Enzymes and Coenzymes: Regulation of Enzyme Action

• Block C: Enzymes and Coenzymes, Lesson 7 is about the Regulation of Enzyme Action
• Regulation of enzymatic activity ensures substrate consumption/product synthesis does not exceed the needs of cells
• The product of a reaction becomes the substrate of the next reaction

Metabolic Pathways

• Control of a pathway is dictated by the rate-determining steps
• Rate determining steps can be one of the first reactions, or in the ramifications
• An irreversible (or nearly so) reaction is a rate determining step
• The slowest reaction of the pathway is the rate determining step

Types of Regulation

• Availability of substrate
• Regulation at enzymatic levels
• Regulation of enzyme activity at limiting steps

Availability of Substrate

• Enzymes depend on the availability of the substrate
• Types of enzyme availability include;
• Passive diffusion,
• Active transport,
• Facilitated diffusion
• Isoenzymes

Enzymatic Levels

• Enzyme activity depends on its concentration, and involves a balance between synthesis and degradation, referred to as long term regulation
• Constitutive enzymes are synthesized at a constant rate
• The synthesis/degradation rate of Inducible enzymes depends on modulators like diet, hormones, ligands, and metabolites

Activity of Enzymes From Limiting Step

• Activity of enzymes from the limiting step is short term regulation, as opposed to long term regulation with enzyme availability and enzymatic levels
• Types of regulation from the limiting step involve:
• Allosteric regulation
• Covalent modification

Allosteric Regulation

• This is a reversible noncovalent bond that increases or decreases the activity of the allosteric enzyme
• Allosteric enzymes are oligomeric
• Allosteric enzymes have more than one active and catalytic site
• Exhibit sigmoidal kinetics
• Their speed is regulated by allosteric modulators bound by noncovalent bonds
• Homoallosterism: Homotropic effects
• Involves the linking of substrates, making it cooperative
• When a substrate binds to the first subunit of the enzyme, there is a conformational change that affects the other subunits, making them more accessible. The R state is then achieved
• Heteroallosterism: Heterotropic effects:
• The binding of allosteric effectors to the allosteric site modifies the affinity of the enzyme for its substrate
• Allosteric effectors/modulators: They Modify T/R equilibrium
• Activators stabilize R State
• Inhibitors stabilize T State
• Can also involve feedback inhibition
• The end product ultimately slows the entire pathway

Covalent Modification

• Type of regulation from limiting step
• Involves reversible or irreversible covalent modification

Reversible Covalent Modification

• Interconvertible enzymes undergo modification by the addition of a chemical group by covalent bond, shifting between an active to inactive form and vice versa
• These enzymes appear in two different forms:
• a form: Higher activity, with or without the chemical group.
• b form: Less active form.
• The modification of the interconvertible enzyme is done by other enzymes that are called converter enzymes which are of two types: one adds the chemical group and the other releases it
• e.g. Phosphorylation/dephosphorylation
• Phosphorylation = binding of a P to the R group of one of the aa (Kinases)
• Dephosphorylation = cleavage of P group (Phosphatases)

Irreversible Covalent Modification

• These enzymes are synthesized as inactive precursors, and are active by proteolysis, called Zymogens or proenzymes
• After cleavage of the molecule, they are converted into their active form

Enzymes and Coenzymes

• Types of enzymes and coenzymes include holoenzymes, apoenzymes, and cofactors

Cofactors

• Cofactors can be inorganic (metal cofactors) or organic (coenzymes)
• Metal cofactors:
• Metaloenzymes
• involved in the catalytic process
• Act stabilizing complex ES
• Act stabilizing active site
• Coenzymes
• Thermostable
• Specificity of reaction (ONLY)
• Chemically modified
• Regenerated to the initial state

Vitamin Coenzymes

• Vitamin B5 (Pantothenic acid)'s active coenzyme is Coenzyme A, involved in acyl group transfer
• Vitamin B8 (Biotin)'s active coenzyme is Biotin-enzyme, involved in CO2 transfer
• Vitamin B1 (Thiamine)'s active coenzyme is Thiamine pyrophosphate (TPP), involved in aldehyde transfer
• Vitamin B6 (Pyridoxal, Pyridoxin, Pyridoxamine)'s active coenzyme is Pyridoxal phosphate (PLP), Pyridoxamin phosphate (PMP), involved in amino group transfer
• Vitamin B12 (Cobalamin)'s active coenzyme is Cob12, methyl cobalamin, desoxyadenosyl cobalamin, involved in methyl group transfer
• Folic acid's active coenzyme is Tetrahydrofolic acid (THF), involved in one-C fragments transfer
• Vitamin B3 (Nicotinamide)'s active coenzyme is Pirimidin nucleotides (NAD, NADP), involved in electron transfer
• Vitamin B2 (Riboflavin)'s active coenzyme is Flavin nucleotides (FMN, FAD), involved in electron transfer
• Vitamin C (Ascorbic acid)'s active coenzyme is Ascorbic acid/Ascorbate, involved in electron transfer

Non-Vitamin Coenzymes

• S adenosyl methionine is involved in methyl group transfer
• Triphosphate nucleosides (NTP) is involved in phosphate transfer
• Lipoic and hydrolipoic acid is involved in acyl and electron transfers
• Coenzyme Q (Ubiquinone) is involved in electron transfer
• Tetrahydrobiopterin is involved in electron transfer