13. PROTEINS Flashcards
This module covers: • Thestructure,functions, dietary sources and factors affecting the bioavailability of protein and amino acids. • The primary role of amino acids in metabolic and physiological processes.
How many different proteins are synthesised by each human cell?
It is estimated that a typical human cell is required to synthesise approx. 10,000 different proteins.
What is a potential pitfall of high protein diets, despite being prized for weight loss, increasing satiety and muscle mass gains?
High ANIMAL PROTEIN diets create acidity in the body, increase the risk of various diseases and are associated with a shortened life expectancy.
What percentage of body weight does protein account for?
About 17%
Which one of the four elements contained in proteins distinguish it from carbohydrates and fats?
Nitrogen
Why is the 3D ‘primary structure’ that a protein folds into so important?
This 3D structure is closely linked to the function of the protein so that it functions like a lock and key.
Which amino acids are required for the synthesis of glutathione?
cysteine, glutamic acid and glycine
How many amino acids are classed as ‘essential’ and what does this mean?
There are 9 amino acids that are termed ‘essential’ since humans cannot synthesise them either at all or in sufficient quantities, so they must be supplied in the diet.
Explain what a complete protein is and provide THREE examples of foods that fit this category.
Food that contains all nine essential amino acids are termed ‘complete proteins’.
- Vegan foods that are complete proteins includes quinoa,
buckwheat, pumpkin seeds, chia seeds, hemp seeds, tempeh.
- Animal foods: meat, poultry, fish, eggs and dairy (except butter).
Name all the essential amino acids
- Phenylalanine
- Valine
- Threonine
- Tryptophan
- Isoleucine
- Methionine
- Histidine
- Leucine
- Lysine
Mnemonic: PVT TIM HiLL
What does “conditionally essential” mean in terms of protein, and why is histidine sometimes considered as falling into this category?
‘Conditionally essential’ amino acids can be synthesised in the body, but become essential under certain circumstances such as acute disease / burns.
Histidine is not synthesised in adults, but can be produced by intestinal bacteria, although unclear if enough. The absence of histidine also does not impair protein synthesis, making it seemingly non-essential.
What causes proteins to be ‘denatured’ and how does this affect their function?
- Exposure to heat, variations of pH, alcohol and heavy metals such as aluminium.
- When proteins are denatured, they lose their 3D structure and hence their function. The structure effectively unravels.
TRUE OR FALSE:
Denaturation and proteolysis change the primary structure of amino acids.
FALSE.
Denaturing does not change the primary structure (the amino acid sequence) – proteolysis does that, but denaturation facilitates the process of proteolysis by making proteins more accessible to be worked on by proteolytic enzymes such as pepsin.
At which pH is pepsin optimally active?
2
RECAP:
Name 5 of the many functions proteins have in the body.
- Structure of body tissues, e.g. Collagen.
- Movement e.g. Actin and myosin fibres (in muscles).
- Carrier molecules, e.g. Haemoglobin.
- Storage molecule, e.g. Ferritin (iron).
- Fluid balance in the blood, e.g. Albumin.
- Enzymes (for reactions in the body).
- Hormones (e.g. Insulin) and cell membranes.
- Immune function, e.g. Antibodies.
- Clotting mechanisms, e.g. clotting factors.
- Alternative energy source.
Name any 5 of the above
Which protein provides building material for ligaments, tendons, blood vessel walls and dermis?
Collagen
Which amino acids are components of:
a) Thyroid hormones
b) Epinephrine, norepinephrine
c) Serotonin & melatonin
a) Tyrosine (+ iodine)
b) Tyrosine
c) Tryptophan
Which proteins, found in blood and bodily fluids are used by the immune system to identify and neutralise foreign materials such as bacteria and viruses?
Immunoglobulins
IgG: abundant throughout the body
IgA: body secretions
IgE: allergies
IgM: first response
Which proteins bind to the following to provide transport:
a) Calcium, zinc and vitamin B6
b) Iron
c) Copper
d) Oxygen
a) Albumin
b) Transferrin
c) Ceruloplasmin
d) Haemoglobin
Name two amino acids, which if contained in proteins make them good buffers, helping to keep the acid-base balance of body fluids such as the blood.
Histidine
Cysteine
What is ‘oncotic pressure’?
Proteins such as albumin attract water. This osmotic pressure from proteins in fluid is called ‘oncotic pressure’.
Name two possible protein-related causes of oedema
- Excessive protein losses due to kidney disease.
- Inadequate protein synthesis due to liver disease.
- Inadequate dietary intake of protein (malnutrition).
This explains the appearance of ascites (a distended, fluid-filled abdomen) in the world’s poorest countries.
Name three glycoproteins in the body and give their functions.
- Mucins – found in mucus and saliva; provide a protective, lubricating barrier.
- ABO blood (type) antigens.
- Hormones – incl. Luteinising Hormone (LH), Follicle Stimulating Hormone (FSH) and Thyroid Stimulating Hormone (TSH)
- Major Histocompatibility Complex – cell surface receptors involved in adaptive immunity (e.g. antigen presentation).
- Proteoglycans (a subclass of glycoprotein) are bound to glycosaminoglycans (GAGs) and found in the extracellular matrix e.g. chondroitin sulphate, found in cartilage – shock absorbs.
What is ‘deamination’?
For individual amino acids to be used as an energy source or to be stored as fat, they need to undergo ‘deamination’.
This involves the removal of the nitrogen-containing amino group from amino acids and occurs primarily in the liver.
What toxic substance forms during deamination and what process occurs that enable it to be safely excreted from the body?
During deamination, ammonia is formed which is very toxic. To convert the ammonia to a water-soluble compound, it must go through a series of transformation reactions in the liver known as the ‘urea cycle’ to ensure that it can be excreted by the kidneys.
