Protein

Protein molecules Proteins   are organic molecules composed of carbon (C), hydrogen (H) and oxygen (O) atoms. They are distinguished from fats and carbohydrates by the presence of   nitrogen (N) atoms in their chemical structure . Although many substances contain nitrogen,   proteins are the largest nitrogen source in the diet and in the body . Amino Acids Proteins are long chains made of  amino acids  as basic units. These chains are often folded or rolled up on themselves, giving a characteristic structure to each type of protein. 1.1.1 Amino Acid Structure: Here are some examples of amino acids: Have you noticed that the only difference between these molecules comes from their side chain, also called radical (R), circled in beige? This radical gives distinct physicochemical properties to each amino acid. 1.1.2 Classification of Amino Acids There are two main classes of amino acids depending on the body's ability to synthesize them or not.

1 st  Class: Essential Amino Acids Essential amino acids are amino acids that either cannot be synthesized by the body at all  or  cannot synthesized in sufficient quantity. We must therefore consume these amino acids in our diet. 9 amino acids are considered essential . 2 nd  Class: Nonessential Amino Acids Nonessential amino acids are amino acids that the body can synthesize in sufficient quantity to meet our daily needs. 11 amino acids are considered non-essential. Some non-essential amino acids may become essential under certain conditions. They are then called  conditionally essential amino acids . These amino acids are identified by an exponent 1  in the activity below. These amino acids are normally synthesized in sufficient quantity by the body. However they become essential under certain conditions in which the body cannot synthesized them in sufficient amount to meet its needs.  Some conditionally essential amino acids become essential if the diet is deficient in certain essential amino acids, some are essential at certain times of life only (ex. premature infancy) and others may become essential under metabolic abnormalities or physical stress (ex. trauma, disease, healing wounds, etc.).  Essential Amino Acids Nonessential Amino Acids Histidine (His) Isoleucine (Ile) Leucine (Leu) Lysine (Lys) Methionine (Met) Phenylanine (Phe) Threonine (Thr) Tryptophane (Trp) Alanine (Ala) Arginine (Arg) 1 Asparagine (Asn) Aspartic Acid (Asp) Cysteine (Cys) 1 Glutamic Acid (Glu) Glutamine (Gln) 1 Glycine (Gly) 1

A first peptide bond create a peptide made of of 2 amino acids, also called  dipeptide . With the addition of a third amino acid, a  tripeptide  is obtained. Many amino acids bonded together are called  polypeptides . One or more polypeptide chains folded into a three-dimensional shape is called a  protein . 1.2.2 Protein Structure During its synthesis, the protein folds back and adopts its final shape which confers its specific function. A protein is made of one or more polypeptide chains folded into a complex,  three-dimensional shape  (see Figure below). This three-dimensional pattern is determined by the order and chemical properties of the amino acids that make the polypeptide chain. The way the polypeptide chains are folding and

bending depend on the attraction or repelling of some amino acids. To learn more read the section  Protein Structure  in your textbook (page 253-256).

Scenario One evening, Melissa decides to make a rosé sauce to accompany her spaghetti. She puts cream in a saucepan and adds freshly cut tomatoes. She notices that lumps are forming in her sauce. Answer: Certain conditions can lead to alteration of the protein specific three-dimensional structure . This is the phenomenon of protein denaturation.  When proteins are denatured, the interactions between the amino acid side chains are broken but the peptide bonds remain intact.  In the case of Melissa's rosé sauce, the proteins naturally present in the cream (casein) are denatured by the acidity of tomatoes and tend to coagulate. This explains the formation of lumps. Other factors such as heat (e. g. cooking meat) and mechanical agitation (e. g. whipped egg whites ) can cause protein denaturation. 1.2.4 Protein in the Body Proteins have several crucial functions in the body. To give you some examples, here are the functions of some proteins involved in the digestion process and in the process of tissue healing and repair. The  proteins names are indicated italics  and  their functions in bold green characters. Example 1: Digestion and Metabolism  Enzymes  are chemical reaction  catalysts . Digestive enzymes are responsible for macronutrients breakdown. For exemple,  salivary amylase  splits the starch into smaller polysaccharides.  Hormones  are chemical  messengers . For example,  insulin , which is secreted by the pancreas allows the uptake of glucose by cells of the body.

