Proteins

Protein-rich foods

Proteins:

Proteins are included in the class of macronutrients. They are large biomolecules and macromolecules that comprises one or more long chains of amino acid residues. The word "Protein" comes from the Greek word protos, which means "to come first." This is an appropriate name, given that proteins are a primary component of all cells throughout the body. 

Besides water, proteins form the major part of lean body tissue, totaling about 17% of body weight. Many of our body proteins are found in muscle, connective tissue, and organs, DNA, hemoglobin, antibodies, hormones, and enzymes are examples of proteins in our body.   Proteins are crucial to the regulation and maintenance of essential body functions. For example, maintenance of fluid balance, hormone and enzyme production, cell synthesis and repair, and vision each requires specific proteins.  

Structure of Proteins:

Like carbohydrates, proteins are made of the elements carbon, hydrogen, and oxygen. However, all proteins also contain the element nitrogen. Some proteins also contain the mineral sulfur. Together, these elements form various amino acids, which serve as the building block for protein synthesis. 

Amino Acids:

The amino acids needed to make body proteins are supplied by the protein-containing foods we eat and through cell synthesis. Each amino acid is composed of a central carbon bonded to 4 groups of elements : a nitrogen (an amino) group, an acid (carboxyl) group, hydrogen, and a side chain (often signified  by the letter R). The basic or "generic", model of an amino acid and the structures of 2 amino acids, glycine, and alanine.

The side chain makes each amino acid unique and determines the structure, function, and name of the amino acid. Some amino acids have chemically similar side chains. These related amino acids form special classes, such as acidic amino acids, basic amino acids, and branched-chain amino acids. For example, the acidic amino acids lose a hydrogen in reactions and become positively charged. This allows them to participate in different enzymatic reactions in the body.

The body needs 20 different amino acids to function. Although all amino acids are needed for life, 11 of them do not need to be obtained from the diet. They are classified as nonessential amino acids because our bodies make them, using other amino acids we consume. The 9 amino acids the body cannot make are known as nutritionally essential amino acids because they must be obtained from foods. Essential amino acids cannot be synthesized in the body because body cells cannot make the carbon skeleton of the amino acid, cannot attach an amino group to the carbon skeleton, or cannot do the whole process fast enough to meet the body's needs.

Complementary Proteins:

When 2 or more plant protein are combined to compensate for deficiencies in essential amino acid content in each protein, the proteins are called complementary proteins. When complementary protein sources are combined, the amino acids in 1 source can make up for the limiting amino acid in the other sources to yield a high quality protein for the diet. Mixed diets generally provide high quality protein because these diets often contain complementary proteins. Complementary proteins need not to be consumed at the same meal but can be balanced over the course of a day to provide a sufficient supply of amino acids for body cells. For nonvegetarians. adding a small amount of animal protein to a plant-based dish (e.g, pizza with cheese or spaghetti with meatballs) is a way of providing adequate essential amino acids.

Synthesis of  Proteins:

Within body cells, amino acids can be linked together by a chemical bond, called a peptide bond, to form needed proteins. Peptide bonds form between the amino group of  1 amino acid and the acid (carboxyl) group of another. Through peptide bonding of amino acids, cells can synthesize dipeptides (joining of 2 amino acids), tripeptides (joining of 3 amino acids), oligopeptides (joining of 4 to 9 amino acids), and polypeptides (joining of 10 or more amino acids). Most proteins are polypeptides, ranging from approximately 50 to 2000 amino acids. The body can synthesize many different proteins by joining different combinations of amino acids with peptide bonds. 

1. The DNA unwinds from its super-coiled state.

2. Unwinding allows the DNA code for the amino acid sequence to be transcribed into a complementary messenger RNA (mRNA).

3. The DNA stays in the nucleus and the mRNA travels to the cytosol.

4. Here the ribosomes read the codons on the mRNA and translate the instructions to produce a specific protein. The summary of protein synthesis shows steps 4 in more detail;

A. Protein synthesis begins at a specific starting point, indicated by AUG. The initiation complex forms when the ribosomal subunits and the first tRNA molecule locks into a strand of mRNA.

