Enzymes

Enzymes are biological catalysts that speed up chemical reactions in cells, while coenzymes are non-protein molecules that assist enzymes by transporting chemical groups or electrons during these reactions.

Enzyme structure

Enzymes are proteins that have a three-dimensional structure composed of one or more polypeptide chains, with an active site specifically shaped to bind substrates and catalyse biochemical reactions.

Features of enzymes:

  • Enzymes are very specific and will only catalyse one reaction.
  • Their name often ends in -ase.
    There are a few exceptions: for example, trypsin and pepsin that break down proteins.
  • They are often named after the substrate that they act on.
    Examples:
    • Maltase catalyses the reaction to break down maltose.
    • Lipase catalyses the breakdown of lipids.
  • They are not used up in the reaction.
Normal reaction

An image of three horizontal sequential circles. The first circle is shown on the left-hand side of the image and depicts a circle with a wedge removed from its top. The circle is labeled “enzyme”, the wedge is labeled “substrate” and there is an arrow pointing to the gap between the circle and wedge labeled “active site”. To the right of this circle is an arrow pointing to the right. To the right of this arrow is another circle, this one depicting the wedge fitting into place at the top of the circle. The circle, with the wedge inserted, is labeled “enzyme-substrate complex”. To the right of this circle is another arrow pointing to the right. To the right of this arrow is a final circle, again with a wedge removed from the top. The wedge piece itself is broken in half. These broken halves are labeled “products”. There is an arrow the gap between the circle and the wedge, pointing toward the wedge.

Enzymes as catalysts

Enzymes act as catalysts to lower the activation energy of chemical reactions.  As a result of lowering the activation energy, enzymes make chemical reactions more likely to occur (called making them ‘energetically favourable’), which makes the reactions appear to run more quickly.  Without enzymes, chemical reactions would proceed too slowly. The diagram below shows the energy changes over the course of a reaction. Both with and without an enzyme energy must first be absorbed by the reaction, but this energy barrier is much lower in the presence of an enzyme.

A graph with the x-axis marked “reaction progress” and the y-axis marked “energy”. Two lines on the graph sit on top of each other and proceed horizontally about mid-way up the y-axis before diverging. Both lines curve upward on the y-axis, but one curves significantly higher than the other. The gap between the two lines at the apex of the curve is denoted by a double-sided arrow. The two lines, when together, are labeled “energy of reactants.” As both lines descend from their apex they gradually come back together to once again sit on top of each other. At this stage they are labeled “energy of products”. As the lines descend from the apex there are two further double-sided arrows, one measuring the height differential between the first line at its starting point and its apex which is labeled “activation energy without the enzyme”, and the other arrow measuring the height differential between the second line at its starting point and its apex which is labeled “activation energy with the enzyme”.

Coenzymes

Coenzymes are organic molecules that assist enzymes in facilitating biochemical reactions. They act as carriers, transferring electrons, atoms, or functional groups from one molecule to another during these reactions.

The diagram below shows how coenzymes work together with enzymes and substrates.

An image of three horizontal sequential circles, with a small wedge removed in the lower right-hand side of each circle. The first circle is shown on the left-hand side of the image and depicts a circle with an additional half-wedge removed from its top. The circle is labeled “inactive enzyme”, and the half-wedge is labeled “coenzyme”. To the right of this circle is another circle, this one depicting the half-wedge fitting into part of the gap left at the top of the circle, and the rest of the wedge piece still removed from the circle. The circle is labeled “coenzyme activates enzyme” and the wedge piece is labeled “substrate”. To the right of this circle is a final circle, with both the half-wedge and other wedge pieces fitting into the circle again. This final circle is labeled “active enzyme-coenzyme-substrate complex.

Roles of Coenzymes:

  • Energy Transfer: Coenzymes like NAD⁺ and FAD are essential in cellular respiration, where they carry electrons and hydrogen atoms to the electron transport chain, aiding in ATP production.
  • Enzyme Function: Coenzymes bind to enzymes and help them achieve the proper shape or active site configuration to catalyse reactions effectively.
  • Metabolic Pathways: Coenzymes like coenzyme A (CoA) participate in metabolic pathways such as the Krebs cycle, facilitating the transfer of acyl groups.

Role of coenzymes in cellular respiration

Coenzymes are crucial for electron transfer and energy release in the process of ATP generation during cellular respiration.

The coenzymes involved in cellular respiration are: NAD+ and FAD. They form NADH and FADH₂ respectively and carry electrons to the electron transport chain from glycolysis and the Krebs cycle where the molecules are generated.

Role of coenzymes in photosynthesis

The primary coenzyme involved in photosynthesis is NADP⁺.

During the light-dependent reactions, NADP⁺ acts as an electron carrier, accepting electrons and hydrogen ions produced when water molecules are split by sunlight. This conversion forms NADPH.

NADPH then carries the high-energy electrons to the Calvin cycle (light-independent reactions), where it provides the reducing power needed to convert carbon dioxide (CO₂) into glucose and other carbohydrates.

In summary, coenzymes enable and enhance the efficiency of biochemical reactions by acting as intermediaries and carriers in various metabolic processes.