Proteins and Amino Acids

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Introduction

Amino acids are the building blocks of proteins. When food containing protein is digested, amino acids are the final product. After being absorbed, these amino acids are used by cells in the body to create chemicals and structures required for life.

There are 21 different amino acids. Amino acids each consist of:1

  • An amino group: -NH2+
  • A carboxylate group: -CO2-
  • An โ€œRโ€ side chain which is specific to that amino acid
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Classes of amino acids

Amino acids can be grouped into four different classes based on their โ€œRโ€ side chains (Figure 1):1

  1. Polar, charged: hydrophilic
  2. Polar, uncharged: hydrophilic
  3. Special cases: hydrophilic
  4. Non-polar, uncharged: hydrophobic
Groupings of amino acids by common structural features
Figure 1. Groupings of amino acids by common structural features.

Protein structure

In order to form a protein, amino acids link together and fold into their final functional unit.

There are four levels of structure that relate to the complexity of the formation (Figure 2):2

  • Primary structure: the bonds that form between each amino acid, known as peptide bonds
  • Secondary structure: the formation of hydrogen bonds between this initial structure
  • Tertiary structure: the folding of the peptide chain to form a 3D shape
  • Quaternary structure: the linking of multiple polypeptide chains together
Folding of protein to form final functional structures
Figure 2. Folding of protein to form final functional structures.
Clinical relevance: haemoglobin

Haemoglobin is the protein which carries oxygen around the body in red blood cells. It consists of four globin chains (two alpha and two beta) and four haem groups.

There are several conditions in which this structure is disrupted:2

  • Thalassaemia: a range of genetic conditions leading to defects in either the alpha or beta-globin chains. This leads to haemoglobin with suboptimal function, causing the production of abnormal red blood cells
  • Sickle cell anaemia: altered beta-globin chain due to a genetic mutation that switches out the amino acid glutamic acid to valine. This leads to a red blood cell with an abnormal shape which causes microvascular occlusion.

Nutrition

Proteins are one of the three major macronutrients in human nutrition (with lipids and carbohydrates the other two).

In humans, protein is critical for the growth, maintenance and structural composition of cells.3

Digestion

During digestion, proteins are broken down into smaller peptide chains in the stomach by reacting with hydrochloric acid and proteases present (Figure 3).

This allows for absorption and the formation of amino acids not able to be made by the body (known as the nine essential amino acids):

  • histidine
  • isoleucine
  • leucine
  • lysine
  • methionine
  • phenylalanine
  • threonine
  • tryptophan
  • valine
Digestion of protein by gastrointestinal system
Figure 3. Digestion of protein by the gastrointestinal system

Sources of protein

Dietary protein can come from animal and plant sources. Animal sources are known to contain all essential amino acids, whereas plant sources generally are missing lysine.3

Animal sources of protein include:

  • Meat (beef, pork, and chicken)
  • Eggs
  • Fish

Plant sources of protein include:

  • Tofu
  • Tempeh
  • Legumes (lentils, peas, and beans)
  • Wholegrains

Protein deficiency

Deficiency in the essential amino acids leads to inhibition of protein synthesis in the body. This leads to deficient growth, upkeep of the immune system, reduced kidney function and reduced organ function.2,3

Protein deficiency is rare in developed regions of the world. Clinical features of protein deficiency may include:

  • Oedema
  • Faltering growth in children
  • Low muscle mass
  • Loss of or poor growth of hair
  • Chronic exhaustion

The amount of protein a person requires differs from person to person depending on their current health situation. For example, growing children, people who are unwell, and those undertaking large amounts of exercise require more protein.


Key points

  • Amino acids are the building blocks for all proteins in the body
  • Proteins undergo four stages of folding from a primary chain of amino acids using peptide bonds, to a quaternary multi-protein structure
  • Some proteins can be made by the body, but the nine essential amino acids must be consumed as they are unable to be made intrinsically

Editor

Dr Chris Jefferies


References

Reference texts

  1. Hughes AB. Amino acids, peptides, and proteins in organic chemistry. 2012. Wiley-VCH. Available from [LINK]
  2. Fry MA. Essential biochemistry for medicine. 2010. Wiley-Blackwell. Available from [LINK]
  3. Langley-Evans SCA. Nutrition health and disease: a lifespan approach. 2015. John Wiley & Sons, Ltd. Available from [LINK]

Image references


 

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