Protein Metabolism - Lecture notes 7 PDF

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20th November 2020

Sophie Green

Protein Metabolism Functions of protein: 1. Growth and maintenance of tissues  Cell membranes, tissues, bones, blood cells, organs  Proteins are synthesised depending on the needs of the body  Normally your body breaks down the same amount of protein it uses When might you need more protein? (When the body has broken down more protein than it creates)      

Injury Illness Surgery Pregnancy Breastfeeding Exercise

2. Biochemical reactions (formation of enzymes  Essential to life, catalyse most of the chemical reactions which occur within the body  Important for digestion, regulation of cellular energy production, synthesis of chemical substances (product) Examples of bodily functions that depend on enzymes:    

Production of energy Digestion of food Muscle contraction Blood clotting

3. Transport  Transport substances around the body and across cell membranes  Proteins can also provide a method of storage by binding to some constituents in the body (e.g., iron with ferratin)

4. Provide structure  Provide cells and tissues with strength and rigidity  Examples of these proteins are keratin, collagen and elastin  Keratin is a structural protein found in hair, skin and nails  Collagen is the most abundant protein in the body and is found in bones, ligaments and skin  Elastin is high in elasticity allowing tissues to return to their original shape after stretching or contracting  Examples include the uterus, lungs and arteries

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20th November 2020

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5. Essential for blood clotting  Some proteins have an essential role in blood clotting  Examples include prothrombin and fibrinogen  If these proteins are not synthesised (created), the outcome can be prolonged bleeding (due to lack of blood clotting)

6. Homeostasis  Acid-base balance  Fluid balance  Bolsters immune health  Formation of hormones  Needed to regulate:  Metabolic rate  Blood glucose  Blood Ca  Digestion  Stress response Protein can be used as an energy source if fat and carbohydrates are not available (secondary source). 3 Homeostasis: 1. Acid-base balance  Protein maintains the acid and base (alkaline) concentrations in our blood by acting as a buffer  E.g. – haemoglobin binds with small amounts of acid to maintain normal pH levels of blood  A constant pH is vital. Even a slight change can be deadly (metabolic acidosis)

Even though someone is having enough protein, what conditions / situations (where too much protein is broken down) could lead to metabolic acidosis?    

Intense exercise (lactic acidosis Lack of insulin through diabetes or low carbohydrate diets (keto acidosis) Kidney disease (Renal tubular acidosis) Diarrhoea (Hyperchloremic acidosis)

2. Balances fluids in the body  Proteins exert an osmotic effect which helps to maintain fluid levels HUG 2319 Nutrition and Metabolism for Health and Disease

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20th November 2020

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Albumin and globulin are proteins that help maintain fluid balance by absorbing and reducing water Lack of protein = lack of albumin and globulin = fluid is forced into spaces between the cells (swelling) = oedema = kwashiorkor Kwashiorkor – enough calories are consumed but not enough protein

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3. Builds immunity  Proteins build antibodies (composed of amino acids) which fight infection  Lack of protein = lack of antibodies = more chance for bacteria and viruses to multiply  Once your body has produced the antibodies to fight the infection, it remembers for next time, so the body never forgets how to make these antibodies (therefore your body develops immunity)  Amino acids associated with immunity – glutamine, arginine and taurine (improve gut barrier and trauma / stress)

4. Hormones  Some proteins act as chemical messengers (hormones) in the body by aiding communication between cells  They transport nutrients to the tissues and organs and bind to the protein receptors on the cell surface

Structure of amino acids and proteins: 

Proteins are formed when amino acids link together with peptide bonds to form long chains (known as polypeptide chains)

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An amine group, a central carbon with a H atom, a side chain, (known as the ‘R’ group) and a carboxyl group Figure 1 - Structure of a single amino acid (monopeptide) has 4 molecules

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Amino acids are joined together by peptide bonds, when 2 amino acids are joined by 1 polypeptide bond it is known as dipeptide. Polypeptide = a long chain consisting of several amino acids and peptide bonds If you have 1 or more polypeptide chain, then you have a protein (polypeptide – another name for protein)

Protein molecule:

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Sophie Green Proteins are large and complex molecules that consist of 4 levels of arrangement: 1. Primary Structure – just simply a long polypeptide chain 2. Secondary Structure – polypeptide chains are arranged in a spiral, pleated type sheet 3. Tertiary Structure – a 3D type structure, formed from the combination of polypeptide chains through disulphide bonds 4. Quaternary Structure – composed of several polypeptides (e.g. haemoglobin)

Amino acid classification:   

More than 300 amino acids in nature but just 20 found in protein 8 of these are essential for adults (9 for babies) – we need to eat these because the body cannot synthesize Non-essential – body can synthesize, still important but do not need to include in everyday diet

Who is at risk of amino acid deficiencies?     

