Unit 2.1 - Metabolic Pathways and their Control

• What is metabolism?

  The biochemical processes that life depends on.
• What are metabolic pathways?

  Integrated and controlled pathways of enzyme-catalysed reactions within a cell
• What are the two types of reactions within metabolic pathways?

  Catabolic and anabolic.
• What is a catabolic reaction?

  Catabolic reactions break down large molecules into smaller molecules and release energy
• What are anabolic reactions?

  Anabolic reactions build up large molecules from small molecules and require energy.
• What are some examples of catabolic reactions?

  Digestion. A cell using ATP for energy.
• What are some examples of anabolic reactions?

  Products of digestion, photosynthesis
• What are the three different steps in metabolic pathways?

  Irreversible steps, reversible steps, alternative routes
• What are irreversible steps?

  Key regulatory points in the pathway. After that point substrates cannot be converted back and are committed to continuing on the pathway.
• What are reversible steps?

  Reversible steps are important if too much of a substrate builds up. It allows it to be converted back to the previous substrate. Some products of metabolism in the body are potentially toxic, this can prevent toxicity.
• What are alternative routes?

  When substrates are plentiful, or in short supply steps in the pathway can be bypassed. This conserves energy.
• What does the cell membrane do?

  Separates internal compartments of cells from the external environment. It allows selective communication between intracellular and extracellular activities and controls what enters and exits the cell.
• What are the three proteins found in the cell membrane?

  Pores, pumps and enzymes.
• What are pores?

  Larger molecules depend on channel forming proteins to form pores to allow movement across the membrane.
• What are pumps?

  Specialised protein pumps that recognise molecules and transfers them across the membrane.
• What is active transport?

  Active transport is the movement of molecules or ions from a low concentration to a high concentration across the phospholipid bilayer. This process requires energy.
• What do enzymes do in the cell membrane?

  Enzymes are biological catalysts. They alter the rate of a biochemical reaction and remain unchanged.
• What do enzymes do to activation energy?

  Reduce activation energy. This conserves energy and speeds up the reaction.
• How are enzymes specific?

  The active site of an enzyme has a particular shape determined by the sequence of amino acids that forms the enzyme. Enzymes only work with one type of substrate. The substrate fits exactly into the active site. There is a chemical attraction (affinity) between enzyme and substrate.
• What is induced fit?

  The active site is flexible and slightly alters its shape when the substrate enters. This close fit ensures the active site is in very close contact with the substrate. End products are released because there is a low affinity with the active site. The enzyme will return to its original conformation.
• To function efficiently, what four factors must enzymes have?

  Temperature, pH, supply of substrate (concentration), inhibitors
• What do metabolic pathways normally involve?

  A group of enzymes
• What features do genes that code for enzymes that operate continuous metabolic pathways have?

  The genes are always switched on so that they are present in cells. Enzymes are then controlled by an inhibitor.
• What is an inhibitor?

  A substance that decreases the rate of enzyme controlled reactions
• What are the three mechanisms of enzyme inhibition?

  Competitive, non-competitive and feedback.
• What are competitive inhibitors?

  Competitive inhibitors compete with a substrate for the active site of an enzyme. These inhibitors are a similar structure to the substrate so can block an active site. This decreases the rate of the chemical reactions. Competitive inhibition can be reversed by increasing substrate concentration.
• What are non-competitive inhibitors?


  They become attached to a non-active (allosteric) site on an enzyme. This alters the shape of the enzyme and, therefore the active site, so that it can no longer combine with the substrate molecule. This type of inhibition is irreversible.
• Describe the effect of increasing substrate concentration on non-competitive inhibition compared with competitive inhibitor.


  Increasing substrate concentration has no effect on non-competitive inhibition, whereas increasing the substrate concentration can reverse competitive inhibition. Non-competitive: Most enzyme molecules have become inactive but some are unaffected by inhibitors so reaction rate remains low.
• What is feedback inhibition?

  Occurs when the end product in the metabolic pathway reaches a critical concentration. The end-product then inhibits an earlier enzyme, blocking the pathway, and so prevents further synthesis of the end-product.