Protein Functions and Metabolism Flashcards
Protein Functions
Each protein has a specific function that is determined during protein synthesis. Functions include:
• 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.
Protein Functions: Growth and Maintenance
• Proteins are the building blocks of muscles, blood, skin, and most body structures.
• Bones are formed of a collagen matrix, which provides a framework for minerals to deposit on.
• Actin and myosin filaments involved in muscle contraction are also proteins.
• Collagen provides building material for ligaments, tendons, blood vessel walls, dermis ,
• Proteins are also needed for the replacement of cells, e.g. of skin, GIT cells etc.
Protein Functions: Hormones and Receptors
• Some hormones are derived from cholesterol, whilst others are made from amino acids. These include:
- Tyrosine + Iodine -> thyroid hormones.
For example: Iodine (e.g. seaweed) and tyrosine (e.g. fish) supports thyroid hormone production.
- Tyrosine -> dopamine, norepinephrine, epinephrine.
- Tryptophan -> serotonin, melatonin.
- Two polypeptide chains -> insulin.
- One polypeptide chain -> glucagon, PTH, calcitonin.
• Cell membrane proteins are receptors for hormones.
Protein Functions: Enzymes
Enzymes are proteins that speed up biological reactions. For example, the digestive enzyme ‘amylase’ digests starch.
Protein Functions: Immunoglobulins (or Antibodies)
• Immunoglobulins are proteins found in blood and bodily fluids.
• They are used by the immune system to identify and neutralise foreign materials such as bacteria and viruses.
• The most abundant antibody in the blood is IgG, whilst IgA is found in bodily secretions (e.g. tears, saliva, mucus, sweat). The first antibody to appear in response to antigen exposure is IgM, whilst allergic reactions are associated with IgE release.
Protein Functions: Transport
• Some proteins combine with other substances in the blood or within cells to provide a mode of transport. Insufficient transport of vital nutrients and products will negatively impact health.
• For example, albumin can bind to calcium, zinc and B6, as well as steroids and fatty acids. Transferrin’s bind to iron. Ceruloplasmin binds to copper. Haemoglobin transports oxygen in the blood.
• Note: Protein malnutrition, hypochlorhydria (low stomach acid)
or liver dysfunction can be the cause of low serum albumin.
Protein Functions: Buffers
• Some amino acids have side chains (R groups) that can easily pick up or let go of hydrogen ions (H+), helping to regulate the acid base balance in body fluids.
• Proteins that contain a lot of these amino acids therefore make good buffers, helping to keep the acid base balance of body fluids such as the blood.
• The amino acid histidine is the best buffer at our healthy physiological pH range (7.35-7.45). Another amino acid, cysteine can also act as a buffer within the normal human pH.
Protein Functions: Fluid Balance
• Proteins attract water. This osmotic pressure from proteins (i.e. albumin) in fluid is called ‘oncotic pressure’.
• If protein levels fall too low, water leaks out of the blood vessels and accumulates in interstitial spaces = oedema .
• Protein related causes of oedema include:
- 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 worlds poorest countries.
Protein Functions: Glycoproteins
• 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.
Protein Metabolism: Deanimation
• For individual amino acids to be used as an energy source or to be stored as fat, they need to undergo ‘deamination’.
• Deamination = removal of the nitrogen containing amino group from amino acids and occurs primarily in the liver.
• When the nitrogen group is removed from the amino acid, ammonia is formed which is very toxic. To then 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.
• 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.
Protein Metabolism: 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)
• Impairments of the liver’s Urea Cycle can lead to symptoms of ‘hyperammonaemia’. These include:
- Chronic fatigue, headache, irritability, nausea and diarrhoea, poor concentration, confusion,
intolerance of high protein foods.
• Hyperammonaemia is a serious metabolic state often associated with liver cirrhosis .
Avoiding toxins that negatively impact liver function such as drugs, alcohol, cigarettes and heavy metals is important.
Protein Metabolism: Transanimation
• This is an important step in the synthesis of some nonessential 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.
• This reaction is dependent on vitamin B6 as a coenzyme, so a B6 deficiency will impair this process. Vitamin B6 is abundant in wholegrains, green vegetables, sunflower seeds, pistachios, walnuts, bananas, lentils, avocados, meat & fish.
Protein Metabolism: Protein Turnover
• Proteins in the body are continually being made and broken down by a process known as ‘protein turnover’. For example, enzymes may be recycled in a matter of minutes.
• When body proteins break down, they free amino acids to join the general circulation; these amino acids + diet derived amino acids = ‘amino acid
• 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.
• The body will break down its own tissues to obtain essential amino acids if needed and not in the current ‘pool’.
• 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). This explains how chronic stress can increase the risk of osteoporosis.
