Mcb 450 Exam Ii

Front 1

Modes of Regulation in Enzyme Activity

Back 1

- Most enzymes have rate set by concentration of substrate present
- Increase or decrease in amount of enzyme in cell
--a longer term, genetic, method because synthesis or degradation of protein is involved
--rate of an enzyme is directly proportional to concentration of enzyme (Vmax=k2[E]knot)
-Activation or inhibition of enzymes already present in cells
--allosteric act. or inh., generally in response to hormonal signals
- Covalent modification
--some enzymes may be modified to alter their activity in response to hormonal signals
- Some enzymes have more than one of the above modes of control

Front 2

Zymogen Activity

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- Digestive enzymes are synthesized as a larger, inactive precursor in the exocrine pancreas and are converted into inactive form in small intestine by removal of a polypeptide
- Examples of such: trypsinogen, chymotrypsinogen, procarboxypeptidase, proelastase

Front 3

Allosteric Enzymes

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- Composed of small subunits
- Subunits may be relaxed (R), high affinity, or tense (T), low affinity
- Have activity reversibly modified by allosteric effectors (activators or inhibitors)
- Do NOT obey Michaelis-Menten kinetics. Typically have sigmoidal vs [S] plots. Another example of non-linear inhibition
- 2 models: concerted and sequential
- Serves as control in metabolic pathways
- Catalyze irreversible reactions

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Positive Cooperative Binding

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When binding of first molecule facilitates binding of second molecule

Front 5

Negative Cooperative Binding

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When binding of first molecule inhibits binding of second

Front 6

Concerted Model

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- If one subunit of enzyme is in the Relaxed form (indicated by circles), all subunits must be in the R form
- If one is in the Tense form, all subunits must be in T form
- There is symmetry in the model
- May explain negative cooperative binding

Front 7

Sequential Model

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- There can be a mixture of R and T forms
- Was proposed to explain negative cooperativity, but may be explained by Concerted

Front 8

Kinetics of Allosteric Enzymes

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See lecture

Front 9

Myoglobin Function

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- Serves as an oxygen-storage pigment in muscle
- Abundant in the muscles of whales and other diving mammals, which requires an on-site supply of oxygen during dives

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Myoglobin Structure

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- Exists as a compact globular protein
- 8 helical regions (75% of Mb) with Pro or beta turns typically separating the helical regions
- Hydrophobic amino acid residues are on the interior
- Hydrophillic AA residues are on the exterior where they H bond with water

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Role of Protein in Mb

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- Prevents oxidation of Fe2+ to Fe3+ (Only will reversibly combine with oxygen
- Modulates binding of O2
- Prevents CO poisoning
- Central exon of Mb binds heme
- Responsible for transport of O2 from lungs to peripheral tissues, and CO2 from tissues to lungs

Front 12

Mb Binding Curve

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Curve is a rectangular hyperbola

Front 13

Hemoglobin

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- A tetrameric molecule with its subunits noncovalently associated
- Allosteric protein
- Each subunit has one polypeptide chain, one heme ring, one Fe2+
- Each subunit can therefore bind one O2

Front 14

Hb Binding Curve

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- Sigmoidal
- Binding of oxygen is said to be positively cooperative. Binding of first oxy facilitates binding of second.

Front 15

BPG Binding

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- One BPG can be bound to Hb in central cavity of Hb molecule
- Effect of BPG: "shift dissociation curve to the right"
- Thus, BPG promotes unloading of oxygen
- See lecture

Front 16

Hb Dissociation Curves

Back 16

See Lecture

Front 17

Sickle Cell Disease & Sickle Cell Anemia

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-Disease: serious genetic disorder characterized by the presence of abnormal hemoglobin, HbS.
-Anemia: presence of an abnormal Hb that differed from HbA by having 2 less negative charges per molecule