Would anyone be able to mark my essay please on the importance of shapes fitting together in cells and organisms. [25 marks]
Enzymes are globular proteins which have a specific tertiary structure that has a complementary shape to that of a specific substrate molecule. The lock and key model is the initial hypothesis used to describe enzyme action,that only one specific molecule could bind to an enzyme.However,a more recent hypothesis is the induced fit model,which found that similar molecules could still be complementary and bind to the active site region of an enzyme, to form an enzyme-substrate complex in order to lower the activation energy of a reaction by providing an alternative pathway.The shape of enzymes can vary due to their structure as a change to the sequence of amino acids in the primary structure can lead to a change in the tertiary and quaternary structure.This is because whilst they can be rigid
due to disulphide bonds between cysteines and other intermolecular bonds, such as hydrogen bonds and van der Waals forces,they can also be flexible,e.g keratin. The shapes that can form in these active sites allow reactions to occur that otherwise would not.
For example, the enzyme lactase has an active site that is complementary only to lactose . Sucrose, a similar disaccharide, has a different shape to lactose so therefore cannot bind to lactase’s active site. On binding to the active site, an enzyme-substrate complex is formed and reaction takes place.In the induced fit model, the active site is not complementary to the substrate, but on binding the shape changes and the active site forms, moulding itself to the substrate.
If the substrate is not complementary to enzymes active site the proteins won’t be able to be digested so won’t be able to make structural proteins in the body this leads to non-functional proteins as enzymes catalyse the formation of bonds to make structural proteins and breaking of bonds to digest the proteins.
These active sites can also act as targets for drugs and inhibitors. Drugs will often work as competitive or non-competitive inhibitors to enzymes of bacteria. As a result, their shape is either similar to the substrate and binds to the active site .Or non-competitive inhibitors bind to an allosteric site and,as a result, change the shape of the active site,so that the substrate can’t bind. This is important because they allow bodies to regulate the enzyme's action as in negative feedback and nerves or for treatment as in the case of antibiotics working on the enzymes that make the murein walls of bacteria.
As part of the immune response shapes fit together to combat infections and diseases to protect the human body.This is because T-lymphocytes can recognise pathogens as being foreign due to the surface antigens. The T-lymphocytes that have specific shapes to the antigen on the pathogen will sensitise via the release of interleukins to the specific B- cells. The sensitised B-cells then divide into B-memory cells and B-plasma cells. These B-plasma cells release antibodies that have specific complementary shapes to the antigens on the pathogen, and can thus form antigen-antibody complexes. As the antibodies have 2 antigen binding sites on their quaternary structure, it allows the agglutination of pathogens, which increases the efficiency of phagocytosis.
Antibodies contain a constant and variable region, to allow two antigens to bind at each variable region. An example is HIV, where it binds to the CD4 cell surface antigen on the T-helper cells. HIV releases the proteins into the host cell surface so the virus can enter. The antigen becomes integrated into the T-helper cell, this manipulates the host cell to replicate as normal but instead, it is replicating as HIV. Eventually, the viral RNA buds of the host cell attack the body from within, however symptoms are dormant for many years.
Antibodies have a tertiary structure that’s complementary to the antigen ,similar to how neurotransmitters have a complementary and specific shape to a receptor. If the structure didn’t fit together then without them we wouldn’t be able to fight infectious diseases which could lead to damage to tissues ,cell death and life-threatening diseases .For example,
monoclonal antibodies can be used for the treatment of diseases caused by the overproduction or inappropriate production of B-cells eg. multiple sclerosis as the antibody (rituximab) binds to cell surface receptor proteins on B-cells and causes the death of the cells.
Starch is taken in the form of food so digestion starts at the mouth .Saliva contains the amylase enzyme that hydrolyses some of the starch into maltose .The pancreas secretes amylase into the small intestine, where remaining starch is digested. Disaccharidases such as maltase break down disaccharides into monosaccharides in the small intestine. Some of these are attached to the epithelial cells lining the ileum and are known as membrane-bound disaccharidases.These monosaccharides are small enough to be transported across the plasma membrane of the epithelial cells lining the ileum.The glucose is absorbed into the blood plasma by sodium-glucose co-transport. The increased glucose concentration causes the blood water potential to reduce which may prove harmful. The increase in glucose is therefore detected by a glucose receptor on the surface of the beta-cells of the islets of Langerhans in the pancreas. The receptor triggers a release of a protein hormone, insulin which in turn travels to the liver. Here it binds to another specific insulin receptor on the cell surface membrane of muscle/liver cells. This alters the shape of the receptor triggering the activation of phosphorylase enzymes which condense glucose into glycogen in glycogenesis leading to the lowering of blood glucose.