Animal Physiology Chapter 2

Part 2

Enzymes Catalyze Rxns in Both Directions

The principles of mass action: if compounds are out of equilibrium, the rxn will proceed in the direction of equilibrium as dictated by the ratios of concentrations

A+B↔C+D rxn will proceed towards reactants with the

lowest concentrations

Enzymes Catalyze Rxns in Both Directions

The catalytic behavior of an enzyme may be different depending on which way the rxn proceeds due to the substrate affinity

In a cell a rxn might always proceed the same directions since substrates might be continually replaced

Enzymes Occur in Several Forms

The catalytic behavior of an enzyme may be different depending on which way the rxn proceeds due to the substrate affinity

In a cell a rxn might always proceed the same directions since substrates might be continually replaced

Isozymes and interspecific enzyme homologs

Isozymes: different enzyme forms in produced by a single species

Interspecific enzyme homologs: different enzyme forms from homologous gene loci in different species

May be functionally and catalytically different but they catalyze the same rxn may be adaptive differences

One species of the thornyhead rock fish lives at 500+m, one lives 500-1500m

Affinity of LDH to its substrate decreases with water pressure adapted a homolog of LDH that is less sensitive

Regulation of Cell Function by Enzymes: metabolic pathways

in a cell require enzymes

1. Types and amounts of enzymes present determine which metabolic pathways are fxn’l in a cell

2. The catalytic activities of the enzymes present can be modulated to affect cellular metabolic pathways

Regulating Enzymes Present by Gene Expression

1. Transcription of DNA coding for an enzyme to form pre-messenger RNA

2. Posttranscriptional processing to form mature mRNA

3. Exit of the mRNA from the nucleus to the ribosomes

4. Translation of the mRNA into a.a.’s 5. Posttranslational processing of a.a.’s to a

mature protein

Regulating Enzymes Present by Gene Expression

1. Transcription of DNA coding for an enzyme to form pre-messenger RNA

2. Posttranscriptional processing to form mature mRNA

3. Exit of the mRNA from the nucleus to the ribosomes

4. Translation of the mRNA into a.a.’s 5. Posttranslational processing of a.a.’s to a

mature protein

Regulating Enzymes Present by Gene Expression

Binding of transcription factors to enhancers and promoters controls the ability of RNA polymerase to transcribe the DNA

-Constitutive enzymes: present in a tissue in relatively high and steady amounts-Inducible enzymes: present at low levels or not at all until induced (i.e. cytochrome P450 enzymes in the liver, kidneys and g.i. tract)

1. Types and amounts of enzymes present determine which metabolic pathways are fxn’l in a cell

2. The catalytic activities of the enzymes present can be modulated to affect cellular metabolic pathways

Modulation of Existing Enzyme Molecules Permits Fast Regulation

of Cell Function

Enzymes that catalyze rate-limiting reactions and branch-point reactions are well positioned to exert control over metabolism

An allosteric modulator follows the principles of mass action in binding with the enzyme it modulates – very rapid

Nonsubstrate molecules bind noncovalently with allosteric sites to affect catalytic activity of the enzyme. Upregulated or downregulated.

Can affect the whole pathway if the allosterically regulated enzyme is also rate limiting.

Phosphofructokinase, an allosterically modulated enzyme, is a key regulatory enzyme for glycolysis

Covalent Modulation of Existing Enzymes

• Chemical rxn’s that make or break covalent bonds between modulator and enzymes\

• Requires the action of enzymes to catalyze the making or breaking of covalent bonds– Phosphorylation by protein kinases (specific to the

enzymes they modulate)– Dephosphorylation by protein phosphatases

Protein kinases often function in multi-enzyme sequences that bring about amplifying effects

Evolution of Enzymes

1. Evolutionary relationships of multiple enzyme forms found in sets of related species

2. Evolution of allele frequencies within a single species

Figure 2.21 Enzymes and other proteins can be arranged into family trees based on their amino acid sequences (Part 1)

Figure 2.21 Enzymes and other proteins can be arranged into family trees based on their amino acid sequences (Part 2)

A single LDH gene duplicated (made 2) at the diamonds

Evolution of Enzymes

1. Evolutionary relationships of multiple enzyme forms found in sets of related species

2. Evolution of allele frequencies within a single species – dynamic and on short time scales, reflect present day action of natural selection

Contemporary evolution of lactate dehydrogenase B alleles in the killifish

Death of Enzymes and Proteins

The ubiquitin–proteasome system tags proteins and then inevitably destroys those that are tagged

Ubiquitin tags are added to a protein (ubiquitination) catalyzed by E1 and E2 enzymes

Polyubiquitination leads the proteasome (also an enzyme) to recognize to break up the protein/enzyme to peptides

Peptidases further break down to a.a.’s

Enzymes play a role in light and color

Reception and Use of Signals by Cells

Extracellular signals initiate their effects by binding to receptor proteins

-Ligand gated channels-G protein-coupled receptors-Enzyme linked receptors-Intracellular receptors

Ligand-gated channel

Acts as receptor and channel

Opens to permit inorganic ions to pass through when they bind their ligands. Ions alter the electrical charge across the membrane.

Common in nerve and muscle cell signaling

Fish-hunting cone snail

G protein-coupled receptor and associated G protein system

-Binds ligand then interacts with 2 other cell membrane proteins to activate intracellular enzyme catalytic sites.-The catalytic activity of the enzyme produces cyclic AMP or other 2nd messengers in the cell.-No chemicals pass through the cell membrane.-1st messengers (i.e. hormones & neurotransmitters) bring the message to the outside of the cell. 2nd messengers carry signal to the inside.

Enzyme/enzyme-linked receptor

1. Structurally and functionally diverse

2. Are enzymes themselves or interact directly with enzymes

3. Binds a ligand4. Activates a catalytic

site on the same molecule

5. Cause production of 2nd messengers (i.e. cyclic GMP)

Intracellular receptor

Not localized at cell surface

Ligand (i.e. steroid hormones, thyroid hormones, retinoic acid, vitamin D, nitric oxide gas) diffuses into the cell.

Complexes with a receptor in the cell and often activate transcription.