Week 3 Flashcards
What role do regulatory enzymes play in pathways?
They effect the overall rate of a serious of metabolic reactions
What causes the levels of regulatory enzymes to change?
They change levels in response to transcriptional regulation
When do regulatory enzymes control rates of enzyme-catalyzed reactions?
When more of a substance is needed so the reaction rates increase
Or when a substance is no longer needed so the rate decreases
What ability allows the cell to adapt to its changing needs?
The ability to control the rate of the overall metabolic pathway
In what part of a pathway do regulatory enzymes catalyze a reaction (speed up)?
The rate-limiting or slowest step in the pathway
How does substrate concentration affect the enzyme rate in a pathway?
The more substrate there is the faster the reactions will occur due to the enzyme’s active site being saturated with the abundant amounts of substrate
What are the two types of regulation?
Stimulatory and inhibitory
What are the two ways to mediate regulation?
Control of catalytic efficiency through protein modification (increasing the speed of reaction more efficiently)
Bioavailability (the amount of the enzyme in different tissues and cellular compartments)
What are the 5 ways enzymes are regulated?
Allosteric control (non-covalent)
Having multiple forms of the enzymes (isozymes)
Reversible covalent modification
Proteolytic activation
Controlling the amount of enzyme present (transcriptional level of control)
Where is the allosteric site?
On the enzyme separated from the active site (not competing with substrate)
What does non-covalent binding to the allosteric site cause for the active site?
A conformational change (shape changes) and can increase or decrease the enzymes affinity for the substrate (ability of binding)
What structure protein is an allosteric enzyme (primary, tertiary, etc.)?
Quaternary structures (remember this means there is more than one subunit)
Are allosteric effectors modified when binding to the enzyme’s allosteric site?
No - this is a NON-covalent interaction so no modifications are made - all weak interactions
What happens when an allosteric effector is a homotropic effector?
The enzyme’s substrate is serving as the effector (homo=same, so the substrate and effector are the same)
Are homotropic effectors usually activators or inhibitors?
Usually activators (having the substrate binded in one active site increases the catalytic properties at the other sites (positive cooperation))
What is happening when the allosteric effector is a heterotropic effector?
The effector is different than the substrate (hetero=different or not same)
The binding to the allosteric site has a different effect than the substrate at the active site
Do heterotropic effectors usually serve as allosteric inhibitors or activators?
Inhibitors (important in negative feedback)
What is positive cooperativity?
When the binding of one substrate makes it easier for other substrates to bind to different subunits
What is T conformation?
When there is low affinity for the substrate (taut or tight)
In a conformation that makes it hard for the substrate to fit into the binding site
What is the R conformation?
The relaxed state - high affinity conformation - easy to bind
Do allosteric activators bind more tightly to the R state or the T state?
When the enzyme is in the R or relaxed state (higher affinity for substrate or activator in this case)
In what way do allosteric activators stimulate a reaction?
They increase the amount of enzymes in the active state
Do allosteric inhibitors bind more tightly to the R state or the T state?
The T tight state
Make it even more difficult for substrates and activators to convert the subunit to the most active conformation
How would one overcome the effects of allosteric inhibitors?
By increasing substrate concentration OR the activator concentration
Is the effect of an allosteric effector fast or slow?
Fast (not happening at a transcriptional level but on the surface of the enzyme so easily accessible)
What do isozymes allow for?
Tissue specific regulation
How do isozymes differ from its regular enzyme?
They catalyze the same reaction but have different physical properties and amino acid sequences
How are glucokinase and hexokinase isozymes?
They both are kinases (phosphorylation) but glucokinase has a low affinity for glucose and located in the liver while hexokinase has a high affinity for glucose and is found in the red blood cells, skeletal muscle, and most tissues
(phosphorylation of glucose can trap it inside cells)
What does lactate dehydrogenase do?
Converts pyruvate to lactate in anaerobic conditions (no oxygen available)
Converts lactate to pyruvate when oxygen is available
How many different isozymes exist of LDH?
