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What are enzymes?

  • All enzymes are globular proteins → spherical in shape (Fig 1)
  • Control biochemical reactions in cells
  • They have the suffix "-ase"
  • Intracellular enzymes are found inside the cell
  • Extracellular enzymes act outside the cell (e.g. digestive enzymes)
  • Enzymes are catalysts → speed up chemical reactions (Fig 2, Fig 3)
    • Reduce activation energy required to start a reaction between molecules
    • Substrates (reactants) are converted into products
    • Reaction may not take place in absence of enzymes (each enzyme has a specific catalytic action)
    • Enzymes catalyse a reaction at max. rate at an optimum state
  • Lock and key theory
    • Only one substrate (key) can fit into the enzyme's active site (lock)
    • Both structures have a unique shape
  • Induced fit theory (Fig 4)
    • Substrate binds to the enzyme's active site
      • The shape of the active site changes and moves the substrate closer to the enzyme
      • Amino acids are moulded into a precise form
      • Enzyme wraps around substrate to distort it
    • This lowers the activation energy
    • An enzyme-substrate complex forms → fast reaction
    • E + S → ES → P + E
  • Enzyme is not used up in the reaction (unlike substrates)


Fig4


Fig1

Enzyme Activity

  • Changes in pH
    • Affect attraction between substrate and enzyme
    • Ionic bonds can break and change shape → enzyme is denatured
    • Charges on amino acids can change → ES complex cannot form
    • Optimum pH (enzymes work best)
      • pH 7 for intracellular enzymes
      • Acidic range (pH 1-6) in the stomach for digestive enzymes (pepsin)
      • Alkaline range (pH 8-14) in oral cavities (amylase)
    • pH measures the conc. of hydrogen ions → higher conc. will give a lower pH
  • Enzyme conc
    • Proportional to rate of reaction, provided other conditions are constant
    • Straight line
  • Substrate conc. (Fig 5)
    • Proportional to rate of reaction until there are more substrates than enzymes present
    • Rate of reaction increases
      • Substrate binds to active site, but more enzymes are available
      • Rate increases if more substrate is added
    • Eventually, curve becomes constant (no increased rate)
      • Substrates occupy all active sites (all enzymes)
      • Adding more substrate won't yield more product, as no more active sites are available

  • Increased Temperature
    • Increases speed of molecular movement → chances of molecular collisions → more ES complexes
    • At 0-42°C rate of reaction is proportional to temp
    • Enzymes have optimum temp. for their action (usually 37°C in humans)
    • Above ≈42°C, enzyme is denatured due to heavy vibration that breaks -H bonds
    • Shape is changed → active site can't be used anymore
  • Decreased Temperature
    • Enzymes become less and less active, due to reductions in speed of molecular movement
    • Below freezing point
      • Inactivated, not denatured
      • Regain their function when returning to normal temperature
    • Thermophilic: heat-loving
    • Hyperthermophilic: organisms are not able to grow below +70°C
    • Psychrophiles: cold-loving

Enzymes - Heroes and Villains

Analytical reagents

  • Made up of 2 enzymes (glucose oxidase and peroxidise) and a colourless hydrogen-donor fixed on a strip
    • The strip is dipped into a test solution (urine)
    • Colour develops which indicates that glucose is present
    • This method is used by diabetics to monitor their blood glucose levels
    • In healthy people, the urine contains NO glucose
  • Glucose oxidase
    • Highly sensitive to low conc. of glucose
    • Highly specific because it only reacts with one specific substrate (glucose)
    • Catalyses the conversion of glucose to hydrogen peroxide (H2O2)
  • Peroxidase
    • Catalyzes reaction between colourless hydrogen-donor molecule and H2O2
    • A coloured molecule is formed

Alpha1-antitrypsin

  • Function
    • White blood cells (neutrophils) in the lung help to prevent infections
    • They also release elastase and protease (trypsin)
    • Those enzymes break down/digest ct. and proteins inside the lungs and damage it
    • NB: Trypsin is also found in the digestive system and digests food!
    • The anti-protease alpha1-antitrypsin protects the lungs from elastase and protease
  • Alpha1-antitrypsin deficiency
    • Genetic disease that causes emphysema
    • Trypsin is no longer inhibited and damages the lungs
    • Walls of alveoli are damaged and surface area for gas exchange is reduced
    • Patients can be treated by infusing alpha1-antitrypsin
  • Smoking
    • Increases the number of neutrophils in the lungs (more trypsin is secreted into the lungs)
    • ALSO inactivates alpha1-antitrypsin
    • This creates an imbalance between proteases (trypsin) and anti-proteases (α1-antitrypsin)
    • Same lung damage as in α1-antitrypsin deficiency but much slower

Lactose intolerance

  • Lactase splits lactose (milk sugar) into β-glucose and galactose
  • Lactose intolerant person lacks lactase → lactose is neither digested nor absorbed
  • High levels of soluble lactose remain in small intestine
  • Supports large populations of bacteria / ferment lactose producing gas / causing abdominal discomfort
  • Water potential becomes more negative / H2O moves into small intestine / not reabsorbed / diarrhoea
  • Adults rarely produce lactase / gene is switched off in adulthood

Pancreatitis

  • Pancreas is found below the stomach
  • Produces digestive enzymes (amylase, lipase, trypsin that break down starch, lipids, proteins, respectively)
  • Acute (sudden onset)
    • Reversible inflammation of a previously normal pancreas
    • Caused by gallstones, alcohol, scorpion bite, trauma
    • Inappropriate activation of enzymes
      • Trypsin becomes active before released from the pancreas
      • Pancreas is made of proteins
      • Trypsin is active and digests/hydrolyses proteins
      • Cell wall breaks down, amylase and lipase escape into the blood
    • Diagnosis
      • Amylase and lipase remain elevated in blood for 3 days
  • Chronic (gradual onset)
    • Inflammation is caused by cystic fibrosis or long-term alcohol intake
    • Pancreas gradually loses its ability to produce digestive enzymes
    • Food is not digested, hence not absorbed (malabsorption)
    • Diagnosis
      • Amylase and lipase in blood normal
      • Low levels of faecal elastase
      • Too much fat in faeces → fat passes through gut without being digested and absorbed
  • Pancreatic enzyme replacement therapy (PERT)
    • Required for life in patients with irreversible pancreatic damage
      • Chronic pancreatitis
      • Cystic fibrosis
    • Tablet contains digestive enzymes (normally produced by pancreas)
    • Degradation of enzymes/proteins by stomach acid prevented by
      • Coating tablet with protective layer
      • Taking extra tablet that inhibits acid secretion