When food is chewed and swallowed it travels down the oesophogus by a muscular action called peristalsis and through the cardiac sphincter into the stomach. In the stomach any bacteria in the food are killed by hydrochloric acid and protein digestion starts here. Proteins are hydrolysed by breaking the peptide bonds between amino acids. The enzymes that carry this out are proteases. There are 2 types of proteases; exoprotidases and endoproteases. Exoprotidases start at the end of the polypeptide chain breaking bonds at the end producing a mix of amino acids and di peptides. Endoprotidases breaks bonds in the middle of the molecule at specific amino acids. These enzymes are hydrolases as the reaction us a hydrolysis reaction (breaking bonds by adding water)
The last stage of protein digestion is the conversion of di peptides into amino acids. The epithelial cells of the small intestine have enzymes in their plasma membrane and cytoplasm which convert di peptides into amino acids.
Proteins are then absorbed into the blood by active transport in the villi of the small intestine, into the capillary network. They are then transported by the hepatic portal vein to the liver where a pool of amino acids are formed. Amino acids are then transported to working cells in the cytoplasm from which new proteins are made. Metabolically active tissues and rapidly growing organisms require a large supply of amino acids for protein synthesis.
Protein synthesis relies on the effective communication of the coded information held in the genes to the sites of protein manufacture, the ribosomes in the cytoplasm. Since DNA is unable to move from the nucleus, messenger RNA molecules are needed. The mRNA is assembled using the enzyme RNA polymerase. This process is called transcription.
The DNA template strand is read from the 3’ to the 5’ end and the mRNA is made from the 5’ to the 3’ end. During transcription only the coding parts of the DNA are copied (the exons). Non coding parts or introns are ignored. The completed mRNA molecule detaches from the DNA template and exits the nucleus via the nuclear pores, moving into the cytoplasm. The mRNA is now ready for translation, which is organised by the ribosomes. The start codon of the mRNA attaches to the information processing region of the ribosome. tRNA has a complementary anticodon to the mRNA and a site where amino acids can be joined. The ribosomes move along the mRNA reading the codons from the start codon. The anticodon slots into place temporarily (held by hydrogen bonds) when they are held there amino acids are joined by peptide bonds, freeing the tRNA which can be re used.
Proteins are important in the structure of collagen, keratin in skin hair and nails and elastin in connective tissue and contractile proteins such as actin and myosin allow contraction and movement. Many hormones have a protein structure such as insulin and growth hormone. Proteins are very important in transport, for example haemoglobin transports oxygen around the body and proteins also assist in transport in and out of cells with carrier and channel proteins in the cell membrane regulating movement across it. Virtually all enzymes being proteins. The majority of the reactions that occur in living organisms are enzyme-controlled. Without them, the rate of the reactions would be so slow as to cause serious, if not fatal, damage. Without enzymes toxins would soon build up and the supply of respiratory substrate would decrease. Enzymes are proteins and thus have a specific shape. They are therefore specific in the reactions that they catalyse - one enzyme will react with molecules of one substrate. The site of the reaction occurs in an area on the surface of the protein called the active site. Since the active site for all molecules of one enzyme will be made up of the same arrangement of amino acids, it has a highly specific shape. Generally, there is only one active site on each enzyme molecule and only one type of substrate molecule will fit into it. Chymotrypsin and trypsin both catalyse the hydrolysis of peptide bonds but due to their shapes, the active site of chymotrypsin only splits bonds after an aromatic amino acid (one containing a ring of atoms) whereas trypsin only splits bonds after a basic or straight chain amino acid.