 B. Transfer RNA units bring amino acids to the ribosomes as needed during protein synthesis. The tRNA carriers have a complementary code to the mRNA__ such that, if the amino acid arginine is needed during synthesis, the AGA on the mRNA would correspond to UCU on the tRNA. Numerous tRNA carriers are present during protein synthesis to continually supply the ribosomes with needed amino acids. ATP is used to supply the energy needed to activate the tRNA in order to form each new peptide bond.

C. Protein synthesis continues by adding 1 amino acid at a time to the growing polypeptide chain until a specific ending codon is reached or when a needed amino acid is not available.

D. The polypeptide is then released from the ribosome when it encounters the ending codon (codon; specific sequence of 3 nucleotide units within DNA that codes particular amino acids needed for protein synthesis) .

Protein Organization:

Four different levels of structure are found in proteins. The primary structure of a protein is the linear sequence of amino acids in the polypeptide chain. Amino acids must be accurately positioned in order for the amino acids to interact and fold correctly into the intended shape for the protein. This, in turn, allows weaker chemical bonds to form between amino acids near each other and stabilizes the structure. This creates a spiral-like or pleated sheet shape called the secondary structure. The unique 3-dimensional folding of a protein, called tertiary structure, determines the protein's overall shape and physiological function. Thus, if a protein fails to form the appropriate configuration, it cannot function. In some cases, 2 or more separate polypeptides interact to form a large, new protein, with quaternary structure. 

Denaturation of  Proteins: 

Exposure to acid or alkaline solutions, enzymes, heat or agitation can change a protein's structure, leaving it in a denatured state. Alteration of a protein's 3-dimensional structure is called denaturation. Although denaturation does not affect the protein's primary structure, unraveling' a protein's shape often destroys its normal biological function.

  Sometimes the denaturation of protein is beneficial. For example, the secretion of hydrochloric acid in the stomach during digestion denatures food proteins, which increases their exposure to digestive enzymes and aids in the breakdown of polypeptide chains. The heat produced during cooking also can denature proteins, making them safer to eat. However, denaturation also can be harmful to physiological function and overall health. During illness, changes in gastrointestinal acidity, body temperature, or body pH can cause essential proteins to denature and lose their function. 

Adaptation of Protein Synthesis to Changing Conditions:

Most viral body proteins are in a constant state of breakdown, rebuilding, and repair. This process, called protein turnover; allows cells to adapt to changing circumstances. For example, when we eat more protein than necessary for health, the liver makes more enzymes to process the waste product from the resulting amino acid metabolism__ namely, ammonia_ into urea. Overall, protein turnover is a process by which a cell can respond to its changing environment by increasing the production of needed proteins while reducing the production of proteins not currently needed.

Food Sources of  Protein:

Some food sources of proteins include:

Lean meat _ beef, mutton

Poultry _ chicken, eggs

Fish and seafood

Dairy products _ milk, yogurt and cheese

All Nuts and seeds

Legumes and beans - all beans, lentils, chickpeas

Some grain and cereals-based products are also sources of protein, but generally not as high in protein as meat and meat-alternative products.

Biological value: 

The biological value of a protein is a measure of how efficiently the absorbed food protein is converted into body tissue protein. If a food possesses adequate amounts of all 9 essential amino acids, it should allow a person to efficiently incorporate amino acids from food protein into body protein.

The concept of biological value has clinical importance whenever protein intake must be limited. This is because it is important that the small amount of protein consumed be used efficiently by the body. For example, protein intake during liver disease and kidney disease may need to be controlled to lessen the effects of the disease. In these cases, most of the protein consumed should be of high biological value, such as eggs, milk, and meat.

Protein Digestion : 

For some foods, the first step in protein breakdown takes place during cooking. Cooking unfolds (denatures) proteins and softens the tough connective tissues in meat. This can make many protein rich foods easier to chew and aids in breakdown during digestion and absorption in the GI tact.

Digestion Steps:

1. Stomach:

Protein is partially digested by enzyme pepsin (a major enzyme protein produced by the stomach, begins to break the long polypeptide chains into shorter chains of amino acids through hydrolysis reactions) and hydrochloric acid.

2. Pancreas:

Further protein digestion by enzymes released by the pancreas into the small intestine.

3. Small intestine:

Final digestion of protein to amino acids occurs in the small intestine.

4. Liver:

Amino acids are absorbed into the portal vein and transported to the liver. From there they enter the general bloodstream.

5. Large intestine: 

Little dietary protein is present in feces.

