Physio: Chapter 4: Enzymes and Energy

Read this list of ways that physiological enzyme activity is tightly controlled.

-Temperature -pH -cofactors -coenzymes -zymogens -phosphorylation -substrate concentration -reversibility -tissue-specific expression

List two models of enzyme active site.

1. Lock and Key 2. Induced Fit

Describe each enzyme active site model.

1. Lock and Key: E and S will fit together like a lock and key (or like pieces of a puzzle) 2. Induced Fit Model: E and S interaction causes a CONFORMATIONAL CHANGE in the active site, inducing the binding of the substrate

Why is end product inhibition/negative feedback inhibition important?

1. Prevents over accumulation of final product 2. Can shift the pathway to production of another intermediate.

Why would the body want to store something like a zymogen that is inactive? (Another way to ask this is... why wouldn't the body just go straight to the active form?)

2 reasons: 1. Inactive form is more chemically stable, so it'll last longer. 2. When something is stored inactively, you don't have to wait to make it when it is needed. Instead, you just activate the inactive form (it's a time thing).

Most physiological enzymes have an optimum temperature at or near ___ degrees C. Why?

37. Body temp

At a temperature higher than __ degrees C, most enzymes are inactivated. Why?

40. The tertiary structure (of the active site) gets destabilized at a high temp, preventing proper functioning of the enzyme

Describe how an isoform can aid in diagnosis.

A tissue specific isoform would be released by damaged or diseased tissue. Monoclonal antibodies can be used against the isoforms to help find diseased portions.

Describe allosteric inhibition

Allosteric inhibition is when an endproduct will inhibit an enzyme by binding to a site on the enzyme SEPARATE from the active site. This binding induces a conformational change in the enzyme, inhibiting the activity. The inhibition is directed by the concentration of the allosteric inhibitor. The allosteric binding site is not active unless the concentration of product exceeds a certain level.

Give an example of an enzyme that catalyzes both a forward and reverse reaction.

Carbonic anhydrase catalyzes the breakdown of carbonic acid. Think of Le Chatelier's Principle from chemistry. Increasing or decreasing either side of the equation will result in either reactants or products being favored. They just call it Law of Mass Action in the powerpoint. H2Co3 <---> H2O + CO2

______ are organic molecules that comprise a subcategory of cofactors. They are derived from water soluble vitamins like niacin, riboflavin, and pyridoxine. What is their function?

Coenzymes. They transport small molecules like H atom to the active site of the enzyme. They may or may not change the active site conformation.

Divalent metal ions like Ca2+, Mg2+, Zn2+, Mn2+, Cu2+, and selenium are all examples of ___________. What is their function?

Cofactors. They make conformational changes in the active site of the enzyme to enhance the formation of the ES complex.

Enzymes act by (increasing/decreasing) the activation energy needed for a chemical reaction to occur.

Decreasing

Cells use ____ to increase the speed of chemical reactions.

Enzymes

Give an example of an isoform and describe it.

Example: Creatine Phosphokinase (CPK, CK) isoforms exist as MM (skeletal muscle), BB (Brain), and MB (Heart). CPK will do the same thing in the different tissues, but the amino acid sequence will be different depending on which tissue it's found in, which helps in differentiation during diagnosis.

(T/F) All coenzymes are cofactors and vice versa.

False. All coenzymes are cofactors (remember that coenzymes are a subgroup of cofactors), but not all cofactors are coenzymes.

(T/F) pH optima of purified enzymes in a test tube = the physiologically relevant pH.

False. The pH optima of the enzymes in the test tube doesn't necessarily match the physiologically relevant pH.

(T/F) Catalysts... -Increase the rate of reactions -Are changed by the reaction -Do not change the nature of the reaction or the final result (the same reaction would have occurred in absence of enzyme but at a slower rate)

False. All of the statements are good except for the second one. Catalysts remain UNCHANGED by the reaction.

(T/F) Enzymes are always proteins.

False. Enzymes are generally proteins, except for ribozymes, which is an RNA molecule capable of acting as an enzyme

The ability to maintain bodily conditions under different environmental stressors is also known as _______.

Homeostasis

pH optima of enzymes spans the pH scale. Give two examples of enzymes that have different pHs discussed in class.

Pepsin: pH 2 Trypsin: pH 8

Give an example of a zymogen.

Pepsinogen (inactive), which aids in digestion, gets cleaved to form pepsin (active)

_______ will gremove phosphate groups.

Phosphatases

Describe how inborn errors of metabolism occur and their consequences.

Separate genes control separate enzymes, so if there is a genetic defect or mutation, it would result in an inborn error of metabolism. Intermediates upstream of the defect would accumulate, whereas intermediates downstream of the defect would decrease. An over or under accumulation of these intermediates would cause a disease state. An example of this is lactose intolerance, where lactose intermediate is accumulated.

This is the molecule or reactant the enzyme changes.

Substrate

Describe end product inhibition (aka negative feedback inhibition).

The concentration of the final product in the pathway will inhibit an enzyme earlier in the sequence.

Describe the mechanism by which the enzyme decreases activation energy.

The enzyme will bring the reactants into close proximity at the active site and get them to react. The formation of the ES complex will have more energy than the individual reactants had.

Make sure to understand the cascade picture on slide 18.

The picture shows a complex, yet common phosphorylation cascade. 1. PKC (phosphorylation kinase C) phosphorylates, and therefore activates, Kinase 1. 2. Kinase 1 phosphorylates inactive phosphatase to activate it. 3. Phosphatase will take away the phosphate from Enz2 (which is not active while phosphorylated) and therefore activate Enz2

What are zymogens?

They are enzymes produced in the inactive form that become activated through proteolytic cleavage.

What are isoforms?

They are enzymes that are the same, but that contain tissue specific amino acid sequences.

As substrate concentration increases, enzyme activity will also increase and eventually lead to a plateau. Why does the enzyme activity plateau?

This is the point at which the enzyme is saturated. No further increase in substrate concentration can change the reaction rate.

(T/F) An enzyme is relatively specific for a substrate.

True. This means that although many enzymes catalyze only one reaction, there ARE enzymes that can catalyze more than one kind of reaction. An example of this is enzymes that can catalyze both forward and reverse reactions.

Phosphorylation (the addition of a phosphate group) is carried out by _____. What does adding a P group do?

protein kinases will phosphorylate. A P group can... - activate or deactivate enzymes -increase or decrease activity

The active site of an enzyme is made of this kind of folding (primary, secondary, etc.)

tertiary. The active site is 3D in nature.