33 第九章 代谢调节.ppt

第1页：Chapter 9 开篇

第2页：Specificity Is the Result of Molecular Recognition

9.1•Specificity Is the Result of Molecular Recognition “Induced Fit” and the Transition-State Intermediate

第3页：Key and lock示意图

第4页：Key and lock Induce

Key and lockInduce fit

第5页：The induced

The induced conformational change in hexokinaseM11.8Induced fit is the main model for changes in the enzyme to fit the transition state and to bring functional side groups of the enzyme to the right location to help the catalysis.

第6页：Controls Over

8.2 •Controls Over Enzymatic Activity—General Considerations

第7页：Regulatory

• Regulatory strategies
• Product accumulates(The enzymatic rate “slows down” as product accumulates and equilibrium is approached);
• Substrates and cofactors(The availability of substrates and cofactors will determine the enzymatic reaction rate);
• Allosteric control(aspartate transcarbamoylase, hemoglobin);
• Multiple forms of enzymes(lactate dehydrogenase);
• Reversible covalent modification(e.g.phosphorylation/dephosphorylation, protein kinase A);

第8页：Proteolytic

• Proteolytic activation(proteases involved in digestion, blood clotting).
• Regulation at protein level often allows a faster response to changes in cellular conditions (substrate concentrations, hormone action, etc.) than regulation of gene expression.
• Changes of 3D-structures of proteins are pivotal in many mechanisms of regulation at protein level.Changes in structure regulate binding of substrates (e.g. formation of active sites in enzymes) or interactions with other proteins (e.g.covalent modifications like phosphorylation in signal transduction).

第9页：Specialized controls

Specialized controls: Enzyme regulation is an important matter to cells, and evolution has provided a variety of additional options, including zymogens, isozymes, and modulator proteins

第10页：zymogens Most

zymogensMost proteins become fully active as their synthesis is completed and they spontaneously fold into their native, three-dimensional conformations. Some proteins, however, are synthesized as inactive precursors, called zymogensor proenzymes,that only acquire full activity upon specific proteolytic cleavage of one or several of their peptide bonds.

第11页：Proteolytic

• Proteolytic activation
• Many proteins/enzymes are made as inactive precursors that are activated by proteolytic cleavage: after this cleavage the enzymes fold into their catalytically active conformation.
• Inactive precursors are called proenzymesor zymogens.
• Proteolytic activation is irreversible -> a different mechanism is required for inactivation of the enzyme.
• Examples: blood clotting, digestive proteases, peptide hormones (insulin), proteins involved in developmental processes (collagen), apoptosis (programmed cell death; caspases) and virus proteases (cuts multidomain viral proteins into their active forms, see 9.1.7).

第12页：Example 1

Example 1

digestive proteasesProteases involved in digestion of proteins in the stomach and small intestine are synthesized as inactive precursors (pancreas). They are activated by proteolytic digestion after secretion into the lumen of the intestine.

第13页：Chymotrypsin.

• Chymotrypsin.
• Made as an inactive precursor (chymotrypsinogen) in the pancreas.
• Activation by cleavage of the peptide bond between Arg-15 and Ile-16 with trypsin -> -chymotrypsin; cleavage results in a small conformational change with large consequences !
• -Chymotrypsin removes 2 dipeptides (residues 14-15 and 147-148) from other -chymotrypsin molecules -> -chymotrypsin. Activity does not change after this second proteolytic step: -chymotrypsin is already fully active.

第14页：Insulin.Some protein

Insulin. Some protein hormones are synthesized in the form of inactive precursor molecules, from which the active hormone is derived by proteolysis. For instance, insulin,an important metabolic regulator, is generated by proteolytic excision of a specific peptide from proinsulin

第15页：Isozymes LDH(lactate

IsozymesLDH (lactate dehydrogenase) isozymesdiffer in theirallosteric regulation by pyruvate.

第16页：The kinetic

The kinetic properties of the various LDH isozymes differ in terms of their relative affinities for the various substrates and their sensitivity to inhibition by product. Different tissues express different isozyme forms, as appropriate to their particular metabolic needs. By regulating the relative amounts of A and B subunits they synthesize, the cells of various tissues control which isozymic forms are likely to assemble, and, thus, which kinetic parameters prevail.

第17页：Example 2

Example 2

Blood clottingBlood clotting results from a cascade of reactions. In a cascade, a signal initiates a series of steps, each of them catalyzed by an enzyme. At each step the signal is amplified.In blood clotting the activated form of one clotting factor catalyzes activation of the next. Very small amounts of the initial factors trigger the cascade, => rapid response to trauma (e.g., damage to a blood vessel).

第18页：Blood clotting

Blood clottingFibrin fiber

第19页：Conversion of

Conversion of fibrinogen to fibrin causes clotting.The final step of clotting is conversion of fibrinogen to fibrin by thrombin, a protease.Fibrinogen has 6 protein chains (2x A, Band ), folded into globular units connected by rods. Thrombin cleaves 4 peptides from the Aand Bchains in the central globule, resulting in fibrin monomer ()2.

第20页：Carboxyl ends of the

Carboxyl ends of the -and chains interact with the newly exposed N-terminal regions => polymerization (protofibrils).

第21页：Fibrils are

Fibrils are stabilized by cross-linking: formation of amide bonds between lysine and glutamine by transglutaminase, which is activated from protransglutaminase by thrombin.The network of fibrils forms the clot.

第22页：Activation of

Activation of thrombin.Thrombin activates fibrinogen, but how is thrombin activated ?Thrombin is activated by proteolytic activation of prothrombin with factors Xa(also a protease) and Va. Activation removes a glaand 2 kringledomains.Modular structure of prothrombin

第23页：Modulator Proteins

Modulator Proteins Modulator proteinsare proteins that bind to enzymes, and by binding, influence the activity of the enzyme.Cyclic AMp-dependent protein kinase is a 150-to 170-kD R2C2tetramer in mammalian cells. The two R (regulatory) subunits bind cAMP (KD= 3 x 10-8M); cAMP binding releases the R subunits from the C (catalytic) subunits. C subunits are enzymatically active as monomersactiveinactive

第24页：Covalent

Covalent modification(Reversible) covalent modification alters properties of proteins (for example: enzyme activity).

第25页：Some modifications

• Some modifications are reversible(e.g. phosphorylation by kinases dephosphorylation by phosphatases).
• Others are irreversible:
• signal-transduction proteins like Ras and Src are attached to the plasma membrane by irreversible attachment of a lipid
• irreversible attachment of ubiquitin to a protein is a signal that the protein should be destroyed
• proteolysis is always irreversibleAll metabolic pathways are partially regulated by covalent modifications.

第26页：Many enzymes/

Many enzymes/proteins involved in signal transduction and membrane channels are regulated by phos