Proteins contain carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and may have other elements such as iron (Fe), phosphorous (P) and sulfur (S).
Proteins are made of amino acids. There are 20 common amino acids from which all proteins in living organisms are made. Nine of them are considered essential amino acids, as they cannot by synthesised in the body from other compounds, and must be obtained from the diet. Amino acids are bonded together by peptide bonds to form peptides. A long peptide chain forms a protein, which folds into a very specific three-dimensional shape. This three-dimensional shape is completely determined by the identity and order of the amino acids in the peptide chain. We often refer to four different levels of protein structure (see figure below):
Because the sequence of amino acids determines the way that a protein folds, if you start with a certain peptide chain, you will always get the same three-dimensional structure!
- Primary structure: This refers to the sequence of amino acids joined together by peptide bonds to form a polypeptide chain. Some proteins have fewer than a hundred amino acids, while others have several thousand.
- Secondary structure: This is the first level of three dimensional folding. It is driven completely by hydrogen bonding. Hydrogen bonding usually results in regions of the chain coiling and other regions forming sheets.
- Tertiary structure: This is the second level of three dimensional folding and is the overall final shape of the protein molecule. The secondary structures and unstructured regions of the chain further fold into a globular shape, driven by hydrophobic interactions (non-polar regions trying to escape the water in the cell environment) and electrostatic interactions (polar and charged regions wanting to interact with the water environment and each other).
- Quaternary structure: Some proteins are complex: two or more peptide chains fold into their tertiary structures, then these complete structures associate together by hydrophobic and electrostatic interactions to form the final protein.
Role in animals and plants
Proteins are important in several crucial biological functions. Proteins are found in hair, skin, bones, muscles, tendons, ligaments and other structures and perform key structural and mechanical functions. Proteins are also important in cell communication and in the immune system. Proteins can also act as an energy reserve when broken down through digestive processes. Certain proteins called enzymes are important in catalysing cellular reactions that form part of metabolism.
Proteins are essential to any diet. A lack of protein results in a disease called kwashiorkor (see figure below) or marasmus (see figure below). Marasmus is caused by a general nutritional deficiency (starvation), and kwashiorkor is caused by a deficiency in protein specifically.
Meat or vegetables: which is a better source of protein?
Both animal protein and protein from vegetables is good for health. But each type comes with other nutrients. So which ‘package’ of nutrients-meat or vegetables is better for health?
- A 180 gram steak provides 40 g of protein BUT also provides 38 g of fat which is more than the Recommended Dietary Allowance
- The same amount of salmon gives 34 g of protein and 18 g of fat.
- A cup of cooked lentils has 18 g of protein and 1 g of fat.
Therefore when choosing protein-rich foods, pay attention to what comes along with protein. Vegetable sources of proteins such as beans, nuts and whole grain are excellent choices and they offer healthy fibre, vitamins and minerals. The best animal protein sources are fish and poultry.