Cell processes

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Cell processes

• Enzyme activity

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Key terms

• Amino acids
• Protein
• Enzyme
• Catalyst
• Metabolism
• Anabolism
• Catabolism
• Active site
• Substrate

• Lock-and-key model
• Induced fit model
• Denature
• pH scale
• saturation point
• Co-factors
• Co-enzymes
• Inhibitor
• Optimum temperature

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Proteins

• Proteins are one of the major food groups in the
  diet of animals.
• They are made of chains of amino acids.
• There are only 20 different types of amino acids.
• The varying combinations of amino acids result in
  the huge diversity of proteins, each having its
  own function.

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Two main types of proteins

• Fibrous proteins
• - long and stringy
• - form structures such as collagen in muscle,
  elastin in skin, keratin in hair, nails and horns
• Globular proteins
• - folded into a 3-D shape
• - perform regulatory functions such as
  hormones, transporting other molecules,
  antibodies for fighting off infections and
  enzymes

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Protein synthesis from DNA
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Enzymes

• Enzymes are proteins that act as biological
  catalysts i.e. they increase the rate of chemical
  reactions in the body.
• Without enzymes, metabolism would occur too
  slowly for life to exist.
• Remember what metabolic reactions are!

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Two main types of metabolic reactions

• Synthesis of large molecules from smaller
  molecules anabolic reactions
• e.g. glucose molecules into starch
• Breakdown from larger molecules into smaller
  molecules catabolic reactions
• e.g. food protein into amino acids for making
  other proteins

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Each enzyme has a specific role

• One enzyme catalyzes only one type of reaction.
• Often named after the main substance in the
  reaction it catabolises.
• Suffix -ase is added.
• e.g. Lipase catalyzes breakdown of lipids (fats)
• Lactase facilitates catabolism of
  lactose from milk
• Protease helps break down proteins
  from food

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Why are enzymes specific?

• This property of enzymes relates to their shape.
• Each enzyme has a specific shape, depending on
  the sequence of amino acids it is made of.
• Shape of an area on the enzyme known as its
  active site is where the substrate fits.

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Two slightly varying models of enzyme action

• Lock - and - key model
• The shape of the substrate corresponds exactly
  to the shape of the active site.
• This model, although useful to gain basic
  understanding, is now considered too simple to
  explain most enzyme action.

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Two slightly varying models of enzyme action

• Induced fit model
• Assumes that the enzyme is partially flexible,
  and that the substrate plays a role in
  determining the final shape of the active site.

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• Enzymes get reused several times before they get
  worn out.
• e.g. Peroxidase catalyzes breakdown of several
  million hydrogen peroxide molecules (dangerous to
  body tissues) into water and oxygen per minute.

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Factors that affect enzyme activity

• Temperature
• pH
• Substrate concentration
• Co-factors
• Inhibitors

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Temperature

• Up to 40 45 C, temperature speeds up enzyme
  activity, as molecules move faster at higher
  temperatures and collide more often.
• If temperature is too high, proteins/enzymes get
  denatured.
• e.g. What happens when you cook an egg?

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• Temperature at which the reaction is fastest is
  called optimum temperature.
• Optimum temperature for enzymes in different
  organisms varies!
• e.g. Antarctic fish, bacteria living in sulfur
  springs, etc.

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pH

• pH scale measures acidity ranges from 1 to 14.
  The closer pH is to 1, the more acidic a
  substance or environment is.
• Most enzymes work within cells where the pH is
  neutral. So, their optimum pH will be approx. 7.
• When pH is outside range for an enzyme (too low
  or too high), enzyme denatures.
• Examples of exceptions
• - Pepsin (works in stomach, where it is acidic,
  optimum pH is low)
• - Pancreatic lipase (works in small intestine,
  where it is basic, optimum pH is high)

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Effect of pH on enzyme acitivity
pH lt 7 is acidic pH 7 is neutral pH gt 7 is basic
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Substrate concentration

• Rate of enzyme activity increases as the
  concentration of the substrate increases.
• This happens up until saturation point i.e. there
  are no more free enzymes/active sites left.

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Co-factors Co-enzymes

• Enzymes often need helpers. 
• Sometimes ions or metal atoms are used. These
  helpers are called cofactors (e.g. iron in
  haemoglobin, calcium in nerve signalling, nickel
  in urease etc.)
• Small molecule helpers are called coenzymes.
• Coenzymes that we can't build ourselves, that we
  need to get from our food in their working form,
  are called vitamins.  (e.g. vitamin B in
  respiration, vitamin C for turning genes on)

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Co-factors Co-enzymes
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Inhibitors

• Inhibitors are substances that prevent enzymes
  from catalysing reactions.
• Many poisons work as enzyme inhibitors.
• Also, unwanted enzyme activity may be controlled
  by inhibitors.
• Sometimes reversible, sometimes not.
• Heavy metals (lead, mercury) prevent enzymes in
  cells of the nervous system from functioning.
• Cyanide prevents the action on an enzyme in the
  electron transfer chain of respiration
• Not always poison look up what ACE-inhibitors
  are used for!

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Competitive inhibitors

• Structure closely resembles the structure of the
  enzymes normal substrate.
• Takes over the enzymes active site.
• e.g. The antibiotic penicillin inhibits an
  enzyme that bacteria use to make cell walls.

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Non-competitive inhibitors

• Bond to another part of the enzyme molecule, but
  this alters the shape of the active site.
• Hence, substrate can no longer bind to the active
  site.
• Often a way in which unwanted enzyme action is
  controlled.