Proteins

General Characteristics of Proteins

  • Proteins are large, complex molecules made up of one or more chains of amino acids.
  • They play an essential role in the structure, function, and regulation of the body’s tissues and organs.
  • Proteins are composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulphur.

Structure of Proteins

  • The basic building blocks of proteins are amino acids. There are 20 different types of amino acids that can be combined to make a protein.
  • Each amino acid consists of an amino group, carboxyl group, a hydrogen atom, and an R-group or side chain.
  • A peptide bond links two amino acids together through a dehydration synthesis reaction, forming a dipeptide.
  • The sequence of amino acids in a protein is known as its primary structure.
  • The secondary structure of a protein refers to the coils and folds in the polypeptide chain, result of hydrogen bonds between amino acids. These structures include alpha helices and beta-pleated sheets.
  • Tertiary structure is the overall three-dimensional shape of a protein, stabilized by various types of interactions, including disulfide bridges, hydrophobic interactions, hydrogen bonds, and ionic interactions.
  • The quaternary structure applies to proteins with multiple polypeptide chains, refers to how these chains interact and are arranged.

Classification of Proteins

  • Proteins can be broadly classified into two categories: globular and fibrous proteins.
  • Globular proteins are generally compact, spherical and soluble in water. These include enzymes, antibodies, and some hormones.
  • Fibrous proteins are elongated and insoluble in water. They play structural roles in organisms, key examples being collagen, keratin, and elastin.

Functions of Proteins

  • Proteins perform an extensive range of functions, including catalysis, defence, transport, support, motion, regulation and storage.
  • Proteins acting as enzymes catalyse biological processes, greatly accelerating the rate of reactions.
  • In defence, antibodies (proteins) bind to specific foreign particles (antigens) helping to protect the body.
  • Proteins provide structural support in organisms. For instance, collagen forms the matrix that strengthens the skin, bones, tendons and blood vessels.
  • Proteins such as actin and myosin play a vital role in muscle contraction and movement.
  • Proteins are involved in the transport of substances. For example, haemoglobin, a protein, transports oxygen in the blood.

Protein Synthesis

  • Protein synthesis is the process by which cells produce proteins, involves two stages: transcription and translation.
  • Transcription is the process where the information in a strand of DNA is copied into a new molecule of messenger RNA (mRNA).
  • During translation, the mRNA is used as a template to assemble a protein from amino acids. Ribosomes, transfer RNA (tRNA), and various enzymes play crucial roles in this process.
  • The sequence of nucleotides in the DNA and thus the mRNA determines the order of the amino acids in the protein, establishing the protein’s structure and function.

Protein Denaturation

  • Denaturation is the process in which proteins lose their structure due to the application of an external stress, such as high temperature, changes in pH, or the presence of certain chemicals.
  • Denatured proteins cannot perform their functions properly, as their active sites have been altered. This process is typically irreversible and can lead to the death of cells or the organism.