Transport proteins  will carry substances between organs, in and out of cells. For example, the protein called  GLUT4  transports the glucose across the cell membranes of muscles and adipose tissue. Contractile proteins  found in muscle such as  myosin  allow the body  movement  and provide  structure  to the body. During prolonged fasting, the amino acids from the myosin may also be used by the liver to synthesize glucose and to produce  energy . Example 2: Healing and Repair of Tissues  In case of open wound injury, some proteins such as  fibrinogen  participate in  blood clotting  process. Other proteins, the  antibodies  such as  immunoglobulin  contribute to the  immune defenses  as they recognize and attack foreign substances. Moreover, other proteins will play an important role in the healing process by allowing the  formation of new tissues. Dietary source of proteins North Americans are big protein consumers. Proteins are found in many types of food. They can be used as an energy source by the body and provide 4 kilocalories per gram. Looking at the table below, what are your main observations regarding the distribution of protein sources in the North American diet? Protein sources % of Total Protein in Canadian Diet Animal Proteins  62.3%       Meats 21.4%

good amounts of protein (legumes, nuts and seeds), they are not widely consumed by North Americans and therefore contribute little to the total protein intake. Interestingly, 21,7% of the protein in the Canadian diet comes from grain products which are relatively low in protein. However, grain products are consumed in large quantities, which explains their important contribution to total protein intake. The increase in meat consumption: A Major Challenge! World meat consumption has greatly increased in recent years. This increase inevitably has negative repercussions on farming methods and the environment. Watch the first 3:00 minutes of the  video Understanding the Real Weight of Meat  to learn more about the impact of our growing meat consumption. Then answer the following questions to test your knowledge.  In addition to containing a lower amount of protein, most plants don't contain the essential amino acids in optimal proportions to support the body protein synthesis. To ensure the body gets all the amino acids it needs, it is important to combine different types of plant products. The essential amino acids don't need to be combined during the same meal but each of them needs to be present in sufficient amount everyday. An example !?:  The combination of legumes (low in methionine and cysteine, but rich in lysine and isoleucine), with grain products, (low in lysine but rich in methionine and cysteine), is a perfect example of protein

complementarity. The weaknesses of one protein are filled by the strengths of the other and vice versa. Long live to peanut butter toast! Food Isoleucine Lysine Methionine Tryptophan* Cystein Legumes    Légumineus es-Isoleucine No   Légumine uses - Lysine No   Légumineuse s - Méthionine Yes!   Légumineuses - Tryptophane No   Légumineuses - Cystéine Yes! Grain products   Grains céréaliers- Isoleucine No   Grains céréaliers -Lysine Yes!   Grains céréaliers - Méthionine No    Grains céréaliers - Tryptophane No   Grains céréaliers- Cystéine No Nuts and seeds   Noix et graines- Isoleucine Yes!   Noix et graines - Lysine Yes!   Noix et graines - Méthionine No   Noix et graines - Tryptophane No   Noix et graines - Cystéine No  Vegetabl es    Légumes- Isoleucine No    Légumes -Lysine Yes!   Légumes - Méthionine Yes!   Légumes - Tryptophane No    Légumes - Cystéine Yes! *Tryptophan can sometimes be a limiting amino acid for some legumes and grain products. Should Lucy and Lea have the same concerns? 

The quality of protein in food can be calculated using two types of methods:  chemical and biological evaluation  methods. Chemical Evaluation Methods  Among the chemical evaluation methods, the  chemical index  (or amino acid score) compares the amino acid profile of a specific food to that of a reference food with a high quality protein profile such as egg or milk casein. It is also possible to take into account the  digestibility of food  (we never absorb 100% of the nutrients we consume!). The U.S. federal government uses the method called the Protein Digestibility-Corrected Amino Acid Score (PDCAAS), which applies a digestibility coefficient to the limiting amino acid score. Biological Evaluation Methods  An example of biological method is the Protein Efficiency Ratio (PER). The Canadian federal government uses this method to measure the quality of proteins found in food products. The method consists in comparing the weight gain of a growing animal with the amount of protein consumed by the animal. It is also possible to compare the amount of nitrogen ingested with the amount of nitrogen excreted by the body in stool and urine (e. g. biological value). Comparison of the essential amino acid profile of a reference protein with a test protein.  Essential amino acids Reference protein Whole egg (mg amino acid/g protein) Test protein (mg amino acid/g protein) Amino acid reference ratio Histidine 145 160 1.1

