Next Lesson - Purine and Pyrimidine Metabolism
Core
Nitrogen is an important molecule within the body. It is used in DNA/RNA, neurotransmitters, and hormones. Therefore humans need a constant supply. The main dietary source of nitrogen is protein. Protein is broken down into amino acids, which are then absorbed at the intestinal wall.
Nitrogen balance compares the nitrogen entering the body (through diet) with the nitrogen leaving it. Nitrogen is lost from the skin and in waste such as urine and faeces.
Nitrogen balance can be:
- In equilibrium, intake = loss. This is a normal nitrogen balance.
- In a positive nitrogen balance: intake > loss. This occurs in growth and pregnancy as more protein is required for growth.
- In a negative nitrogen balance: intake < loss. This occurs with trauma, malnutrition, or infection, and can also be seen during short-term fasting.
In addition to being used in protein synthesis, amino acids can have special functions of their own. For example, glycine and glutamate are both amino acids and important neurotransmitters.
Amino acids are also needed for the production of other chemicals in the body:
- Tyrosine - production of noradrenaline and melanin, the pigment made by melanocytes (the pigment-producing cells of the skin).
- Cysteine - glutathione production, which is important for “mopping up” free radicals (highly reactive molecules) that could go on to cause cell damage.
- Tryptophan - producing serotonin, which is an important neurotransmitter in the brain.
- Histidine - producing histamine, an important chemical involved in the inflammatory process.
- Glutamate - production of GABA (gamma-aminobutyric acid), which is an inhibitory neurotransmitter in the brain.
- Glycine - production of haem (used to make haemoglobin), collagen and purines (the adenine and guanine bases of DNA and RNA).
There are three different types of amino acid:
- Glucogenic amino acids can be converted to glucose through gluconeogenesis, e.g. Alanine, Aspartate.
- Ketogenic amino acids can be converted into Acetyl CoA (which can be used in respiration and is a precursor of ketone bodies), e.g. Lysine, Leucine.
- Amino acids can also be both ketogenic and glucogenic, meaning they can be converted to glucose or ketone bodies, e.g. Threonine, Tyrosine, Tryptophan.
While this list does not include every amino acid of each type, it is helpful to remember one or two examples. An easy way to remember them is:
- Glucogenic begins with A - Alanine and Aspartate
- Ketogenic begins with L - Lysine and Leucine
- Those in both classes begin with T - Threonine, Tyrosine, Tryptophan
Unfortunately, not all amino acids fit this pattern, such as phenylalanine, which is both glucogenic and ketogenic (important, see later).
The body does not store amino acids as it does carbohydrate and fat. Amino acids can be liberated from proteins and used for energy, so the balance between protein synthesis and protein breakdown needs to be carefully controlled by hormones.
- Insulin ("the hormone of plenty") increases protein synthesis and decreases amino acid release. This is because high concentrations of insulin indicate that there is lots of glucose available in the blood, so no gluconeogenesis is needed. Growth hormone also promotes protein synthesis, like insulin, but unlike insulin it causes insulin resistance (it reduces the uptake of glucose into muscle in response to insulin).
- Glucocorticoids (like cortisol) increase amino acid release for gluconeogenesis and decrease protein synthesis. This is because high levels of cortisol indicate that more glucose is needed in the blood. This relates to Cushing’s Syndrome, where excess cortisol causes excessive protein breakdown, causing characteristic thin skin and purple stretch marks (striae) on the abdomen.
To be used in the production of energy, an amino acid first has to lose its amino group (the nitrogen-containing NH2 part). Only the carbon skeleton (the carbon chain) then remains. This usually happens in two linked steps:
Transamination is the first step. It moves the amino group (NH2) from an amino acid to a keto acid (a molecule with a keto group, C=O), usually alpha-ketoglutarate (a Krebs cycle intermediate), which becomes glutamate. The amino acid itself becomes a keto acid. Different amino acids use different aminotransferase enzymes. Two important ones are alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Both of these reactions are reversible. In one direction they break amino acids down to give substrates for energy production (pyruvate, or oxaloacetate, which is a Krebs cycle intermediate). In the other direction they make alanine or aspartate from pyruvate or oxaloacetate, with glutamate donating the amino group.
- ALT catalyses the movement of the amino group from alanine to alpha-ketoglutarate, producing pyruvate and glutamate.
- AST catalyses the movement of the amino group from aspartate to alpha-ketoglutarate, producing oxaloacetate and glutamate.
The second step is deamination. This usually involves glutamate, which releases its amino group as free ammonia (NH3, a toxic waste product). The ammonia is eventually turned into urea through the urea cycle.
Urea is a molecule in the body with a high nitrogen content. It is relatively unreactive at normal levels, but at the high levels seen in chronic kidney disease it can generate cyanate, a reactive chemical that can modify proteins. Urea is produced mainly in the liver, following deamination of glutamate, a step that releases ammonia. As ammonia is toxic to human cells, it is converted to urea which is a safer (water soluble) waste product which is released in the urine.
Ammonia is converted to urea during the urea cycle, a process involving five main liver enzymes. Two of these enzymes work inside the mitochondria and three work in the cytosol (the fluid inside the cell).
Diagram - The Urea Cycle with enzymes and products. You do not need to know the enzymes. You should understand that they can be up- or down-regulated (made more or less active), and how this might affect the production of urea from ammonia.
