Microbial Metabolism Enzymes

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What is metabolism?

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The sum of chemical reactions within a living organism

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Catabolism

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-The breakdown of complex organic molecules into simpler molecules
-Generally hydrolytic
-exergonic (produce energy)
-energy stored in chemical bonds is released

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Anabolism

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-The synthesis of complex organic molecules from simpler molecules
-Generally dehydration synthesis reactions (release water)
-Endergonic (consume energy)

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Role of ATP in metabolism

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ATP (adenosine triphosphate) stores the energy generated by catabolic reactions and makes it available for anabolic reactions

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Enzymes-the driving force of metabolic reactions

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An enzyme is a biological catalyst that speeds up chemical reactions is the cell (but is not consumed in the reaction)
Specific for a particular substrate and reaction
The unique three-dimensional shape of an enzyme allows it recognize its substrate
Very efficient-can increase the rate of a chemical reaction 108-1010 times
Enable metabolic reactions to proceed at a speed compatible with life

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How do enzymes work? (Insight 8.1)

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By increasing the activation energy, the energy required to initiate a chemical reaction
Enzymes act as a physical site on which reactants or substrates can be positioned for various interactions
Enzymes present a unique active site which fits only a particular substrate

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Turnover number

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Enzymes participate in chemical reactions but are not consumed by them (can function over and over again)
An enzymes speed or turnover number is the maximum number of substrate molecules an enzyme molecule can convert to product each second
Enzyme speeds can range over several orders of magnitude but are characteristic of a particular enzyme

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Enzyme components (Figure 8.2)

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Simple enzymes-entirely protein
Conjugated enzymes consist of;
Apoenzyme-the protein component
Cofactor-non protein component e.g., Mg2+ or Ca2+ ions. If cofactor is an organic molecule it is called a coenzyme
Apoenzyme + cofactor = Holoenzyme
In the absence of the cofactor, the apoenzyme is inactive

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Coenzymes (Figure 8.5)

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Can act in catalysis by accepting a chemical group from one substrate and transferring it to another substrate
Some act as electron carriers
Many are derived from vitamins

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Important coenzymes in cellular metabolism

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Nicotinamide adenine dinucleotide (NAD+)
Nicotinamide adenine dinucleotide phosphate (NADP+)
Derivatives of vitamin B1 (niacin)
Function as electron carriers
NAD+ involved in catabolic reactions
NADP+ involved in anabolic reactions

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The flavin coenzymes

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-Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD)
-Derivatives of vitamin B2 (riboflavin)
-Also act as electron carriers

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Coenzyme A

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-Derivative of vitamin B5 (pantothenic acid)
-Important roles in fat metabolism and the TCA cycle

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Naming enzymes (table)
Enzyme names usually end in -ase

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Oxidoreductase: Oxidation reduction (redox) reactions
Transferase: Transfer of functional groups, such as phosphate or amino groups
Hydrolase: Cleaves bonds on molecules with the addition of water (hydrolysis)
Lyase: Removal or addition of groups of atoms without hydrolysis
Isomerase: Rearrangement of atoms within a molecule
Ligase: Joining two molecules

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Factors affecting enzymatic activity

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-temperature
-pH
-substrate concentration

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Exoenzymes

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Usually active only outside the cell
Breakdown of nutrients that are too large to enter the cell. e.g., lipases and proteases
Some play a role in disease e.g., Streptokinase and phospholipase C

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Endoenzymes

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Most metabolic enzymes are endoenzymes

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Control of metabolic pathways (Figure 8.9)

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Metabolic pathways are controlled at the level of their enzymes
Control of enzymes
-Synthesis
-Activity

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Production of enzymes in the cell (Figure 8.7)

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Enzymes can be produced at constant levels in the cell OR
Their production can be regulated in response to substrate or product concentrations. Occurs at the genetic level

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Enzyme Inhibitors

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An effective way to control the growth of bacteria is to control their enzymes
Certain poisons such as cyanide, arsenic and mercury combine with enzymes and inhibit their activity
Enzyme inhibitors can be classed as
-Competitive inhibitors
-Noncompetitive inhibitors

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Competitive inhibitors (Figure 8.10)

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Fill the active site and compete with substrate
Similar in shape and chemical structure to the substrate
Does not undergo any reaction to form products
May bind reversibly or irreversibly.

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Noncompetitive inhibitors (Figure 8.10)

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Interact with a site other than the active site (allosteric or regulatory site)
Binding of the inhibitor causes a change in the shape of the active site, making it nonfunctional (allosteric inhibition)
May bind reversibly or irreversibly

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Feedback inhibition

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The end-product of the reaction allosterically inhibits one the enzymes in the pathway, often the first enzyme
Prevents the cell from wasting energy

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Ribozymes-unconventional enzymes

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Unique type of RNA
Function as catalysts
Specifically act on strands of RNA by removing section and splicing them together
Only act on RNA
Ancient molecule?