Protein is one of the 3 macronutrients which are required by our body to survive. Proteins are made up of long chains of small molecules called amino acids, which form the complex ‘globular’ structures we call proteins. There are many amino acids which can be found in nature, and it is thought that not all have yet been identified. However, there are only 22 which are commonly found in our diet and used by our body, 9 of which are considered to be essential. A list of amino acids and their role can be fold at the bottom of this article.
Food
Protein is found in all foods to some extent (with the exception of some highly refined foods), but it’s most abundant in meats, eggs and dairy. The quality of protein in a food is based on the ratio of amino acids in the food compared to the ratio of amino acids required by humans, and our ability to absorb them. Protein quality is scored on a scale of 0-1, and is called the Protein Digestibility Corrected Amino Acid Score (PDCAA), a score of 1 indicated a perfect amino acid ratios for humans. Generally, vegetables have a low score of between 0.2-0.5, and animal products generally score 0.7+. Very few foods such as chicken eggs have a PDCAA score of 1.
Roles in the body
The potential sequences and length that amino acids chains can form is extremely variable, and so there are a vast number of structures and shapes that can be formed. This makes proteins extremely diverse and able to fulfil a wide variety of complex roles in the body. Here are some common examples of the role of specific proteins in the body:
Enzymes – primarily made up of proteins and minerals, enzymes are needed to facilitate the majority of biological reactions. Enzymes themselves are very variable in their role with many types of enzymes being produced in our body. They are needed to neutralize radicals, break down food, destroy foreign bodies (such as pathogens) and destroy foreign cells.
Transport – Once food has been digested by enzymes into smaller and more absorbable units, they must be transported out of the digestive system. This is achieved by a different type of protein, which transports nutrients into the blood stream. These specialized proteins form channels between the cell lining of our digestive system, which allow specific nutrients to pass through. As with enzymes, there are a variety of these protein channels which exist in our intestines, each specifically designed to facilitate the transport of specific nutrients out of the digestive system (e.g sugars). Protein transport channels are not exclusively found in the digestive system, and are also essential for nerve function where they are required to uptake minerals which stimulate nerve impulses, as well as re-uptake neurotransmitters.
Signalling – Hormones and neurotransmitters are made up of proteins. There are a wide variety of both of these signalling molecules, and can vary from 2 or 3 amino acids to hundreds, with hormones generally being larger and more complex than neurotransmitters. Hormones and neurotransmitters are needed for everything from moving our body, to stimulating feelings such as hunger.
Red blood cells – Red blood cells are made up of 4 proteins bonded together with iron. Red blood cells are responsible for binding to oxygen in the lungs, and then releasing it in high demand areas throughout the body.
Immune – Protein structures found on our cell membrane allow our immune cells to recognize that they are our cells. An example of this is the blood groups, where each blood group has a different protein on their blood cells. Protein structures are also found on the cells of pathogens, and allow the immune system to distinguish pathogen cells from ‘self’ cells. If the pathogen has been encountered before, then these protein structures may be recognized by our immune system, allowing our immune system to defend the body rapidly and eradicate the pathogen more efficiently.
Amino acids
Non-essential amino acids
The body is able to form these amino acids from other amino acids. Although they are not essential, a healthy diet will still contain all amino acids. Aside from being used to make proteins amino acids play important roles in the body as single molecules.
Alanine (ala) – The most basic amino acid, and is relatively abundant in both meat and vegetables. Alanine is often thought of as the ‘template’ amino acid.
Arginine (Arg) –Arginine is particularly important in the healing process, releasing hormones and cell division. Arginine is a precursor molecule to the synthesis of nitric oxide, which is a vasodilator, and can help reduce blood pressure.
Aspartic acid (Asp) –Aside from natural protein sources, you can also obtain aspartic acid from the artificial sweetener aspartame. Aspartic acid can be converted into aspartate, which can mildly activate the NMDA receptor – a neural pathway responsible for memory.
Cysteine (Cys) – Although found in most protein sources, some diets in the western world are thought to be slightly deficient in cysteine. Cysteine has a chemical group call (thiol) attached to it, which give cysteine antioxidant properties. Cysteine can also be converted into glutathione which also a very powerful antioxidant. Cysteine is one of the two amino acids which provide the body with organic sulphur, which is important for joint health.
Glutamic acid (Glu) – Glutamic acid is used as a controversial food additive to enhance flavour in some meals in the form on monosodium glutamate (MSG). Glutamic acid is a neurotransmitter in a number of nervous pathways, and is more influential in NMDA receptor pathways than aspartic acid. As such, glutamic acid is very important for functions such as learning and memory.
