chapter 4 Flashcards
Why are enzymes important?
Most of the processes necessary to life involve chemical reactions, and these reactions need to happen very fast.
In the laboratory or in industry this would demand very high temperatures and pressures.
These extreme conditions are not possible in living cells - they would damage the cell components.
Instead, the reactions are catalysed by enzymes.
What are enzymes:
Enzymes are biological catalysts.
They are globular proteins that interact with substrate molecules causing them to react at much faster rates without the need for harsh environmental conditions.
Without enzymes many of the processes necessary to life would not be possible.
The role of enzymes in reactions:
Living organisms need to be built and maintained.
This involves the synthesis of large polymer-based components.
For example, cellulose forms the walls of plants cells and long protein molecules form the contractile filaments of muscles in animals.
The different cell components are synthesised and assembled into cells, which then form tissues, organs, and eventually the whole organism.
The chemical reactions required for growth are anabolic (building up) reactions and they are all catalysed by enzymes.
Catabolic reactions:
Energy is released from large organic molecules, like glucose, in metabolic pathways consisting of many catabolic (breaking down) reactions.
Catabolic reactions are also catalysed by enzymes.
These large organic molecules are obtained from the digestion of food, made up of even larger organic molecules, like starch.
Digestion is also catalysed by a range of enzymes.
Reactions rarely happen in isolation but as part of multi-step pathways.
Metabolism:
Metabolism is the sum of all of the different reactions and reaction pathways happening in a cell or an organism, and it can only happen as a result of the control and order imposed by enzymes.
Speed of cellular reactions depends on:
dependent on environmental conditions.
The temperature, pressure, and pH may all have an effect on the rate of a chemical reaction.
Enzymes can only increase the rates of reaction up to a certain point called the Vmax (maximum initial velocity or rate of the enzyme-catalysed reaction).
How enzymes work:
Molecules in a solution move and collide randomly.
For a reaction to happen, molecules need to collide in the right orientation.
When high temperatures and pressures are applied the speed of the molecules will increase, therefore so will the number of successful collisions and the overall rate of reaction.
specificity of the enzyme:
Many different enzymes are produced by living organisms, as each enzyme catalyses one biochemical reaction, of which there are thousands in any given cell.
This is termed the specificity of the enzyme.
activation energy:
The energy that’s needed to be supplied for most reactions to start
Sometimes, the amount of energy needed is so large it prevents the reaction from happening under normal conditions.
Enzymes help the molecules collide successfully, and therefore reduce the activation energy required.
There are two hypotheses for how enzymes do this.
Lock and key hypothesis:
An area within the tertiary structure of the enzyme has a shape that is complementary to the shape of a specific substrate molecule.
This area is called the active site.
In the same way that only the right key will fit into a lock, only a specific substrate will fit the active site of an enzyme - This is the lock and key hypothesis.
When the substrate is bound to the active site an enzyme-substrate complex is formed.
The substrate or substrates then react and the product or products are formed in an enzyme-product complex.
The product or products are then released, leaving the enzyme unchanged and able to take part in subsequent reactions.
Lock and key hypothesis chemical formation:
The substrate is held in such a way by the enzyme that the right atom-groups are close enough to react.
The R-groups within the active site of the enzyme will also interact with the substrate, forming temporary bonds.
These put strain on the bonds within the substrate, which also helps the reaction along.
lock and key image
Induced-fit hypothesis:
More recently, evidence from research into enzyme action suggests the active site of the enzyme actually changes shape slightly as the substrate enters.
This is called the induced-fit hypothesis and is a modified version of the lock and key hypothesis.
Induced-fit hypothesis chemical formation:
The initial interaction between the enzyme and substrate is relatively weak, but these weak interactions rapidly induce changes in the enzyme’s tertiary structure that strengthen binding, putting strain on the substrate molecule.
This can weaken a particular bond or bonds in the substrate, therefore lowering the activation energy for the reaction.
What is an Intracellular enzyme
Enzymes that act within cells
Intracellular enzyme example:
Hydrogen peroxide is a toxic product of many metabolic pathways. The enzyme catalase ensures hydrogen peroxide is broken down to oxygen and water quickly, therefore preventing its accumulation.
It is found in both plant and animal tissues.
Cellular Reactions and Nutrient Supply:
All of the reactions happening within cells need substrates (raw materials) to make products needed by the organism.
These raw materials need to be constantly supplied to cells to keep up with the demand.
Nutrients (components necessary for survival and growth) present in the diet or environment of the organism supply these materials.
Nutrients are often in the form of polymers such as proteins and polysaccharides.
These large molecules cannot enter cells directly through the cell-surface membrane.
They need to be broken down into smaller components first.
Enzymes are released from cells to break down these large nutrient molecules into smaller molecules in the process of digestion.
These enzymes are called extracellular enzymes.
What are extracellular enzymes:
Enzymes that work outside the cell that made them.
In some organisms, for example fungi, they work outside the body.
Both single-celled and multicellular organisms rely on extracellular enzymes to make use of polymers for nutrition.
Single-celled organisms, such as bacteria and yeast, release enzymes into their immediate environment.
extracellular enzymes examples:
The enzymes break down larger molecules, such as proteins, and the smaller molecules produced, such as amino acids and glucose, are then absorbed by the cells.
Many multicellular organisms eat food to gain nutrients.
Although the nutrients are taken into the digestive system the large molecules still have to be digested so smaller molecules can be absorbed into the bloodstream.
From there they are transported around the body to be used as substrates in cellular reactions.
Examples of extracellular enzymes involved in digestion in humans are amylase and trypsin.
Digestion of starch:
The digestion of starch begins in the mouth and continues in the small intestine. Starch is digested in two steps, involving two different enzymes.
Different enzymes are needed because each enzyme only catalyses one specific reaction.
Starch polymers are partially broken down into maltose, which is a disaccharide. The enzyme involved in this stage is called amylase.
Amylase is produced by the salivary glands and the pancreas.
It is released in saliva into the mouth, and in pancreatic juice into the small intestine.
Maltose is then broken down into glucose, which is a monosaccharide.
The enzyme involved in this stage is called maltase. Maltase is present in the small intestine.
Glucose is small enough to be absorbed by the cells lining the digestive system and subsequently absorbed into the bloodstream.
diagram of human digestive system
Digestion of proteins:
Trypsin is a protease, a type of enzyme that catalyses the digestion of proteins into smaller peptides, which can then be broken down further into amino acids by other proteases.
Trypsin is produced in the pancreas and released with the pancreatic juice into the small intestine, where it acts on proteins.
The amino acids that are produced by the action of proteases are absorbed by the cells lining the digestive system and then absorbed into the bloodstream.
Factors Influencing Interaction with Substrates and Investigative Approaches:
For enzymes to catalyse a reaction, they must come into contact with the substrate, and the enzyme must be the right shape (complementary) for the substrate.
Enzymes are complex proteins and their structure can be affected by factors such as temperature and pH.
These can cause changes in the shape of their active site.
Enzymes are more likely to come into contact with the substrate if temperature and substrate concentration are increased.
Factors affecting enzyme action can be investigated by measuring the rate of the reactions they catalyse.
Temperature:
Increasing the temperature of a reaction environment increases the kinetic energy of the particles.
As temperature increases, the particles move faster and collide more frequently.
In an enzyme-controlled reaction an increase in temperature will result in more frequent successful collisions between substrate and enzyme.
This leads to an increase in the rate of reaction.
