What is Protein?

Kate Onissiphorou

by Kate Onissiphorou

1st December 2021

Proteins are biological macromolecules composed of amino acid chains. They have physiological and structural functions in the body, and exist in a wide variety of forms, such as enzymes, hormones, blood, antibodies, muscles, and skin. 

They also function as transporting molecules within and in between cells. Proteins are considered macronutrients, which are needed by the body in fairly substantial quantities from dietary sources.

Proteins mainly differ based on their amino acid sequence. The precise sequence of amino acids is called the ‘primary structure’ of a protein. There’s also a secondary protein structure, which refers to the looped forms such as the α-helices and β-structures. Finally, a tertiary structure refers to the way the secondary structures form sheets and fold into three-dimensional structures.

What Does Protein Do?

On average, proteins comprise about 14-16% of the body mass of a human adult. Adult males generally have higher protein body mass compared to females. These proteins have various roles depending on their type. Here are some of the important roles they play in humans and in other organisms in general:

  • Fertilisation

Proteins are involved in the human body from the moment of conception. For example, in order to prevent multiple sperms from fertilising the egg cell, the cell protein structure of the egg cell changes upon the entry of a sperm. This is driven by the fusion of cortical granules with the plasma membrane, and creates an impenetrable barrier for other sperms.

  • Embryonic development

Once the mature egg cell is fertilised, it becomes an embryo that rapidly divides into specialised cells. The rapid cell division fuels embryonic development and is guided by the cascade of various types of proteins that are ‘switched’ on and off by the genes at particular intervals.

Protein helps a mature egg cell divide into specialised cells

Protein helps a mature egg cell divide into specialised cells

Thousands of proteins are produced during the developmental stage of an embryo. These include structural proteins, such collagen and elastin, and physiological proteins, like enzymes and hormones. One important hormone indicator, for example, is hCG or human chorionic gonadotropin. This is an early indicator of pregnancy in humans and is created by the trophoblast tissue of the early embryo. Later, it becomes part of the placenta.

  • Metabolic-biochemical reactions

Proteins in the form of enzymes have very crucial roles in many metabolic processes and other biochemical reactions in the body. From digestion to the Krebs cycle, various types of enzymes facilitate biochemical reactions. For instance, the salivary amylase assists in the digestion of starch, breaking it down to maltose.

  • Biochemical messengers

Multicellular organisms like humans have differentiated and specialised cells that are organised into tissues, organs, and organ systems. They perform specific functions, but they also need to communicate with the rest of the body. They can do this through hormones. The physiological functions of various organs are also regulated by hormones. For example, pituitary hormones regulate bone and muscle development.

In terms of molecular types, hormones can be classified into three main categories:

  • Protein and peptides: These hormones are composed of amino acid chains that range from a few amino acid units to several hundreds.
  • Steroids: Steroid-based hormones are made from fat cholesterol. Some examples of these are testosterone and estrogen, which are sex hormones.
  • Amines: These are the simplest types of hormones. They’re composed of individual amino acids, which are either tryptophan or tyrosine. They help regulate the sleep pattern.
  • Structural support and mobility

Bones, muscles, skin, and teeth are mainly made of proteins. All types of muscles, for instance, are composed of actin and myosin filaments. These filaments are responsible for the contractions of muscles, including the internal organs, the cardiac muscles of the heart, and the skeletal muscles.

  • Immunity response

The immune system of the body has the capacity to remember pathogenic invaders, such as bacteria and viruses. Immunity is based on the immune system’s memory of the protein structures on the surface of the cell membrane. It’s like a lock-and-key mechanism that identifies the protein receptors, enabling the immune system to synthesise specific antibodies.

The immune system can also identify genetic codes of pathogens and react accordingly. From white blood cells to antibodies, proteins are the primary components.

  • Transport and storage

Proteins can transport and store various types of nutrients within the cell and among the different parts of the body. For example, haemoglobin in the blood is a type of protein that carries oxygen throughout the body via the circulatory system.

What Are the Different Types of Protein?

Between 80,000 and 400,000 unique protein species can be found in the human body. There are about 42 million protein molecules per cell, according to this research paper.

However, not all of these proteins are simultaneously present. Many are produced only at specific durations and are based on specific needs, such as in the case of some hormones and antibodies. Some proteins are short-lived and exist only in trace amounts, while others are abundant and permanently active throughout the lifetime of the individual.

Proteins can be classified into eight categories based on their functions and structures:

  • Hormonal 
  • Enzymatic 
  • Structural 
  • Defensive 
  • Storage 
  • Transport 
  • Receptor 
  • Contractile 

What is the Difference Between LBV and HBV Proteins?

The difference between high biological value (HBV) and low biological value (LBV) proteins is that HBV proteins are needed in more abundance. 

Organisms need dietary sources of protein for various reasons such as growth and development, damage repairs, immunity, locomotion, and metabolic processes. Some types of proteins, however, are not as necessary as other types of proteins.

In terms of dietary biological value, the high biological value (HBV) proteins contain all the essential amino acids. Meanwhile, the low biological value (LBV) proteins contain only a few of the essential amino acids. Typically, plant proteins are considered as having low biological value compared to animal proteins that have high biological value.

How Much Protein Should You Eat?

Just like other types of nutrients, proteins from dietary and food supplement sources have recommended daily allowances based on reference nutrient intake, or RNI. In the UK, the RNI for protein is 0.75 g/kg of body weight. You can plan your diet based on the RNI.

Foods containing high levels of protein, including chicken, red meat, salmon, and legumes

Foods containing high levels of protein, including chicken, red meat, salmon, and legumes

What Meat Has the Most Protein?

Not all types of meat are created equal. Some have better protein content than others. If you eat meat, you should choose lean meat in order to get the best HBV proteins. For example, chicken breast has 109% daily value of protein. Meanwhile, lean pork has 105% of the daily value of protein.

How to Get Protein As a Vegetarian

Although vegetables do contain HBV proteins, there are specific types that are particularly good dietary sources of protein. If you’re a vegetarian, you can get 87% of your daily value of protein per cup of firm tofu.

Similarly, you can also get 37% of your daily value of protein from lentils per cup. However, these vegetarian sources of protein are not enough, especially for high biological value proteins. You may need to take food supplements. These are some good sources of protein for vegetarians:

A graphic containing 8 sources of protein for vegetarians


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