Isoleucine 340 360 1.06 Leucine 540 515 0.95 Lysine 440 420 0.95 Methionine 355 330 0.93 Phenylalanine 580 565 0.97 Threonine 294 310 1.05 Tryptophan 106 120 1.13 Valine 410 430 1.05 Total 3210 3210 Score de : 0.93 Identify the limiting amino acids in the test protein. In other words identify the amino acids present in smaller amounts in the test protein in comparison in the reference protein (these amino acids limit the body's protein synthesis). Leucine, lysine, methionine, and phenylalanine.  Which   amino acid is the most limiting?   Methionine What is the name of the biological evaluation method that consists of measuring the weight gain of a growing animal and comparing it to the amount of protein consumed? Protein Efficiency Ratio (PER) Soya and quinoa are two plant-based foods that contain all essential amino acids in sufficient proportion. However, the amount of protein present in quinoa is lower than in soybeans and animal products. This means that you have to eat a larger serving of quinoa to get the same amount of protein that would be provided by a smaller serving of soybeans or animal products!

Small intestine  Chymotrypsinogen  Trypsinogen  Enterokinase  Trypsin  Trypsin  Peptidase  Chymotrypsin Th e proteins of the stomach wall   are not digested by pepsin. How can this phenomenon be explained? Gastric chief cells release pepsin in its inactive form, pepsinogen, therefore preventing the stomach self-digestion. Other cells in the stomach lining produce mucus to protect the stomach wall from pepsin present in the stomach lumen. Nature is well made! Food intolerance vs. Food Allergies Food intolerance and food allergy are often confused. However, they are two very different conditions. Referring to the American Academy of Allergy, Asthma & Immunology article  Food Allergy versus Food Intolerance , indicate if the statements are true or false.  1. Allergy is an inappropriate and disproportionate immune response after exposure to a food allergen .  True    False   2. Allergic reactions are immediate, i.e. 20 minutes after exposure to the allergen. Reactions to intolerance occur several hours after exposure.  o True   o False Allergic reactions can also be delayed and may occur in the 24 hours following exposure. 3. A food intolerance mainly stimulates a reaction in the digestive system, while a food allergy causes a reaction of the immune system.

 True    False   4. A very small amount of food can trigger a food allergy or intolerance and this food must be eliminated from the diet. o True   o False   This statement is true for allergies, but false for a food intolerance. In case of a food intolerance, it is not always necessary to completely eliminate the food from the diet. Reactions are usually triggered when a certain amount of the food is consumed. People affected by a food intolerance must identify their tolerance level to avoid uncomfortable symptoms. Amino Acids Metabolism Once proteins are digested and absorbed as amino acids, they can either be 1) used to synthesize new proteins or non protein molecules, 2) be stored or 3) be excreted. 1. Amino acid pool Newly digested amino acids (exogenous source) mix with amino acids from the catabolism of the body's proteins (endogenous source) that are already present in the blood. 2. Protein synthesis (anabolism) Anabolism or protein synthesis is carried out from the pool of amino acids available to the body's cells. It is therefore important that our body has all the essential and non-essential amino acids it needs at its disposal. For more information on protein synthesis in the cell, refer to section 3.1.1  ( Protein Synthesis ).

protein - 3D , read the "Protein Synthesis" section (pages 260-262) of your textbook and complete the following activity. Order Steps  1 Transcription of the genetic code for a given protein into mRNA    2 Transport of mRNA from the nucleus to the cytoplasm and then to the ribosome of the cell    3 Delivery of amino acids to the ribosome via tRNA    4 Assembly of amino acids and folding of the functional protein chain    During certain stages of life, such as in growing children and adolescents and pregnant and lactating women, protein requirements increase not only to maintain the normal renewal of endogenous proteins, but also to allow the formation of new tissues or milk production. The excretion of urea is then reduced since the body uses almost all its amino acids to synthesize new proteins. Other conditions can increase the synthesis of new tissues, such as healing after surgery, wounds healing, re-feeding after malnutrition and physical training and  muscle building . Amino acids cannot be stored just like glucose can be stored as glycogen in the liver and muscle or fatty acids can be stored as triglycerides in fat cells.  Instead, they are subjected to catabolism and then converted into glucose or fatty acids by the liver.

Nitrogen Balance Nitrogen balance is the comparison between the nitrogen (N) consumed (as proteins) and excreted (mainly as urea) by an organism.  Positive nitrogen balance The nitrogen supply must be higher than its excretion to allow the formation of new tissues or production of breast milk (N ingested > N excreted). Neutral nitrogen balance Nitrogen supply is equivalent to excretion (N ingested = N excreted). Usually in healthy people. Negative nitrogen balance The nitrogen supply is lower than its excretion. May be due to deficient protein intake or increased protein utilization (N ingested < N excreted). Positive nitrogen balance  Neutral nitrogen balance Negative nitrogen balance