Cell processes

Enzyme activity

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

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.

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

Protein synthesis from DNA

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!

Two main types of metabolic reactions

• Synthesis of large molecules from smaller molecules – anabolic reactionse.g. glucose molecules into starch

• Breakdown from larger molecules into smaller molecules – catabolic reactionse.g. food protein into amino acids for making other proteins

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

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.

• 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.

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.

Two slightly varying models of enzyme action

• 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.

Factors that affect enzyme activity

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

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?

• 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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Temperature (in °C)

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)

Effect of pH on enzyme acitivity

pH scale

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pH < 7 is acidicpH = 7 is neutralpH > 7 is basic

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.

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”)

Co-factors & Co-enzymes

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!

Competitive inhibitors• Structure closely

resembles the structure of the enzyme’s normal substrate.

• Takes over the enzymes active site.

e.g. The antibiotic penicillin inhibits an enzyme that bacteria use to make cell walls.

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.