Higher Human Biology Subtopic 6 (a) Metabolic Pathways Higher Human Biology Subtopic 6 (a)

Learning Intentions State the difference between an anabolic and catabolic metabolic pathway Discuss reversible, irreversible and alternative pathways in the metabolism Describe the role of enzymes in the control of pathways

Cell Metabolism The collective term for all the biochemical reactions in a living cell. Controlled by enzymes

Two types of metabolic pathways Anabolic - biosynthesis (build up) - requires energy Catabolic breakdown releases energy

Anabolic Pathway glucose-1-phosphate phosphorylase starch This reaction requires the input of energy to proceed.

Catabolic Pathway starch amylase maltose This reaction releases energy. NOTE ATP plays an important role in the transfer of energy between the 2 types of reaction

Steps in the Pathway Glucose A pathway may contain reversible and irreversible steps. e.g. glycolysis in respiration The reversible step is useful if the cell has too much of intermediate 2. It can convert it back to 1 and use 1 in another pathway. Enzyme A Intermediate 1 Glycogen or starch Enzyme B Intermediate 2 Enzyme C Intermediate 3 Many enzymes Pyruvate

Steps in the Pathway Glucose Intermediate 1 Intermediate 2 The irreversible step here keeps the glucose concentration inside the cell LOW so glucose keeps diffusing in. The irreversible step from 2 to 3 is a key regulatory point in the pathway. Enzyme A Intermediate 1 Glycogen or starch Enzyme B Intermediate 2 Enzyme C Intermediate 3 Many enzymes Pyruvate

Alternative Pathway Glucose These allow steps in the pathway to be bypassed. This is used when the cell has plenty of sugar. Enzyme A Alternative route Intermediate 1 Enzyme B Sorbitol Intermediate 2 Enzyme C Many enzymes Intermediate 3 Many enzymes Pyruvate

Cell Metabolism Summary Metabolism describes all biochemical reactions which occur within a cell. Metabolic pathways involve synthesis (building up of molecules) reactions, termed anabolism, and breakdown reactions, termed catabolism. Synthetic pathways require the input of energy whereas break down pathways usually release energy. Some pathways can be reversible, others irreversible. Pathways may also have more than one route.

Enzymes

What is an Enzyme? BIOLOGICAL CATALYST Remain unchanged at the end of the reaction (can be re-used) Increase the rate of a reaction by lowering activation energy Activation energy is the energy needed to break bonds and start the reaction. Without enzymes reactions would be too slow for life!

Enzyme Action Globular proteins. Have an active site determined by the protein structure. Only work on one specific substrate which has a complimentary shape to the active site.

Enzyme Action INDUCED FIT Active site is flexible and can change slightly to fit the substrate very closely. Increases the chance of a reaction due to the close contact. induced fit short film

Induced Fit

Enzyme Action ORIENTATION When a reaction involves 2+ substrates the shape of the active site determines the orientation of the reactants.

Enzyme Action THE REACTION Substrate is held in an induced fit while reaction occurs – this reduces activation energy. After the reaction the affinity is lower so the products are released.

Factors Affecting Enzyme Activity For enzymes to function efficiently they need : Suitable pH Suitable temperature Enough substrate

pH B A C As pH increases to the optimum pH the reaction rate increase. Too acidic or alkaline conditions denature the enzyme. B A C

Temperature Increasing temp (until the optimum) increases reaction rate. High temp breaks bonds and changes the shape of the active site. The enzyme is permanently damaged …….. ….DENATURED The substrate can’t bind.

Substrate Concentration Low Too few substrates to use all active sites Low reaction rate Higher More substrates fill more active sites Increased reaction rate Reaction rate Substrate concentration NOTE : the reaction rate remains constant when all active sites are full. There are more substrate molecules than active sites

Enzyme concentration is the limiting factor

Direct Enzyme Action Complexes Metabolite W Metabolite Z Metabolite Y Metabolic pathways usually include groups of enzymes. The product for one reaction is the substrate for the next. Metabolite Z Enzyme2 Metabolite Y Complexes Enzymes often work in groups – multi enzyme complex e.g. DNA polymerase

Reversibility Metabolite W Metabolite X Too high Metabolite Y An enzyme can catalyse a reaction forward and back. This depends on concentrations of reactants and products. It restores equilibrium. Enzyme 1 Metabolite X Enzyme2 Too high Metabolite Y Enzyme 3 Metabolite Z

Enzymes Summary Globular proteins that speed up a reaction but remain unchanged. Lower activation energy needed. They are specific – active site is complimentary to ONE substrate. Induced fit increases the chance of reaction. Reaction rate is affected by pH, temperature and substrate concentration.

Learning Intentions State the difference between an anabolic and catabolic metabolic pathway Discuss reversible, irreversible and alternative pathways in the metabolism Describe the role of enzymes in the control of pathways