Metabolism Overview and Enzymatic Reactions

Background Information on Metabolism

  • Metabolism consists of all chemical reactions in living organisms, particularly microbes.

  • Focus lies not just on burning calories or sugars but understanding the entire chemical process.

  • Two main subdivisions of metabolism: Catabolism and Anabolism.

Catabolism

  • Definition: Catabolic processes involve the breakdown of molecules.

    • Involves breaking chemical bonds, specifically those formed by electrons.

    • Energy Release: Breaking bonds releases energy; hence, catabolic processes are known as exergonic processes.

    • Considered as a mechanism for producing energy that can further drive other biological processes.

Anabolism

  • Definition: Anabolic processes involve the building of larger molecules from smaller ones.

    • Requires energy input to construct these larger molecules, thus considered endergonic processes.

    • Example: Building proteins or nucleic acids from amino acids or nucleotides respectively.

  • Metabolically, cell respiration comprises a series of endergonic and exergonic reactions that are coupled together.

    • Energy is invested in anabolic pathways, generating a return on energy by breaking down molecules in catabolic pathways.

Nutritional Classification of Organisms

  • Organisms can be classified based on their nutritional requirements, particularly focusing on:

    1. Energy source

    2. Carbon source

  • Detailed understanding of these classifications will follow later in the curriculum.

Electron Carriers in Metabolism

  • Electron carriers are crucial for transporting electrons during metabolic reactions.

  • Key players:

    1. Nicotinamide adenine dinucleotide (NAD+)

    • When it picks up electrons and protons (H), it is reduced to NADH.

    1. Flavin adenine dinucleotide (FAD+)

    • When it captures electrons and protons, it is reduced to FADH.

  • These participate in oxidation-reduction (redox) reactions, involving the exchange of electrons and protons.

    • Remember the acronym OIL RIG:

    • Oxidation Is Loss (of electrons)

    • Reduction Is Gain (of electrons)

Oxidation and Reduction of Electron Carriers

  • When NAD+ gains electrons and protons, it is said to be reduced, forming NADH.

  • Conversely, when NADH loses electrons and reverts to NAD+, it undergoes oxidation.

Phosphorylation

  • Definition: The process of adding a phosphate group to a molecule, often converting ADP (Adenosine Diphosphate) to ATP (Adenosine Triphosphate).

  • Phosphate bonds in ATP are high-energy bonds, critical for cellular energy processes.

  • Breakdown of ATP by releasing the phosphate group is exergonic, where energy is released for cellular work.

  • Coupling of endergonic (creating ATP) and exergonic (using ATP) reactions enables ongoing cellular processes.

Enzymes in Metabolism

  • Definition: Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required.

  • Each enzyme targets a specific substrate for biochemical reactions, following a lock and key model where:

    • Substrate: the molecule upon which the enzyme acts.

    • Active Site: specific region of the enzyme where substrate binds.

  • Enzymes are recyclable, facilitating multiple reactions without being consumed in the process.

Effects of Environmental Conditions

  • Since enzymes are proteins, their activity can be influenced by:

    • pH levels

    • Temperature

    • Concentration of substrates and products

  • Understanding these factors is pivotal for clinical microbiology, as all cellular reactions rely on enzyme activity.

Enzyme Activation Energy Example

  • Analogy: Rolling a boulder up a hill demonstrates activation energy.

    • Without help, significant energy is needed.

    • With a group (enzyme), less energy is required to achieve the same result (products).

Cofactors and Coenzymes

  • Cofactors: Non-protein components that assist enzymes; often include vitamins and minerals.

    • Apoenzyme: Enzyme without its cofactor.

    • Holoenzyme: Complete enzyme with its cofactor attached, ready to bind substrate.

Enzyme Inhibition

  • Enzyme inhibitors can prevent enzymes from functioning effectively:

    • Competitive Inhibition: Inhibitor competes with the substrate for the active site.

    • Analogy: A physical object (bubble gum) blocks the active site, preventing substrate binding.

    • Non-competitive Inhibition: Inhibitor binds to an alternate site (allosteric site), changing the enzyme's shape and rendering the active site ineffective.

Enzyme Activation

  • Activation can also occur through processes similar to non-competitive inhibition where a substance binds to the allosteric site, enhancing activity:

    • A normally inactive enzyme becomes active upon binding of the activating molecule.

  • This regulation of enzyme activity ensures resources are not wasted when enzymes are produced without necessity.

Feedback Mechanisms

  • Feedback inhibition demonstrates how product levels can regulate enzyme activity in metabolic pathways:

    • Pathway Example: Enzyme 1 converts substrate to Product A, which is further processed by Enzyme 2 into Product B, then through Enzyme 3 to produce an end product.

    • When end product accumulates, it can act as an allosteric inhibitor on the first enzyme, altering its shape and preventing substrate binding.

    • Conversely, low levels of the product keep the enzymes active, promoting continued production.

  • This establishes a self-regulating cycle in metabolic pathways to prevent waste of resources.