5 different isozymes that are all tissue specific and differ in structures and kinetic properties
What protein carries out phosphorylation (adding an inorganic phosphate to an enzyme through hydrolysis of ATP to ADP - using energy)?
Protein kinase
What protein carries out dephosphorylation (removing an inorganic phosphate group from an inactive enzyme to activate it)?
Protein phosphatase
What is the role of serine/threonine kinases?
They transfer a phosphate group from ATP to OH group of serine and sometimes threonine on the target enzyme
What is the role of tyrosine kinases?
To transfer a phosphate group to the hydroxyl group of a specific tyrosine residue
How does the binding of phosphate groups affect the active site?
They are negatively charged and bulky so they change the conformation or shape of the active
How would you reverse effect of phosphorylation?
By removing the phosphate group with a phosphatase
What is the main source of phosphate groups for kinases?
ATP
Do phosphorylation and dephosphorylation occur fast or slow?
Fast (seconds)
What kind of interactions does the addition of phosphoryl groups alter?
Electrostatic interactions (it is negative so it alters ionic interactions)
What kind of bonds can a phosphoryl group form?
Hydrogen bonds (it is a strong electronegative atom)
How is glycogen phosphorylase activated?
Phosphorylation of the Ser side chains
How is glycogen phosphorylase inactivated?
Dephosphorylation of Ser residues by phosphorylase phosphatase
What are zymogens?
Inactive precursors of enzymes
How do zymogens become fully functional?
It must undergo proteolytic cleavage
Does proteolytic cleavage require energy?
No (it can occur outside the cell - in blood)
Is proteolytic cleavage reversible?
No it is irreversible regulation
How would you tell if a zymogen was in its inactive form from its name?
if the prefix is “pro-“ or the suffix was “-ogen”
What are 2 inactive zymogens that have to do with blood clotting?
FibrinOGEN and PROthrombin
When would fibrinogen and prothrombin become activated?
When they are cleaved by proteases and proteases are activated by their attachment to a damaged region in the vessel wall
(keeps clot formation to the site of injury
How would you reach maximal capacity of a tissue to change?
Increased protein synthesis
Increased protein degredation
Is enzyme synthesis fast or slow?
Slow (making enzymes - DNA-mRNA-amino acid-folding) takes hours or days
What does cytochrome P450-2EI do?
Oxidizes ethanol to acetaldehyde
What happens to proteins in the skeletal muscle during fasting?
They are degraded in order to increase the levels of amino acids in the blood for gluconeogenisis (creating glucose)
What happens to proteins during an infection?
They are degraded in order to free amino acids to make antibodies and other proteins needed for the immune response
What are 2 pathways that cause protein degredation?
Lysosomal degradation and ubiquitin-proteasome system
How does the ubiquitin-proteasome system work?
By attaching a ubiquitin polypeptide to proteins targeted for degradation by proteasome
What kind of reactions are metabolic pathways?
Sequential reactions (product of one is the substrate for the next and can have branch points)
What is usually the first committed step of a pathway?
The rate limiting step (slowest step and not easily reversible)
How does feedback regulation happen?
When the end product is high, it can bind to the regulatory enzyme of the pathway and inhibit formation of intermediate substrates in a pathway
When the end product is low, the allosteric inhibitor dissociates from the allosteric site allowing the regulatory enzyme to become active again
(too much product- it inhibits, too little product- inhibitor comes off)
Why would a cell compartmentalize an enzyme?
Provide unique conditions
Limit access of enzyme to substrates
Separate anabolic pathways from catabolic pathways (build up from break down)
What are inhibitors and how do they work?
They are substances that decrease the rate of an enzyme-catalyzed reaction by binding to the enzyme
What are reversible inhibitors?
Substances that non-covalently bind to and enzyme and so can be released
Usually similar in structure to the substrates or products
(competitive, uncompetitive, and mixed competitive)
What are irreversible inhibitors?
Substances that cause covalent alterations of the enzyme
(either alters the enzyme itself or irreversibly binds to the enzyme)
PERMANENT SHUT DOWN OF ENZYME
Where do competitive inhibitors bind on the enzyme?
To the active site (they are competing with the substrate for the active site)