Some vegans could be at risk, including those with nut allergies, soy allergy, those with a lack of knowledge of essential amino acid sources, fussy eaters Maple syrup urine disease – inherited disorder where the body cannot process some amino acids, can lead to seizures or death if untreated Developing countries – Marasmus – lack of protein, carbohydrates and minerals causing stunted growth Kwashiorkor – inadequate protein intake (fluid retention, lethargy, weakness) Phenylketonuria (PKU) – phenylalanine builds up in the body. Symptoms = tremors, shaking, eczema

Digestion of protein:  

Aim of digestion of proteins, is to break the peptide bonds holding the proteins and amino acids together The enzymes that break down proteins are called proteases (e.g. pepsin, trypsin and chymotrypsin)  Pepsin – stomach  Trypsin – small intestine or pancreas  Chymotrypsin – small intestine or pancreas

HUG 2319 Nutrition and Metabolism for Health and Disease

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Process:

Protein digestion:

Protein metabolism Main metabolic pathways:     

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Glycolysis Gluconeogenesis Glycogen metabolism (glycogenesis, glycogenolysis) Fatty acid metabolism Krebs cycle / citric acid cycle / Tricarboxylic acid cycle (TCA cycle) Oxidative phosphorylation Amino acid metabolism Urea cycle

Main aims of protein metabolism:   

Breakdown of amino acids (known as deamination – removal of the amine group from the molecule) Removal of ammonia from the body by synthesis of urea Synthesis of non-essential amino acids

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Synthesis of clotting factors for blood coagulation

Protein metabolism occurs in the liver 

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Proteins are broken down into amino acids by enzymes which are transported to the liver When protein is broken down into amino acids (deaminated) it produces nitrogen which produces ammonia (NH4) So…NH4 is produced when amino acids are ‘deaminated’ (removed of an amine group from an amino acid due to Figure 2 - protein and amino acid metabolism shown in green surplus) for energy NH4 is a waste product which is toxic The liver converts excess ammonia into urea (less toxic / harmless) via urea cycle Transported to kidneys and excreted (some excreted via sweat)

Urea Cycle Series of biochemical reactions which converts ammonia → urea 

Is excreted in urine

Urea Cycle summary:    

Degradation of amino acids occurs when there is surplus to requirements

Occurs in the liver The amino group is removed → formation of ammonia (NH4) This is converted into urea in the urea cycle → excretion in the urine 10-20g per day excreted in a healthy adult

When can ammonia build up?

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If your body cannot get rid of urea (usually through kidney or liver function) Genetic disorder (body is missing the enzymes to remove ammonia from the blood) If you enter a coma or have a seizure – you often have an ammonia test

Effects of excess ammonia: Plasma concentration (µmol / L)

Effect on body

50

Normal plasma level

80-100

Disturbance of consciousness

200

Coma, convulsions, death

Also in the liver 

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Produces heparin, which is an anticoagulant (prevents blood clots). This is often used in surgery Often used: Hepatocytes – cells found in the liver and perform most of the liver’s functions For protein digestion the hepatocytes are responsible for synthesis of plasma proteins (albumin, globulin, fibrinogen)

Synthesis of non-essential amino acids  

All of the non-essential amino acids are synthesized from intermediates of major metabolic pathways Their catabolism produces:  Pyruvate  Or one of the intermediates of the Krebs cycle

7 intermediates of the Krebs Cycle: 1. 2. 3. 4. 5. 6. 7.

Oxaloacetate Pyruvate X-ketoglutarate Fumarate Succinyl coenzyme A Acetyl coenzyme a Acetoacetate

HUG 2319 Nutrition and Metabolism for Health and Disease

Sophie Green

20th November 2020

Synthesis of clotting factors for blood coagulation

Main aims for protein metabolism:    

Breakdown of amino acids (known as deamination – removal of the amine group from the molecule) Removal of ammonia from the body by synthesis of urea Synthesis of non-essential amino acids Synthesis of clotting factors for blood coagulation

Marasmus:       

Undernourishment of children resulting in low weight Caused by lack of protein, carbohydrates and calories May also be caused by malabsorption problems More common in children than adults Common amongst children in developing countries such as Africa and parts of Asia where food is scarce Some cases have also been identified in developed countries – for example the elderly who may live alone and have difficulties preparing food and caring for themselves If exclusive breastfeeding countries for > 6 months children could be at risk, particularly if the mother is malnourished

Marasmus symptoms:        

Weight loss Diarrhoea Dehydration Poor growth Protruding ribs Sunken eyes Dry skin Brittle hair

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20th November 2020

Treatment for marasmus:            

Diet based treatment: Liquid protein based drinks Often includes dried milk powder, egg powder Any protein and calorie dense foods are helped to treat Oil is often added to increase energy content Good choices – milk, fish, nuts as these contain additional minerals that are likely to be deficient in the marasmus sufferer Medical based treatment: Intravenous (IV) feeding plan Allow food to be delivered quickly and directly into the body Used if the marasmus is severe or the patient has absorption problems However, not always possible in 3rd world countries Antibiotics or other medications will also be needed if the patient has an infection (this can be likely due to low immunity)

Kwashiorkor       

A severe form of malnutrition Caused by a severe lack of protein and vitamins Usually starts between 6 months – 4 years (after breastfeeding stops) May also be caused by malabsorption problems Found in children Found in developing countries such as Africa and parts of Asia where food is scarce In very rare cases seen in developed countries due to severe neglect, long term illness or an extremely restricted diet

Kwashiorkor Symptoms Initial symptoms:     

Fatigue Irritability Lethargy Reduced immunity Diarrhoea

Prolonged symptoms:      

Stunted growth Muscle wastage Oedema (swelling) Hair loss Protruding belly Anaemia

HUG 2319 Nutrition and Metabolism for Health and Disease

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20th November 2020

Treatment:            

Diet based treatment: Initially normally given more calories in the form of CHO and fats as these are easier to digest Once energy levels are built up slightly protein foods are introduced Also given fruit juices for vitamins Ready to use therapeutic food (RUTF) Made up of peanut butter, milk powder, oil and sugar Medical based treatment: Intravenous (IV) feeding plan Penicillin is given routinely for the first couple of weeks to treat infections Iron tablets Specially formulated rehydration solutions

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