Glutamine (Gln) – Glutamine is the most abundant amino acid in foods. Although it is considered non-essential, in some circumstances glutamine can be considered to be conditionally essential. This is when there is an increased demand for glutamine (such as intense exercise, or when you have an infection) in the body; glutamine is needed in the production of energy, the safe removal of ammonia from organs and as a nitrogen donor for anabolic processes. There is also some research to suggest that glutamine can aid with the recovery from injury and surgery.
Glycine (Gly) – Glycine is abundant in many sources of protein, but can also be added to foods as a sweetener. Like aspartic acid and glutamic acid, glycine is a neurotransmitter, and is primarily found in neural pathways in the spine. Glycine also acts as a strong agonist (activator) for the NMDA pathway.
Proline (Pro) – Proline has a very unique structure compared to other amino acids, and is very ridged. As a result of this, proline often fulfils an important role in maintaining protein shapes and structures – allowing complex globular shapes to form and be maintained. Proline is one of the most abundant amino acids in the collagen structure, with many bonded together to form a polyproline helix. Collagen is the elastic protein, which gives our skin its ’ping back’ properties, and it is also an important protein in connective tissues.
Serine (Ser) – Many amino acids can be easily synthesised from serine, making it important in maintaining optimum amino acid ratios in the body. This will be of particular importance for vegetarians/ vegans whose diet may provide a more unbalanced amino acid ratio than an omnivore. Serine is also an important amino acid in enzymes, and is often found in the active site (location for chemical reactions) in enzymes.
Tyrosine (Tyr) – tyrosine is found a precursor to a number of thyroid hormones and neurotransmitters, including dopamine. This makes tyrosine important in regulating our mood.
Asparagine (Asn) – Due to its structure, asparagine is often used as a terminal or ‘full-stop’ in protein chains – making it important for the majority of protein complexes.
Selenocystine (Sec) – Selenocystine is structurally similar to cysteine, with the only difference being a selenium group instead of a sulphur group – meaning selenocystine doesn’t offer a valuable source of organic sulphur. However, it does offer organic selenium, which is a vital mineral for immune cells, and so selenocysatine is often found in specialized immune cells and enzymes.
Alanine (Ala) – Alanine plays an important role in the production of energy when needed, alanine is produced in muscles and converted to energy in the liver. Alanine also plays a role in transporting toxins away from the liver.
Essential amino acids
The body is unable to synthesise these amino acids, and so they must be obtained in significant quantities from your diet.
Histine (His) – Histine is the precursor molecule to a group of compounds called histamines. These mediate allergic responses and cause inflammation at target locations. Histine is also the precursor to a molecule called carnosine, which is a small protein molecule that accumulates in the brain and muscle tissue. Carnosine is associated with improving brain function, neuroprotective properties, and may be able to prevent cancer development.
If histine is taken as a supplement it has been shown to increase zinc excretion up to 6 times the regular amount, and may cause zinc deficiency.
Isoleucine (Ile) – Isoleucine is needed for the regulation and creation of energy molecules in processes such as gluconeogenesis (creation of glucose in the body). Isoleucine can also be converted into leucine (see below), but this conversion requires biotin (vitamin B7). Without biotin, this reaction will not occur and can lead to muscle weakness and poor cognitive function.
Leucine (leu) – Although leucine can be converted from isoleucine in the body, it is still considered to be an essential amino acid. Leucine is used in the formation of sterols in fatty tissue and has been shown to stimulate protein synthesis and prevent muscle degeneration. However, there is still some controversy about its anabolic properties.
Lysine (Lys) – Lysine is a very important amino acid, and fortunately makes up a large proportion of many protein sources. Lysine is associated with protein synthesis and an increased recovery from surgery. Lysine is also required for the absorption of calcium in the digestive system.
Methionine (Met) – Methionine is also an organic source of sulphur, making it biologically valuable. Methionine is also is a donor of the methyl group, allowing it to be converted into other amino acids such as taurine and cysteine.
Interestingly, there is evidence to suggest that diets high in methionine reduce life expectancy and increase the risk of disease. These findings are more likely to be due a combination of nutrients, and not just methionine in isolation, and more research is still needed.
Phenylalanine (Phe) – Phenylalanine is a precursor molecule to dopamine and adrenaline – making it a vital nutrient for a healthy nervous system and mood regulation.
Threonine (Thr) – Threonine is associated with the production of antibodies, which make up a major part of the immune system. It is also associated with reducing the build-up of fatty deposits in the liver and fatty acid synthesis.
Tryptophan (Trp) – The primary role of tryptophan is to be converted into serotonin, which is a neurotransmitter. Tryptophan or its metabolites are thought to promote deeper and undisturbed sleep because of this conversion.
Valine (Val) – Valine is most recognised for its role in protein sysnthesis, and is thought to possess anabolic properties similar to leucine.
Summary
Proteins are made up of chains of amino acids, and are the most complex of the macronutrients – fulfilling a vast array of roles in the body. Animal sources, such as eggs, milk and meat contain higher quality protein that vegetables source.
