World Health Organization gives us some very interesting information in this regard:

• In 2014, a total of 422 million diabetic people were estimated in the world, compared to the 108 million registered in 1980.
• Between the years 2000 and 2016, deaths from premature diabetes have increased by 5%.
• Diabetes can cause severe clinical pictures, such as blindness, kidney failure, myocardial infarction, stroke, and limb amputation.
• For all this, it’s estimated that, in 2016, diabetes was the direct cause of more than 1.6 million deaths.

Of course, this prevalence highlights the need to know exactly the mechanisms of insulin and the requirements of each diabetic patient. It should be noted that there are two types of diabetes, with the majority of adults affected being type 2, that is, their body uses insulin ineffectively.

Even so, as indicated by the National Library of Medicine of the United States, the injection of human insulin has been devised with the purpose of controlling blood glucose levels in diabetic patients. Therefore, this pathology is under control as long as the affected individual can afford the relevant treatment.

An essential hormone

As we have seen, insulin is a hormone of essential importance for the metabolic routes of the human being. Thanks to it, blood glucose levels are controlled, and this is a monosaccharide that’s undoubtedly the main fuel for all cellular processes.

In its natural form, it exists as a product of the pancreas, but human beings have also developed forms of artificial production to address diabetic pathology. Its link with metabolism shows how basic it is to regulate its use.