The Biochemistry Behind Bees

by Will Hartridge

(Most of the article is based on a certain species of bee: the Western honey bee- Apis mellifera.)

Bees are, in my opinion, fascinating insects with a complex social caste system and the ability to perform a multitude of intriguing functions. There is a lot of science behind the many activities bees are involved in and I will be discussing areas like the synthesis and composition of honey, how queen bees arise but are genetically identical to other female bees and the mechanism of action of bee venom. Because bees are such interesting insects there is also more areas that are also worth reading about but I will not be discussing, for example, the mathematical structure of beehives and pheromones: a way for bees to communicate with each other through the air.

To begin, I will discuss honey: the sweet, golden liquid bees are known for.

The anatomy of a worker bee
(proboscis and honey stomach can be seen)

It all starts with floral nectar- a substance secreted by plants to attract pollinators. It is around 80% water and main constituents are the monosaccharides glucose & fructose (so-called “simple sugars”) and the disaccharide sucrose (a sugar composed of 2 monosaccharides bonded together- in this case, glucose and fructose). It also contains extra compounds that serve to attract pollinators like amino acids, carbohydrates and VOCs (organic compounds that evaporate at low temperatures, therefore easily become gases) that differ depending on what species is the pollinator of that plant. When a bee lands on a flower, it collects this floral nectar using its proboscis and stores it in its “honey stomach”- a part of the digestive tract located in the pre-digestive region. Now the chemical conversion begins. While collecting the nectar, worker bees have already added invertase to it; an enzyme secreted by the salivary glands that hydrolyses sucrose into the monosaccharides glucose and fructose.

The conversion of nectar into honey

Honey
Then when the bee is back in the hive, this mixture is regurgitated, transferred and ingested between multiple bees over a period of around 20 minutes until the final quality of honey is reached and stored in the honeycombs. During this process, even more enzymes are added: amylase that catalyses the hydrolysis of amylose (a part of starch) into glucose for easier digestion, glucose oxidase that catalyses the oxidation of glucose also producing hydrogen peroxide in the process (thought to contribute to honey’s antibacterial properties) and catalase that ensures the hydrogen peroxide concentration isn’t too high by converting it into oxygen and water. Finally, bees in the hive constantly flap their wings to maintain a temperature of roughly 35°C and to circulate air- this results in further evaporation of water from the honey and the heat prevents crystallisation.

Bee venom

A diagram showing the importance of phospholipids in cell membranes

The bee sting: a last-minute suicidal defensive response to protect the rest of the hive. When the barbed stingers of female worker bees puncture the skin a venom is pumped in. Due to its barbed structure, this stinger gets stuck in the skin, causing the bee to die as body parts are held back by the stinger and ripped off the bee. To ensure maximum pain for the sting recipient, the stingers and venom sac have the ability to continue pumping venom into the victim for around 10 minutes after injection. But why does this venom cause pain and what is its mechanism of action? Honey bee venom (apitoxin) has 63 components including but not limited to: enzymes, peptides, amino acids and histamine. Phospholipase A2 is the second most abundant and most destructuctive component of apitoxin making up 10-12%. It is an enzyme that breaks down phospholipids: the key components of cell membranes, resulting in lysis of cells- degradation of cell membranes leading to cell destruction. This reaction is shown on the diagram- it releases a fatty acid and a lysophospholipid.

The reaction catalysed by Phospholipase A2 (PLA₂)

Melittin is the main constituent, making up 40-60% of the dry mass of apitoxin, it is a small and basic polypeptide chain composed of only 26 amino acids. On an interesting but unrelated note; it is also one of the smallest polypeptides that has the ability to fold spontaneously (into 2 alpha-helices) without being exposed to a polar medium, for example water. It causes pain by the direct or indirect activation of nociceptor cells (pain receptors) via a complex series of events.

The structure of the melittin protein (2 melittin proteins shown here)

To summarise the mechanism of direct action:

Melittin activates the enzyme phspholipase A2 to catalyse the same reaction as shown before (see previous diagram)
The released fatty acid (arachidonic acid- AA) is then catalysed by more enzymes (lipoxygenases & cyclooxygenases)
This produces compounds that have an effect on TRPV1: a receptor protein channel present in nociceptors that is responsible for the sensation of pain.

The direct mechanism of action of melittin to cause pain

I will not go into the details of the indirect mechanism of action as it is more complex.

Queen bees

Larvae being fed royal jelly

The queen bee performs a critical role in beehives, laying up to 1500 eggs per day they are usually the mothers to every bee in the hive as they are the only females with fully developed ovaries. Given their abilities and behaviour, you would think that they are very different to regular female worker bees. However, they both are born as genetically identical larvae and it is the presence of the aptly named substance “royal jelly” (a mixture of proteins, sugars and lipids) that transforms it into a queen bee.

How royal jelly does this is to do with the field of science called epigenetics: the study of changes in gene expression (turning genes “on” or “off”). This works by a process called DNA methylation: where a methyl group (CH3) is bound to a base in DNA (typically a CpG site- a cytosine base followed directly by a guanine base) that causes gene repression proteins to be recruited or inhibits transcription factors- effectively “turning off” the gene.

A diagram showing the process of DNA methylation

It was discovered that over 550 queen bee genes displayed significant methylation differences to those of the regular female worker bee, with most occurring in genes that provide critical cellular functions. This change in gene expression is also found in the brains of the queen bees- explaining their behavioural difference. Methylation does not actually change the underlying base sequence of DNA- hence why queen bees are genetically identical to female worker bees.

A study has been carried out where a key enzyme in the methylation process (DNA methyltransferase Dnmt3) was silenced in bee larvae, causing them to develop into queen bees with fully developed ovaries- the same as bees that were fed on royal jelly, proving that the way in which royal jelly acts is epigenetic. However, it is not yet clear what component of royal jelly causes these effects, some have been suggested but none proven.

I hope this has provided an insight into some of the science behind the many interesting actions bees are capable of.

Bibliography:

Honey:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2802787/

https://www.beeculture.com/the-chemistry-of-honey/

http://www.chm.bris.ac.uk/webprojects2001/loveridge/index.html

https://www.thoughtco.com/how-do-bees-make-honey-1968084

Apitoxin:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5563768/#!po=14.0000

https://www.sciencedirect.com/topics/neuroscience/melittin

Queen bees:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3521964/