SA protein and amino acids

Protein and Amino Acids

• Proteins are large, complex molecules made up of hundreds or thousands of amino acids in a linear fashion.

• Amino acids are composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur and phosphorus.

• Only 20 amino acids are found as components of protein.

• Proteins are linear polymers of amino acids.

• The amino group of one amino acid and the carboxyl group of another are joined by a peptide bond.

• Amino acids that are bonded together are called peptides.

• Two bonded amino acids are a dipeptide, three are tripeptides, and more than three are polypeptides.

Amino Acids

Once hydrolysis occurs, simple proteins are broken down into amino acids or their derivatives.

• Proteins can bond to other molecules, which allows them to be placed into simple classification.

Examples of broken down amino acids

• Albumins: globular proteins found in egg white, blood plasma, and milk.

• Collagens: fibrous proteins found in connective tissue and convert to gelatin with heat.

Elastins:  fibrous elastic proteins found in the walls of arteries and skin.

Proteins

Conjugated proteins give rise to substances and amino acids.

• Examples:

• Glycoproteins contain carbohydrates and are found in mucus.

• Lipoproteins contain lipids and carry fat in the blood.

• Phosphoproteins contain a phosphorus group, such as casein in milk.

• Chromoproteins contain a pigment group, such as heme in hemoglobin.

Nucleoproteins combine proteins and nucleic acids, DNA and RNA.

Purpose

• Protein is required in feline and canine diets to provide amino acids.

• Amino acids are used to build, repair, and replace body proteins (tissue, and muscle.)

• Dietary protein also supplies nitrogen for the synthesis of nonessential amino acids and other compounds.

• Amino acids are either nonessential or essential.

• Nonessential amino acids can be synthesized by the animal’s body, so they are not required in the diet.

• Essential amino acids cannot be synthesized by the animal’s body, so they are required in the diet.

Functions of Protein

• Proteins are the main structural component of hair, feathers, skin, nails, tendons, ligaments, and cartilage.

• Contractile proteins such as myosin and actin regulate muscle action.

• Some proteins are enzymes used in metabolic reactions.

• Some proteins are necessary for nutrient digestion.

• Proteins make up hormones such as insulin and glucagon, which both regulate blood glucose levels.

• Proteins in the blood act as transporters, such as hemoglobin, lipoproteins, and transferrin.

• Plasma proteins help maintain acid-base balance.

Proteins are used to make immunoglobulins, which are important antibodies in the immune system.

Carnivore Protein Requirements

Obligate carnivores, cats, have the highest protein requirement.

• Cats have high protein needs due to nitrogen catabolic enzymes in the liver that are adapted to handle a high amount of dietary protein.

• Cats cannot conserve nitrogen from the body’s nitrogen pool.

They require a constant source of meat protein because there is no on-off switch for protein catabolism.

Protein Facts

• Proteins in the animal body are constantly renewing and degrading.

• Additional proteins are needed for growth and reproduction.

• A constant supply of protein and nitrogen is needed for metabolic processes, tissue repair, and growth.

• The body can make proteins as long as all required amino acids are available.

A high rate of protein synthesis occurs during:

•  red blood cell production, white blood cell production, epithelial cell production, cellular lining of the GI tract, and cellular lining of the pancreas.

Gluconeogenic:

• Most amino acids, except lysine and leucine, are glucogenic and can be used to make glucose.

• Glucogenic amino acids can be converted into glucose through gluconeogenesis.

• This process occurs in the liver.

• Gluconeogenesis is a normal process in cats.

• In dogs, it occurs during periods of catabolism, when they are not ingesting sufficient carbohydrates.

Ketogenic:

• Two amino acids are ketogenic: lysine and leucine.

• Lysine and leucine are degraded into precursors of ketone bodies.

• These ketone bodies can be used for energy when there is insufficient dietary carbohydrate or when the body cannot utilize carbohydrates, such as in diabetes mellitus.

Dietary Protein

• Does not have a specific protein requirement, but it does have specific amino acid requirements.

• These amino acids can be provided in any source as long as they are supplied.

• Absorbed amino acids, dipeptides, and tripeptides are reassembled into new proteins within the liver and other tissues.

After absorption, amino acids are used for:

• tissue synthesis, enzyme synthesis, albumin synthesis, hormone synthesis, synthesis of nitrogen-containing compounds, deamination, carbon skeletons for energy.

Stored Protein

• Structural proteins in almost every tissue, especially:

• in the muscle, liver, and serum albumin (considered stores of amino acids.)

• These stores are not like fat or carbohydrate stores because they are active and functioning.

• Using these stores to meet amino acid requirements decreases the function and abilities of the animal.

• Muscle stores are the largest storage of amino acids.

• If too much muscle storage is used, muscle impairment occurs, including in the heart and respiratory system.

Digestibility

Protein quality and digestibility by the dog or cat greatly affect how well they can use protein as a source of amino acids and nitrogen.

• Highly digestible proteins that contain all essential amino acids in required amounts are high-quality proteins.

• Low digestibility proteins are limited in essential amino acids and are lower-quality proteins.

• The higher the quality of protein consumed by the cat or dog, the less protein they will require.

Digestibility is NOT required on a label (80% DM crude protein might be seen)

Protein Quality

The chemical score compares the amino acid profile of a given protein with the amino acid profile of a reference protein of very high quality.

• Egg protein is usually used as the reference protein and is given a chemical score of 100.

• The limiting amino acid is the essential amino acid with the largest deficit.

• This amino acid can limit the amount of new proteins being synthesized because all amino acids must be present in sufficient amounts.

The three most common limiting amino acids are

methionine, tryptophan, and lysine.

Biological value i

• is the percentage of absorbed protein that is retained by the body.

• It measures the body’s ability to convert absorbed amino acids into bodily tissues.

Biological value does not account for protein digestibility.

How quality is determined

Protein quality can only be determined through feeding trials, and digestibility is not normally present on the product label.

• Within one pet food, there are often multiple protein sources to improve the amino acid profile.

• Protein complementation  is combining proteins in a diet based on their amino acid excesses and deficiencies to create a higher-quality protein profile.

• Amino acid fortification improves protein quality by adding one or more amino acids to a food that is limited in protein content.

• Methionine and lysine are usually added.

Taurine

• Taurine is an essential amino acid in cats.

• Most mammals can synthesize taurine from methionine and cysteine. (cats cannot) 

• Taurine belongs to a group of amino acids called amino-sulphonic acids.

• It is needed for bile acid conjugation to aid in fat digestion.

• It is necessary for retinal function.

• It aids in reproduction and myocardial function.

• Taurine is only found in animal tissues.

Deficiencies can be seen in animals fed high levels of plant products and grains.

• Cats on a canned food diet will have a higher need for taurine.

• The heat used during manufacturing causes an increase of indigestible proteins, which increases taurine lost in the feces.

Conclusion

• Amino acids are joined together by a peptide bond.

• Glycoproteins contain carbohydrates and are found in mucus.

• Cats require a constant supply of meat because they cannot store the nitrogen received in the diet, so protein catabolism occurs no matter how much is present in the diet.

• Highly digestible proteins are usually high-quality proteins.

• A limiting amino acid can limit the amount of new proteins that will be synthesized.