Protein Structure and Metabolism

April 24, 2011

Metabolism, Proteins

Proteins are necessary for building the structural components of the human body, such as muscles and organs. You also need proteins to keep your immune system healthy, synthesize neurotransmitters, create and signal hormones, and much more. A balanced diet supplies you will all of the protein you need. Meats, eggs, and dairy products are significant sources of protein, but you can also get protein from a variety of grains, legumes, nuts, and seeds.

Just a Tiny Bit of Protein Chemistry

Proteins tend to be large molecules made up of several building blocks called amino acids. The general structure of any amino acid molecule includes a carboxyl group of atoms, an amine group and a side chain. The carboxyl group contains one carbon, two oxygen, and one hydrogen atom. The amine group contains one nitrogen atom with two hydrogen atoms attached to it.All 20 amino acids have different side chains, which vary in shape including straight chains of atoms, branched chains of atoms and rings of atoms. The side chains may include carbon, hydrogen, sulfur, nitrogen and oxygen atoms. The configuration and molecules found in the side chain is what differentiates one amino acid from another. The branched-chain amino acids are isoleucine, leucine, and valine. These amino acids are necessary for muscle structure. Tyrosine, phenylalanine and tryptophan are called aromatic amino acids. Each one contains a side chain with a ring-shaped formation. These three amino acids are needed for neurotransmitter production.

There are 20 different amino acids. Amino acids are linked together to form peptides, which are small chains of amino acids. The peptides are then linked together to form larger proteins.

There are thousands of different proteins that carry out a large number of jobs in the human body. Even though so many different proteins are at work in your body, you don’t have to worry about consuming each individual protein from the foods you eat. Your body will make those proteins. All you need to do is to make sure your body has a healthy supply of all 20 of the different amino acid “building blocks.” Having enough of those amino acids is easy because your body can make 11 of them from other compounds already in your body. That leaves eight amino acids that you must get from your diet. Those eight amino acids are called “essential amino acids.”

Non Essential and Essential Amino Acids

The 11 non-essential amino acids are not called “non-essential” because they are not important. They are important and your body requires them to perform several functions. These amino acids are called “non-essential” because you don’t need to get them from your diet. Your body can build those 11 amino acids from chemicals already present in your body. The non-essential amino acids include:

  • Alanine
  • Arginine
  • Asparagine
  • Aspartic Acid
  • Cysteine
  • Glutamic Acid
  • Glutamine
  • Glycine
  • Proline
  • Serine
  • Tyrosine

The amino acids arginine, cysteine, glycine, and tyrosine are sometimes also considered to be “conditionally essential.” That means most people manufacture them on their own, but some with certain illness or genetic abnormalities don’t and need to get them through their diets.The nine essential amino acids are called “essential” because you can’t manufacture them, you have to eat proteins that contain those amino acids. Those essential amino acids include:

  • Histidine
  • Isoleucine
  • Leucine
  • Lysine
  • Methionine
  • Phenylalanine
  • Threonine
  • Tryptophan
  • Valine

Do you really need to worry about essential amino acids when you plan your daily meals?

Not really. Animal sources of protein such as meat, eggs, and dairy products are “complete proteins.” That means that each protein found in an animal product contains each of the nine essential amino acids. Vegetarians and vegans may need to pay a little more attention to the dietary proteins. Plant proteins are called “incomplete proteins.” Each plant protein is missing at least one of the nine essential amino acids. However, every amino acid is found in some type of plant, so you can combine different plant proteins to get all of the amino acids you need. We will get back to this later in the lesson.

Protein Functions in the Body

There are many different proteins in your body, and they perform different functions. Proteins functions include:

  • Contributing to enzyme activity that promotes chemical reactions in the body
  • Signaling cells what to do and when to do it
  • Transporting substances around the body
  • Keeping fluids and pH balanced in the body
  • Serving as building blocks for hormone production
  • Helping blood clot
  • Promoting antibody activity that controls immune and allergy functions
  • Serving as structural components that give our body parts their shapes

Protein Digestion and Metabolism

The digestion of protein begins in the mouth with chewing. Chewing not only makes food easier to swallow, it also helps with digestion by chopping food up into smaller bits. Remember that it really is important to chew your food thoroughly; don’t gulp your food down in huge bites.Protein digestion continues in the stomach with the release of hydrochloric acid and pepsinogen. Hydrochloric acid converts pepsinogen into pepsin, which begins to break down the bonds between the amino acids. This process takes place while the muscles surrounding the stomach squeeze and squish the foods and stomach fluids together.

The next step occurs in the small intestine where the hydrochloric acid is neutralized with bicarbonates released from the pancreas. The pancreas also releases an enzyme called trypsin. Trypsin continues to break apart the amino acids, which are then absorbed into the blood stream. Once in the bloodstream, the amino acids are carried to the cells in various parts of your body. Your body uses the individual amino acids that were broken down during digestion to build the proteins needed for the various functions.

You may not think of protein as an energy source, but proteins do contribute calories to you diet. Those calories need to be acknowledged if you are watching your weight. Each gram of protein you eat has four calories. As this sample menu shows, the USDA suggests that you get about 15% to 20% of your calories from protein. For someone who needs 2000 calories per day, that would equal 75 to 100 grams of protein.

It isn’t difficult to get enough protein in your diet. One chicken leg alone will provide you with about 30 grams of protein. One salmon fillet has about 40 grams of protein, a cup of oatmeal has six grams of protein, and a cup of asparagus even has two grams of protein. Since most people get enough protein from their diet, protein deficiency is rare in developed countries. In underdeveloped countries, protein deficiency is more common. Severe protein deficiency is called kwashiorkor. Children with kwashiorkor tend to have very thin arms and legs and large, distended bellies. Lack of protein can cause growth failure, loss of muscle mass, a depressed immune system, lung problems, heart problems, and death.

This Week’s Assignment

This week I want you to pay attention to the types of protein you eat every day. You will learn about choosing healthy protein sources next week, so right now I want you to keep track of the types of meat, nuts, seeds, legumes, eggs, dairy, cheeses, fish and seafood you eat. Make note of how they are prepared and how much you eat every day.

This Week’s Quiz

You can test your knowledge of proteins with this quiz: Quiz Three – Proteins, Amino Acids and MetabolismThis is lesson three of the basic nutrition – macronutrients e-course. Up next, lesson four is about choosing healthy proteins. You may sign up for the whole e-course at Basic Nutrition – Macronutrients


“Protein.” Nutrition Source, Harvard School of Public Health. March 20, 2007.

This site is for information and support only and NOT a substitute for professional diagnosis and treatment!
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