The less toxic, water-soluble compound formed is ‘urea’, which is filtered out by the kidneys as part of urine
What is the end result of amino acids that have undergone ‘deamination’?
Following deamination (the removal of the amine group), the remaining fragments of amino acids may be used to produce glucose or ketones. These can be used as energy or to become energy storage.
Explain what happens in the urea cycle.
- Ammonia formed by deamination needs to be converted to ‘urea’ to ensure its safe removal from the body. This involves the Urea Cycle, which takes place in hepatocytes (liver cells).
- The Urea Cycle is the sole endogenous source of the amino acids arginine and citrulline, plus ornithine (an important liver support and detox agent)
What metabolic state can impairments of the liver’s Urea Cycle lead to. List three possible symptoms thereof.
‘Hyperammonaemia’.
Symptoms include:
- Chronic fatigue
- headache
- irritability
- nausea and diarrhoea
- poor concentration
- confusion
- intolerance of high protein foods.
What is the importance of ‘transamination’?
Transamination an important step in the synthesis of some non-essential amino acids. If a particular non-essential amino acid is not available, the body can make it from another.
Briefly outline how ‘transamination’ occurs
Transamination is an important step in the synthesis of some non-essential amino acids. If a particular non-essential amino acid is not available, the body can make it from another.
The amino group of an amino acid is transferred onto an enzyme. The enzyme then transfers the amino group on to a ketoacid, thus forming the new amino acid.
Which vitamin is a coenzyme for transamination? Name three food sources rich in this nutrient.
Vitamin B6 which is abundant in:
* wholegrains
* green vegetables
* sunflower seeds
* pistachios
* walnuts
* bananas
* lentils
* avocados
* meat & fish.
What is the ‘amino acid pool’?
When body proteins break down (‘protein turnover’), they free amino acids to join the general circulation. These amino acids + diet-derived amino acids = the ‘amino acid pool’.
TRUE OR FALSE:
Amino acids are readily stored in the body.
FALSE
Amino acids will be utilised or excreted – they are not stored. Essential amino acids generally have a longer half-life as they are more critical to the body in terms of supply.
(It is therefore important to have a regular supply of protein in the diet – particularly proteins containing the essential amino acids.)
Explain how chronic stress can increase the risk of osteoporosis
Stress causes protein losses in areas such as skeletal muscle, due to the catabolic actions of stress hormones (e.g. cortisol).
Chronic stress also impacts the framework of bones, as protein losses can also occur in the extracellular matrix (i.e. collagen).
Outline the process of muscle wasting that could occur from prolonged fasting or starvation
- When glucose and fatty acids supply is limited, cells are forced to use amino acids for energy production.
- Since amino acids are not stored, the only available source is from body component proteins such as those in lean tissue.
- These are then dismantled to be used for energy and in the long term cause tissue wasting.
List two life stages / circumstances during which higher protein diets or specific amino acids could be therapeutic.
- Pregnancy
- High performance athletes
- Injury recovery
Discuss:
Animal sources provide more protein than plant sources.
Meat provides more protein than non-meat sources, but requires more energy to digest than plant sources. Heavy animal protein can accumulate in the intestinal wall, impairing absorption. In addition what is often added to meat dishes reduces its benefit. e.g., a hamburger with bacon and cheese brings with it high calories, trans-fats, heterocyclic amines and nitrosamines (cancer risk factors).
Name two anti-nutrients that could affect plant protein digestibility. What could be done to reduce their impact?
Phytates and lectins.
To support the digestibility of plant protein sources, consider soaking, sprouting and fermenting which can lower anti-nutrient factors.
Compare and contrast animal with plant protein
- Protein from animals are ‘complete’ sources whilst plant sources are generally lacking in one or two amino acids.
- Animal sources contain more protein but requires more energy to digest.
- Heavy animal protein can accumulate in the intestinal wall, impairing absorption whilst plant sources contain fibre that is beneficial for digestion.
- Animal protein sources often bring with it high calories, trans-fats, heterocyclic amines and nitrosamines whilst plant sources contain fibre, prebiotics, phytonutrients and other ‘qualities’ which can make them a superior choice for everyday healthy eating.
What advice can be given for optimising protein absorption?
- Chew thoroughly and avoid drinking with meals.
-
Support stomach acid levels:
– Zinc and B6-rich foods (need for HCl production).
– Apple cider vinegar in a little water before meals.
– Bitter herbs and foods before meals (e.g. dandelion, rocket, watercress, artichoke, gentian, barberry bark, goldenseal). These also promote the release of pancreatic juice.
– Use betaine hydrochloride supplements with at least 600mg per capsule. Start with one when starting to eat, before increasing one capsule per meal up to a total of 5 max or until a feeling of warmth. If warm, cut one pill back.
List three fermentation products of protein are detrimental to health. What effects do these have in the body?
Ammonia, amines, sulphides, and N-nitroso compounds.
They exert effects including systemic toxicity, nephrotoxicity and carcinogenesis.
What is a ‘limiting’ amino acid?
If an essential amino acid is in shorter supply than the amount needed to support protein synthesis, it is called a limiting amino acid.
Which vegan foods are ‘complete proteins’?
Vegan foods that are complete proteins includes quinoa, buckwheat, pumpkin seeds, chia seeds, hemp seeds, tempeh.
Name the limiting amino acid in:
a) Beans
b) Nuts & seeds
c) Grains
d) Vegetables
e) Corn
a) Beans = Methionine
b) Nuts & seeds = Lysine
c) Grains = Lysine & threonine
d) Vegetables = Methionine
e) Corn = Tryptophan & lysine
How can all the essential amino acids be obtained on a plant-based diet?
Plant foods can be combined so that the limiting amino acid in one food is included in a food it is combined with e.g. vegetables and grains.