Creative commons source OpenStax College, edited by Dr. Marcus Judge [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]
Having a high protein diet (meaning there are a lot of amino acids to metabolise) means that the enzymes for the urea cycle are upregulated. The increased amount of protein can then be metabolised faster, so ammonia does not usually build up in a healthy liver.
Having a very low protein diet can lead to down-regulation of these enzymes because they are not needed to metabolise protein.
Little or no nutritional intake for an extended period of time is dangerous because it raises the risk of refeeding syndrome. This is a dangerous shift in fluids and electrolytes that can occur when nutrition is reintroduced after a period of malnutrition, particularly if feeding is restarted rapidly. NICE counts little or no nutritional intake for more than 10 days as a high-risk feature on its own. More than 5 days counts as high risk if another risk factor is also present (for example a BMI under 18.5).
When feeding restarts after a period of malnutrition, glucose triggers a surge of insulin. This drives the massive attempted synthesis of glycogen, fat and protein in cells, which draws potassium and phosphate into the cells and uses up magnesium. The result is low blood levels of potassium, magnesium and phosphate. Low phosphate (hypophosphataemia) is the hallmark feature. Raised blood ammonia (hyperammonaemia) has been described in case reports of refeeding syndrome, but the cause is not settled. One suggestion is that restarting feeding delivers a high load of nitrogen that the urea cycle, limited in the setting of severe malnutrition, cannot convert into urea fast enough. Some of the patients reported had an underlying urea cycle disorder (an inherited deficiency of a urea cycle enzyme). High ammonia is harmful because it interferes with the Krebs cycle, protein synthesis, pH and the blood-brain barrier (the tightly sealed lining of the brain's blood vessels that controls what can enter the brain). Ammonia is basic, meaning it can also affect local pH.
Refeeding syndrome can cause cardiac, pulmonary and neurological symptoms. If the low serum levels of potassium, magnesium and phosphate are severe enough, they can be fatal.
Inherited deficiencies of the enzymes of the urea cycle (urea cycle disorders) can also cause high blood ammonia, with confusion, drowsiness and coma. A low protein diet is part of long-term treatment, but it is not enough on its own. Treatment can also include vitamin and mineral supplements, essential amino acid supplements (amino acids that the body cannot make for itself) and nitrogen scavenger medicines (medicines that increase the excretion of waste nitrogen). For some people, a liver transplant is also an option. A very high ammonia level is an emergency. Protein is stopped, intravenous glucose is started and nitrogen scavenger medicines are given, and the person is transferred without delay to a specialist centre. In adults, haemodialysis or haemofiltration (techniques that filter ammonia out of the blood) is also a first-line treatment. Ideally protein is stopped for no more than 24 to 48 hours.
There are many other disorders that occur due to inadequate levels of enzymes needed to metabolise amino acids.
Phenylketonuria (PKU) is a deficiency in phenylalanine hydroxylase meaning phenylalanine cannot be converted to tyrosine. This causes phenylalanine to build up in the blood. Some of the excess is diverted into alternative metabolic pathways, including transamination, which produces phenylketones that appear in the urine. Untreated PKU can cause developmental delay, a small head (microcephaly), seizures and hypopigmentation (paler skin and hair than expected). PKU, along with other inborn errors of metabolism (inherited defects in the body’s chemical reactions), is tested for in the newborn blood spot test (formerly called the heel prick test). This test is offered to all babies in the UK, usually on day 5, and parents can choose to decline it. The test means that PKU can be found and treated early, which can prevent its long-term problems. The main treatment for phenylketonuria is a diet low in phenylalanine, together with a phenylalanine-free protein substitute (an amino acid supplement) that has tyrosine added. Some people also benefit from tetrahydrobiopterin (BH4), a cofactor that phenylalanine hydroxylase needs in order to work.
Homocystinuria is another syndrome due to enzymatic deficiency. Homocysteine (with an ‘e’) is made from methionine (an essential amino acid from the diet) and is normally broken down by the enzyme cystathionine beta-synthase. In homocystinuria this enzyme is deficient, so homocysteine (with an ‘e’) cannot be broken down properly and builds up in the blood, along with methionine. Some homocysteine is oxidised to homocystine (without an ‘e’: this is two homocysteine molecules bound together). This homocystine (without an ‘e’) is excreted in the urine. The symptoms of homocystinuria can sometimes be confused with Marfan syndrome (an inherited disorder of connective tissue). Both can cause lens dislocation (the lens of the eye moves out of position), skeletal deformities (such as a tall, slender build) and defective connective tissue. They can be told apart by other features. Homocystinuria can also cause learning difficulties, osteoporosis and an increased risk of blood clots in the vessels (thromboembolism). The lens in homocystinuria often moves downwards or towards the nose, whereas in Marfan syndrome it most often moves upwards. Marfan syndrome does not cause high blood levels of homocysteine. Homocystinuria is also included in the newborn blood spot test.
Several of these words look and sound alike, so I have included a little list for clarity:
- Cysteine - an amino acid that is made after the step blocked in homocystinuria, so its level can fall rather than rise
- Cystine - 2 x cysteine bound together with a disulphide bond (a link between two sulphur atoms). It is not the molecule that builds up in homocystinuria
- Homocysteine - made from methionine
- Homocystine - 2 x homocysteine (oxidised form)
Edited by: Dr. Ben Appleby and Dr. Marcus Judge
Reviewed by: Dr. Marcus Judge
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