Proteins are polymers of amino acids that make up about 50 percent of organic content in living organisms. They’re involved in every life activity. Each protein has a unique three-dimensional structure that carries out a specialized function. This article introduces 7 functions of protein： motion, structure, catalysis, signal recognition, defense, transportation, energy source.
Proteins and Motion
When people think of proteins, the first image comes to mind is often a muscular fitness enthusiast. Proteins are most abundant substance in muscles, aside from water. Myofibrils through muscle cells are the units for muscle contraction. A thick myosin filament is surrounded by six thin actin filaments. Muscle contraction results from the ATP-consuming sliding motion between these two proteins, although both filaments length remains unchanged. This’s similar to drawing and sheathing a sword. The sword and sheath remain same length, but the overall length increases.
Protein and enzyme
Almost all enzymes are proteins. Even catalytical RNAs, they also need proteins to maintain their structure. Enzymes are remarkable for their ability to greatly accelerate the biochemical reactions. The activation energy reduced by enzyme is an energy barrier that must be overcome when reactant convert to product. Without enzymes, most physiological reactions would proceed so slowly that life wouldn’t be sustainable. Due to the unique three-dimensional structure of enzyme, especially the active site where substrate binds, it catalyzes only one particular reaction or type of reaction.
Proteins and Structure in Organisms
Most of the organic matter in bone is collagen. It forms a three-dimensional network in bone where hydroxyapatite is embedded. Collagen makes bones more resilient, otherwise, they would be as hard but brittle as seashells. Collagen also forms the network structure that gives skin toughness, while the embedded elastin and proteoglycans keep skin moist and elastic.
Proteins and Signal Recognition
Water-soluble signal molecules that can’t pass through the cell membrane need receptors to transmit information. These receptors are glycoprotein located in cell membrane. Their protein backbones are embedded within cell membrane. The oligosaccharides attach to their heads that exposed on the outside of membrane. The specific structure of sugar chain can be recognized by signal molecules to trigger the corresponding physiological response. Some signal molecules are also proteins. For example, growth hormones that promote bone formation and muscle synthesis are proteins. Like acetylcholine, neuropeptides, one of protein in our brain can also transmit electrical impulses between nerve cells. Their effects are generally slow and lasting. The biological functions such as pain, mood, memory and learning are all regulate by neuropeptides.
Proteins and Defense
When bacteria or viruses invade, the proteins from complement system stick to invaders surface to make them easier to capture. These proteins also form membrane attack complexes that punch holes in membranes of invaders. Fragments resulting from hydrolysis of complements also attract macrophages to join the fight against invasion. After engulfing bacteria, macrophages release some protein cytokines to summon immune cells to the infected area. They also present the invaders' antigens on their surface to activate lymphocytes. When B cells are activated, they secrete specific antibodies (protein to inactivate pathogens.
Proteins and Nutrient Transport
Hemoglobin is a protein in red blood cells containing an iron ion (Fe2+) in its heme group. When hemoglobin passes through lungs, the high concentration of oxygen encourages iron to bond with oxygen temporarily. This binding reversible means that the oxygen molecules are released from hemoglobin in oxygen poor tissues.
Proteins as Nutrients and Energy Sources
Proteins are required for various life activities, so they’re generally not used as energy and nutrients, except in certain cases. For example, the proteins in seeds and eggs are provided for embryo's nourishment. They’re gradually broken down into amino acids the embryo uses to synthesize its proteins for life activities. In extreme cases, they can also serve as energy source. When glycogen and fats are depleted, proteins are broken down into amino acids that are converted into glucose for energy through gluconeogenesis.