Functions of Proteins:

Protein function in many crucial ways in metabolism and in the formation of essential compounds and structures;

Maintaining Fluid Balance:

The blood proteins albumin and globulin are important in maintaining fluid balance between the blood and the surrounding tissue space. Normal blood pressure in the arteries forces blood into capillary beds (minute blood vessels; they are 1 cell thick, that create a junction between arterial and venous circulation ). Role of protein in maintaining fluid balance,

 A. Blood proteins help draw fluid forced into interstitial spaces by blood pressure back into the capillary bed.

B. Without sufficient protein in the bloodstream, edema develops because the counteracting force to blood pressure provided by blood protein declines. Fluid then remains in the interstitial spaces between cells.

C. Examples of feet with edema, in some cases, applying pressure to the swollen area causes an indentation that persists after the release of the pressure 

Producing Vital Body Structures:

One of the primary functions of protein is to provide provide support to body cells and tissues. The key structural proteins constitute more than a third of body protein and provide a matrix for muscle, connective tissue, and bone. During periods of growth, new proteins are synthesized to support the development of vital body tissues and structures. In periods of malnutrition or disease, body proteins are often broken down to supply energy.

Act as Buffers:

Proteins play an important role in regulating acid-base balance and body pH. For example, proteins located in cell membranes pump chemical ions into and out of cells. The ion concentrations that result from the pumping action help keep the body slightly alkaline. In this way, proteins act as buffers__ compounds that help maintain acid-base balance within a narrow range. Proteins are especially good buffers for the body because they have negative charges, which attract positively charged hydrogen ions. This allows them to accept and release hydrogen ions as needed to prevent detrimental changes in pH.

Contributing to Immune Function:

Antibody protein are a key component of the immune system. Antibodies bind to foreign proteins that invade the body and prevent their attack on target cells. In a normal, healthy and individual, antibodies are very efficient in combating these antigens to prevent infection and disease. However, without sufficient dietary protein, the immune system lacks the material needed to build this defense. Thus, immune incompetence (called anergy) develops and reduces the body's ability to fight infection. Anergy can turn measles into a fatal disease for a malnourished child. It also increases the risk of illness and infection in protein-deficient adults.

Transporting Nutrients:

Many proteins functions as transporters for other nutrients, carrying them through the bloodstream to cells and across cell membranes to sites of action. For example, the protein hemoglobin carries oxygen from the lungs to cells. Lipoproteins transport large lipid molecules from the small intestine through the lymph and blood to body cells. Some vitamins and minerals also have specific protein carriers that aid in their transport into and out of tissues and storage proteins. Examples include retinol-binding protein.

Forming Hormones, Enzymes & Neurotransmitters:

Amino acids are required for the synthesis of most hormones in the body. Some hormones, such as the thyroid hormones, are made from only 1 amino acid, whereas others, such as insulin, are composed of many amino acids. Hormones act as messengers in the body and aid in regulatory functions, such as controlling the metabolic rate and the amount of glucose taken up from the bloodstream. Amino acids also required for the synthesis of enzymes. Cells contain thousands of enzymes that facilitate chemical reactions fundamental to metabolism. Many neurotransmitters, released by nerve endings, also are derivatives of amino acids. This is true for dopamine, norepinephrine, and serotonin .

Forming Glucose:

The body must maintain a fairly constant concentration of blood glucose to supply energy, especially for red blood cells, brain cells, and other nervous cells that rely almost on glucose for energy. If carbohydrate intake is inadequate to maintain blood glucose levels the liver (and kidneys, to a lesser extent) is forced to make glucose from the amino acids present in body tissues. This process is called gluconeogenesis.

  Making glucose from amino acids is a normal backup system the body utilizes to supply needed glucose. For example, when you skip breakfast and have not eaten since 7pm. the preceding evening, glucose must be synthesized from amino acids. However, when this occurs chronically, as in starvation, the conversion of amino acids into glucose results in the development of widespread muscle wasting in the body (called cachexia).

Providing Energy: 

Proteins supply very little energy for healthy individuals. Under most conditions, body cells use primarily fats and carbohydrates for energy. Although proteins and carbohydrates contain the same amount of usable energy__ on average, 4kcal/g_ proteins are a very costly source of energy, considering the amount of metabolism aid processing the liver and kidneys and perform to use this energy source.